09:00 - 10:00
09:00 -
Welcome Address
ESA, ESA - ESA-ESRIN, Italy
09:10 -
Workshop organisation and objectives
Engdahl, Marcus - ESA-ESRIN, Italy
09:20 -
Sentinel1 Mission Status
Potin, Pierre
Potin, Pierre - ESA-ESRIN, Italy
09:40 -
S-1 constellation performance status
Miranda, Nuno
Miranda, Nuno - ESA-ESRIN, Italy
10:00 - 10:40
10:00 -
The Sentinel-1 Mission Performance Center activities and support for the SAR Oceanography community
Hajduch, Guillaume
Hajduch, Guillaume (1); Vincent, Pauline (1); Cordier, Karen (1); Meadows, Peter (2); Piantanida, Riccardo (3); Small, David (4); Mouche, Alexis (5); Johnsen, Harald (6) - 1: C.L.S, France; 2: BAE System Applied Intelligence; 3: Aresys; 4: Remote Sensing Laboratories (RSL), Univeristy of Zurich; 5: IFREMER; 6: NORUT
The Sentinel-1 core ground segment allows all Sentinel-1 data to be systematically acquired, processed and distributed [1]. Within the core ground segment, the S-1 Mission Performance Center (MPC) is responsible for calibration, validation, quality control and end-to-end system performance assessment of the products [2]. The MPC includes expert teams for specific Cal/Val, online and offline quality control, algorithm correction and evolutions for the SAR processor both for Level 1 (SLC and GRD) and Level 2 (OCN) products.
The purpose of this presentation is threefold:
First, to present the organization of the MPC and the team covering both L1 and L2 experts, and how the QC/Cal/Val activities on Level 1 products benefits to the L2 products.
Second, to present the main evolutions of the SAR processing baseline for both Sentinel-1A and 1B since their respective start of operations and the planned evolutions. More detailed information of the Level 2 product status will be presented by the members of the Expert Support Laboratories of the MPC, also attending to the workshop.
Finally, to present how the Sentinel-1 product user can access the most up to date information on the Sentinel-1 status and performances and contact the MPC if they want to raise specific questions. This includes presentation of links to the appropriate documentation (IPF release note for S-1 product end users, Cyclic and Annual Performance reports, etc), web sites (QC Web Site, Sentinel Online, etc) and forum / social medias.
This presentation aims to complement the presentation of the mission status by ESA and to provide contact points for the oceanographic users.
[1] https://sentinel.esa.int/web/sentinel/missions/sentinel-1/ground-segment/core-ground-segment
[2] https://sentinel.esa.int/web/sentinel/missions/sentinel-1/ground-segment/core-ground-segment/pdgs
10:20 -
Sentinel-1 WV High-Resolution Image Cross Spectra–perspectives for surface current retrieval from Sentinel1 Doppler
Collard, Fabrice
Collard, Fabrice (2); Johnsen, Harald (1); Engen, Geir (1); Recchia, Andrea (3); Chapron, Bertrand (4); Bras, Sergio (5) - 1: Norut, Norway; 2: Ocean Datalab, France; 3: Aresys srl; 4: IFREMER, France; 5: ESA, ESTEC
The S1 Level-2 WV OCN product contains three geophysical components: the radial surface velocity (RVL), the ocean surface wind field (OWI) and the ocean swell wave spectra (OSW). The OCN product contains also co-located wind speed and direction from the global atmospheric model of ECMWF. The main difference between the former ASAR WM Level 2 product and S1 OCN product is higher range bandwidth (74.5 MHz versus 16 MHz) and larger imagettes (20x20km2) versus 5x5km2) allowing improved detection of very short waves and very long swell systems. In addition, the S1 OCN product contains also an RVL component providing estimates of Doppler centroid (DC) frequency and the corresponding radial velocity. As part of the Level 2 processing, full resolution cartesian cross spectra (CCS) are generated by inter-looking the SLC image providing complex spectra with range spectral components down to around 5 meters. The CCS product is an internal product of the Sentinel 1 IPF OCN processor, but not currently provided within the L2 netCDF product due to the size of the data. It is well known that the CCS can provide ambiguity free estimate of the ocean wave spectra, and with the high bandwidth of Sentinel 1 WV mode, directional properties of short wave (wind driven) spectra can be revealed. The CCS is shown to provide useful proxy for the range wind speed component, and thus also for the Doppler centroid anomaly (DCA). In this paper we describe an optimal resampling of the CCS such as to reduce the data volume to fit within next version of L2 OCN netCDF product, and still preserve the spectral shape in the region of interest. We also present some perspectives for estimation of new spectral metrics which can be applied for Doppler calibration, NRCS monitoring, wind estimation, and RAR MTF estimation.
11:10 - 11:50
11:10 -
Sentinel-1 WV OSW Validation
Johnsen, Harald
Johnsen, Harald (1); Mouche, Alexis (2); Grouazel, Antoine (2); Husson, Romain (3) - 1: Norut, Norway; 2: Ifremer, France; 3: CLS, France
The Sentinel -1 Level-2 WV OCN product contains three geophysical components: the radial surface velocity (RVL), the ocean surface wind field (OWI) and the ocean swell wave spectra (OSW). The main difference between the former ASAR WM Level 2 product and S1 OSW component is higher range bandwidth (74.5 MHz versus 16 MHz) and larger imagettes (20x20km2) versus 5x5km2) allowing improved detection of very short waves and very long swell systems. In addition, the S1 OCN product contains a RVL component providing estimates of Doppler centroid (DC) frequency and the corresponding radial velocity. The S1 Level 2 WV OCN product contains also co-located wind speed and direction from the global atmospheric model of ECMWF. As part of the Sentinel-1 Mission Performance Center activities, an automatic collocation system is set-up at Ifremer providing collocated wave model (WW3), scatterometer (ASCAT-A, ASCAT-B) as well as altimeter (Cryosat-2) data in near real-time and on global basis. Collocation with buoys is also undertaken. A flexible reprocessing system including the S1 IPF Level 2 processor is installed at the cluster at Ifremer. The system is capable of reprocessing of one month of S1 WV OCN products within few hours. The system is used to upscale and evaluate on massive data, evolution of the Level 2 processing algorithms.
The operational 2D ocean wave spectra of the OSW product is validated partition by partition against WW3, and also for effective Hs. For effective Hs a standard deviation of around 0.45 m is achieved for WV1 (incidence angle 23o), while around 0.55m for WV2 (incidence angle 36o). In general the WV1 performs better than WV2 due to larger SNR. The validation results show that the main deviations are related to none-wave low frequency signal contaminating the SAR wave spectra. These are likely to appear at low winds. As of this improved filtering of low frequency signal is implemented and impact is assessed using the reprocessing capacity at Ifremer. The Level-2 algorithm improvements and upgrades will be part future S1 IPF releases.
11:30 -
History on Sentinel-1 L2 wind product performance (OWI): Calibration/ Validation strategy, status and coming evolution
Vincent, Pauline
Vincent, Pauline (1); Hajduch, Guillaume (1); Husson, Romain (1); Grouazel, Antoine (2); Mouche, Alexis (2) - 1: CLS, France; 2: Ifremer, France
The OCN products from Sentinel-1 routinely produced by the PDGS and circulated to the users consist on 3 main components: surface wind retrieval (OWI), radial velocity (RVL), swell (OSW) [1]. This conference paper is focused on the validation and the status of performance for the OWI component, to be shared with the Sea Sar community.
First, it will be reminded the strategy put in place to validate those products at Mission performance center. In a second part, we will summarize the performance status of those products (past and present for the Sentinel 1 missuion). Then, it will present the short-term evolution for the OWI products.
The validation of these products is made as a collaboration between CLS and Ifremer, in the framework of Mission Performance Center. The strategy to assess the accuracy of the wind retrieval is to compare it with an auxiliary wind source which is used as a reference. This source could be in-situ data from buoy, other satellite data (ex: scatterometer) or atmospheric model outputs. It is important to outline the importance to multiple the type of data used as reference, due to their coverage, resolution or possible bias. In this scope, Ifremer has performed systematic collocations with such data with L2 products generated by the ESA-IPF by PDGS.
It can be noticed the strong correlation of the SAR-derived wind speeds with the wind references. The bias and the RMS are less important for the comparison with ECMWF re-analysis wind model, this is expected since the wind inversion is based on the ECMWF forecast as an a priori wind input. A nominal RMS of 1.5m/s to 2m/s is observed, for all the wind reference. A bias of around 0.3 to 0.7 m/s depending on the wind reference is present. This bias may be related to the choice of the current geophysical model function (GMF) used for the wind retrieval. As expected, at low wind speeds, the NESZ impacts the SAR wind measurement. Then, a noticeable impact of the noise can be observed on HH polarization and high incidence angle (over estimation, beam shaped profile). The performances are similar for both satellites. Some work to improve the performance are currently conducted and will be presented.
With the next version of the processor IPF 2.90, the format of L2 data has been extended in order to include additional information on cross-polarization (NRCS, NRCS corrected and NESZ annotation on the OWI grid) [4]. This is a good opportunity for the community working on very strong wind-speed conditions (extreme wind phenomena: ex hurricanes…). The products that will be processed with this IPF version will also contain the new NESZ annotation (variations on the azimuth).
We would like by this paper take the opportunity to share with to the user Community the strategy of the validation of the OWI component of Sentinel-1 OCN products, the status on their performances (present and past), and the short-term evolution planned for the next year.
11:50 - 12:20
13:20 - 15:00
13:20 -
Using Sentinel-1 dual-polarization incoherent images to detect iceberg
Marino, Armando
Marino, Armando (1); Dierking, Wolfgang (2,3); Soldal, Ingri Halland (3) - 1: The Open University, United Kingdom; 2: Alfred-Wegener-Institut fuer Polarund Meeresforschung, Germany; 3: Tromso University, Norway
The presence of icebergs represents a danger to navigation and activities in cold waters near and in the Polar Regions. With the aim of improving safe navigation, in the last decades satellite data have been used to detect and track large icebergs. Specifically, the potentials of scatterometers, altimeters and synthetic aperture radar (SAR) systems have been successfully tested. However, small icebergs are still hard to identify especially when these are embedded in sea ice.
In this work, we present the latest developments on a recently proposed iceberg detector, namely the intensity Dual-Pol Ratio Anomaly Detector (iDPolRAD) [1]. The algorithm is based on the use of incoherent (or Ground Detected) dual-pol HH and HV (or VV and VH) images. These are routinely acquired by Sentinel-1 in the Polar Regions.
The detector considers a bank of box cart filters for the HV and HH images averaging over sizes selected as test and training areas. The difference between the HV intensity over the test and the HV intensity over the training areas is divided by the intensity of HH over the training area [1]. Developing the maths, it is possible to demonstrate that the detector returns a high value if the HV intensity and the depolarisation ratio are increasing from train to test area. This happens if we have a positive anomaly (i.e. an increase) in volume or oriented multiple reflections, which forms the main scattering mechanisms of icebergs.
To validate the algorithm, we used Sentinel-1 data acquired on the East Coast of Greenland, near the Kangerlussuaq glacier where a large number of grounded icebergs are visible in the images. The grounded icebergs were used to perform a quantitative validation. The result of the analysis is that the iDPolRAD contrast between sea ice clutter and icebergs increases to about 80 times compared to the contrast of the filtered HV intensity alone.
Reference:
A. Marino, W Dierking, and C. Wesche, “A depolarization ratio anomaly detector to identify icebergs in sea ice using dual-polarization SAR images,” vol. 54, no. 9, pp. 5602–5615, 2016.
13:40 -
On the use of Sentinel-1 (IW/EW and WM modes) for iceberg detection applied to sail racing around the world
Viard, Jimmy
Viard, Jimmy (1); Longépé, Nicolas (1); Vincent, Pauline (1); Husson, Romain (1); Tournadre, Jean (2); Kerbaol, Vincent (1); Hajduch, Guillaume (1) - 1: CLS, France; 2: IFREMER, France
The routes of vessels participating to the yacht races around the world (Vendée Globe, Barcelona World Race, Volvo Ocean Race, Jules Vernes Trophy [1-4], and Brest Ultim in 2019 [5]) cross the southern parts of the Atlantic, Indian and Pacific oceans.
The icebergs infesting those areas are a major threat for the safety of the navigators. Iceberg monitoring is thus of vital interest to set up no-go zones, optimize routes and warn sailboats of infested areas. This paper presents the benefit of Sentinel-1 (and more generally SAR imagery) for operational iceberg monitoring service around Antarctica. We specifically provide a feedback on the last 2016-2017 Vendée Globe race
An initial situation map of icebergs was built before the race by using sparse sampled but systematic observations of altimeters (Jason 2, SARAL/Altika). For the first time, all the acquired wave mode (WM) S-1 imagettes were used jointly to produce cumulative iceberg maps over long period (from August to October 2016) enabling the identification of potential iceberg fields. To this end, a dedicated WM processing chain has been developed. More specifically, it enabled the detection and basic characterization of icebergs from both Wave mode 1 and Wave mode 2 acquisitions, respectively at 23° and 36° incidence angle. Whereas the backscattering of icebergs is generally higher than the surrounding ocean sea surface at 36° (double bounce and/or multiple bounces), icebergs observed by WM1 may appear darker.
During the 2016 VG race, 317 S-1 images in Extra Wide Swath have been specifically planned and acquired by ESA, few days ahead the first boat from mid-November 2016. In addition to these images, 91 IW images have been integrated in the analysis as they were already part of the planned observation strategy, imaging areas with potential iceberg risks (e.g. Kerguelen, Crozet islands…). All these images have been processed with automatic iceberg detection, and then compiled by ice analysts to provide iceberg situation reports to the Vendée Globe race direction. In addition, a drift model has been used to forecast the location of EO-based detected icebergs. Exclusion zones could thus be defined.
In this paper/talk, the following topics will be addressed:
We would like to thank the ESA Sentinel-1 mission manager Pierre Potin, and all the mission planners for their kind cooperation. In addition, this study is being partly funded by the CNRS/LOPS via the 2018 SARICE project.
[1] Vendée Globe: https://www.vendeeglobe.org
[2] Barcelona Race : http://www.barcelonaworldrace.org/
[3] Volvo Ocean Race : http://www.volvooceanrace.com/fr/home.html
[4] Trophée Jules Vernes: http://www.tropheejulesverne.org/
[5] Brest Ultim http://www.brest-ultim.fr/
14:00 -
Synergies between SAR and AIS data for the surveillance of the maritime traffic
Vadaine, Rodolphe
Vadaine, Rodolphe (1); Nguyen, Duong Van (2); Longépé, Nicolas (1); Hajduch, Guillaume (1); Fablet, Ronan (2) - 1: C.L.S; 2: IMT Atlantique
The surveillance of the maritime traffic is a major issue for security and monitoring issues. Spaceborne technologies, especially satellite AIS ship tracking and high-resolution imaging, open new avenues to address these issues. The free access to Sentinel Earth Observation data streams (high-resolution Sentinel-1 SAR and Sentinel-2 optical imaging, up to a few TB daily [1]) offers novel opportunities for the analysis and detection of ship behaviours, including AIS-Sentinel data synergies.
Regarding the joint usage of spaceborne SAR data and AIS information, its relevance for the characterisation of SAR vessel detection performances using interpolated/extrapolated AIS tracks as ground truth has been already demonstrated [2]. It is operationally used for the monitoring of oil spills at sea and the identification of potential polluter sources [3] in the framework of the European Maritime Safety Agency (EMSA) CleanSeaNet Service [4].
In this context, the SESAME (*) initiative aims to develop new big-data-oriented approaches to deliver novel solutions for the management, analysis and visualisation of multi-source satellite data streams going beyond the current implementation.
Preliminary results highlight the relevance of the synergies between SAR and AIS information in terms of geographic coverage and of detection and characterisation of abnormal activities at sea. We also demonstrate the feasibility to construct large-scale datasets of SAR echoes acquired in various configuration corresponding to a subset of known vessels with a view to applying machine-learning-based detection and classification strategies.
Considering a four-month dataset of Sentinel-1 A satellite data over Europe from March to June 2017, we collected 5414 SAR images. They were systematically processed using CLS vessel detection algorithm. The detected SAR echoes were then matched with interpolated/extrapolated AIS data to build a unique dataset. We then explore the development of novel learning-based ship vessel detection and identification strategies.
(*) Acknowledgements :
This work was supported by public funds (Ministère de l'Education Nationale, de l'Enseignement Supérieur et de la Recherche, FEDER, Région Bretagne, Conseil Général du Finistère, Brest Métropole) and by Institut Mines Télécom, received in the framework of the VIGISAT project managed by "Groupement Bretagne Télédétection" (BreTel – Brittany Remote Sensing) and by DGA (French Military Procurement Agency) through the ANR/Astrid Program.
[1] A.G. Castriotta, “Sentinel data access report 2016”, Tech. Rep.
[2] R. Pelich et al, “Performance evaluation of sentinel-1 data in SAR ship detection,” in IEEE IGARSS, 2015.
[3] N. Longépé et al, “Polluter identification with spaceborne radar imagery, ais and forward drift modeling,”Marine Pollution Bulletin, vol. 101, no. Issue 2, pp.826–833, 2015.
[4] European Maritime Safety Agency, “Clean sea net service,” https://portal.emsa.europa.eu/web/csn.
14:20 -
Analysis of the Ship-Sea Contrast using Quasi Simultaneous Space-borne Single and Quad Polarization Data
Velotto, Domenico
Velotto, Domenico (1); Nunziata, Ferdinando (2); Marino, Armando (3) - 1: German Aerospace Center (DLR), Germany; 2: Università di Napoli Parthenope, Italy; 3: Open University, U.K.
Polarimetric SAR (PolSAR) technology has been proven to be very effective in many areas of Earth Observation (EO) including several maritime applications. Firstly developed for airborne platforms, PolSAR has been later adopted also in space-borne missions, e.g. RADARSAT-2, TerraSAR-X, etc. There are certainly a number of technical and hardware constraints in the space-borne case, such as limited power and antenna size, which limit the radar performances.
Spatial resolution, signal to noise ratio, azimuth ambiguity to signal ratio and spatial coverage of a space-borne SAR system are characteristics that affect the capability to effectively detect and classify ships in the sea environment. Compared to single-pol acquisitions, where only one polarization state is transmitted and only one is received at a designed pulse repetition frequency (PRF), a PolSAR system record the 4 linear combinations of Horizontal (H) and Vertical (V) linear polarized pulses, hence, the system has to operate with an ideally doubled PRF. As consequence: a) the Doppler bandwidth per channel is reduced, which causes a loss in azimuth resolution, b) the ambiguous range region increases, which causes the across-track swath to be narrowed down and c) the azimuth length of the antenna is halved in reception, which causes a wider main lobe and higher side lobes.
In the past, the performances of space-borne PolSAR system for the detection of maritime targets have been assessed be direct comparison among the channels acquired. Although this comparison strategy is quite fair, it has never been possible to compare the ship-sea contrast of a target imaged by PolSAR with the contrast resulting from a conventional single-pol SAR that observes the same target with the same acquisition geometry. This is the fairest comparison one can imagine since it does include on one side the unprecedented scattering details coming from a PolSAR acquisition; however, on the other side, it accounts for the lower spatial resolution that characterizes PolSAR acquisitions. It is obvious that such experiment conducted in space requires two twins’ satellites operating simultaneously (or close to be simultaneous). We have taken the unique opportunity provided by the pursuit monostatic configuration of the TanDEM-X mission to accomplish such experiment. In such a configuration, the twin satellites TerraSAR-X (TS-X) and TanDEM-X (TD-X) can operate independently by each other while flying in a close formation with the two orbits separated of circa 76km (that would correspond to approximately 10sec). Operating the first satellite in the standard StripMap single-pol mode and the second satellite with the experimental Dual Receive Antenna (DRA) quad-pol mode, it has been possible to observe the same scene with the same geometry and almost simultaneously.
This paper summarizes the results obtained in the ship-sea contrast and discrimination, processing real SAR data and collocated ship ground truth information provided by automatic identification system on board the ships present in the selected geographical region.
14:40 -
Preliminary assessment of a wake-based ship detection process
Graziano, Maria Daniela
Graziano, Maria Daniela (1); D'Errico, Marco (2); Grasso, Marco (2) - 1: University of Naples "Federico II", Piazzale Tecchio, 80, 80125, Naples, Italy; 2: University of Campania "Luigi Vanvitelli", Via Roma, 29, Aversa (CE), Italy
Recently, the European Board and Coast Guard Agency Frontex has showed the necessity to monitor the traffic of illegal goods in different parts of the Mediterranean Sea [1] . The smugglers typically use very fast and small ships, made by fiberglass and carry out their traffic during the night time. This makes the detection of ships by electro-optical sensors almost impossible and the detection by SAR-based algorithm very difficult, due to weak radar cross section of the used ships. To overcome such difficulties, the Agency suggests the detection of the illegal ships by detecting their wakes, which will be very prominent.
In this ambit, a technique for ship wake detection has been recently proposed and applied to X-band SAR images provided by COSMO/SkyMed and TerraSAR-X [2]-[3]. Moreover, to assess the method’s robustness with respect to wake appearance in C-band images, the algorithm has been also applied over 28 wakes imaged by the Sentinel-1 mission with different polarizations and incidence angles [4].
The detection performance has been analyzed in order to distinguish between true confirmations, i.e., imaged features classified as true; true discards, i.e., not-imaged features classified as false; false confirmations, i.e., not-imaged features classified as true; and wrong positioning, i.e., feature locations mismatched with the true locations. The results showed that the wakes were correctly classified in 78.5% of cases, whereas false confirmations occurred in 18.5% of the components.
The above described technique has been conceived to be applied when the ship position is already known in order to estimate its route.
In this paper, the application of the wake detection technique will be properly adapted to be used as a validation of ship presence. In fact, the algorithm correctly validates the visibility of the wake even when it is not present. This means that the algorithm has the potentiality to recognize when the wake is imaged or not, and, in the former case, to identify the wake vertex and the ship location.
[1]. On-line Source: http://frontex.europa.eu/assets/About_Frontex/Procurement/2017-OJS191-390537-en.pdf
[2]. Graziano, M.D, Grasso M., D’Errico, M., Performance Analysis of Ship Wake Detection on Sentinel-1 SAR Images, Remote Sens. 2017, 9(11), 1107;
[3]. M.D. Graziano, M. D’Errico, G. Rufino, Ship heading and velocity analysis by wake detection in SAR images, Acta Astronautica, Volume 128, November–December 2016, Pages 72–82.
[4]. M.D. Graziano, M. D’Errico, G. Rufino, Wake component detection in X-band SAR images for ship heading and velocity estimation, Remote Sensing, Volume 8, Issue 6, 2016.
15:30 - 16:30
15:30 -
Characterization and detection of icebergs in open water and sea ice using spaceborne Fully polarimetric SAR
Akbari, Vahid
Akbari, Vahid (1); Lohse, Johannes Philipp (1); Eltoft, Torbjørn (1); Dierking, Wolfgang (1,2) - 1: UiT The Arctic University of Norway, Norway; 2: Alfred Wegener Institute for Polar and Marine Research (AWI)
Icebergs are pieces of freshwater ice that have broken off from marine glaciers and ice sheets. Calving of icebergs is part of the mass loss of glaciers and ice sheets in Polar regions and play an important role in the global freshwater cycle by delivering freshwater to the regions that are very far from the glaciers and ice sheet margins. Studying the regional distribution of icebergs, their volumes, motions, and their interactions with the ocean, atmosphere, and cryosphere is of interest. This interest is driven by the need for: 1) short-term iceberg drift predictions as a key to prevent damages to ships or oil rigs and 2) more knowledge about the role of icebergs in long-term climate change. Since spaceborne synthetic aperture radars (SARs) can image the vast Polar regions independently of light during the polar night and harsh weather conditions, they are the preferred sensors for iceberg detection and monitoring. Many studies in the literature have demonstrated the potential of single-channel SAR images for icebergs detection and characterization. However, RADARSAT-2 (RS-2) and the recently launched Sentinel-1 provide quad- and dual-polarization SAR data, respectively. It has been shown in several studies that the additional information contained in multipolarization data improves characterization, classification, and detection of icebergs.
In this study, we first demonstrate the potential of different polarimetric features with respect to improved iceberg discrimination from the surrounding sea ice or open water. To find the feature set to best separate icebergs (large in size) from sea ice and open water, we run a Sequential Forward Feature Selection (SFFS) with a Bayesian classifier. Training regions for different classes (icebergs, sea ice, and open water) are manually selected from the radar polarimetric images. We estimate the probability density functions of the features using Parzen windows with a multivariate Gaussian kernel function and calculate classification accuracies with cross-validation, such that the result is independent of the training. Based on the classification accuracy, we add features one by one to the chosen feature set, until the accuracy starts to decrease. The SFFS automatically provides the best feature set for the separation of icebergs from other classes. We use 5 RS-2 quad-pol images in the Bellingshausen Sea and southern Weddell Sea regions for iceberg characterization. However, the detection of small icebergs floating in nonhomogenous sea clutter environments is a challenging task. Based on the best feature set from the SFFS for iceberg characterization, we use our segmentation-based iceberg detection algorithm to show its performance for detecting small icebergs in real complex situations. We test the algorithm with a series of quad-pol RS-2 images containing numerous icebergs broken off from glaciers in Kongsfjorden on Svalbard which cover different sea states, wind conditions, and incidence angles in open and ice-infested water background.
15:50 -
A Ship Wake Detectability Model and its Application to Wake Detection
Tings, Björn
Tings, Björn; Velotto, Domenico - DLR, Germany
The opportunity to use satellite-based SAR sensors for the monitoring of ship traffic has been researched extensively during the last decades. Most commercial ships are well visible on SAR images due to their excellent backscattering capabilities. However, the direct detection of any kind of maritime vessel is not possible, as non-metallic maritime objects made of materials like rubber or wood are nearly invisible on SAR images. The reason for this is the inherent property of SAR of being most sensible to the availability of conductive materials. Nevertheless, the anomalies on the imaged conductive sea surface structure induced by moving ships can be recognized. For this reason, the consideration of ship wake signatures for the monitoring of ship traffic is of importance. While most automatic ship detection methods search for strong backscattering on the SAR images to identify possible positions of ships, the automatic detection of the ship wake signatures is a more complicated task. Simultaneously to automatic ship detection in the recent decades also the detection of the signatures of ship wakes has been studied. Published approaches often apply Hough transform or image convolution by filter banks.
The executed study elaborates on the detectability of ship wake signatures on TerraSAR-X, RADARSAT-2 and Sentinel-1 images. A binary logistic regression classifier is applied to build a new data-driven detectability model. The classifier is used to calculate probabilities of the visibility wake signatures in the surroundings of verified positions of moving ships, by parameters describing environmental conditions, image acquisition settings and ship properties. As the parameters describing environmental conditions and image acquisition settings are available for each SAR images, the wake detectability model can be applied to control the sensibility of operator-based and automatic wake detection methods. Also drawbacks to the ship velocity can be estimated by reversing the model. An example application will be presented.
Similar to the wake detectability the detectability of ship signatures can be modelled. Such a data-driven approach for ship detectability is also presented and compared to the state-of-the-art simulation-based ship detectability model, which was developed by Vachon. For high and low resolution images the models show similar dependencies of ship detectability.
16:10 -
Analysis on Vessel Velocity Estimation in Synthetic Aperture Radar Image Domain.
Panico, Alessandro
Panico, Alessandro; Renga, Alfredo; Graziano, Maria Daniela - University of Naples, Italy
Maritime surveillance is a very worthwhile application for space remote sensing systems. Indeed, it typically interests wide areas and needs a frequent observation of those areas. In particular, SAR systems fit perfectly these requirements being active sensors that work in any weather and lighting condition and because the resolution of current systems is really impressive even if the swath of the observed scene can be several km wide. Within this framework, the estimation of the moving target velocity in maritime environment represents a point that is still partially open. Despite the existence of several methods and algorithms that encompass both signal and image processing, all of them have different pros and cons. For sure, from the user point of view a single SAR product is generally used for several uses (objects detection, target coordinates extraction, classification…) and therefore the image domain methods can be more affordable. The author developed an innovative method to retrieve the vessel speed that is based on the observation and analysis of the Kelvin wake, that is the hydrodynamic perturbation generated by any target sailing in deep sea conditions (no second order perturbation caused by waves that are reflected by the seabed). The proposed algorithm works efficiently also when the wake structure is partially observable in real SAR images. The literature state of the art in this field was Zilman’s technique based on the application of the FFT on a general line chosen within the Kelvin cone, possibly in proximity of the edges. The uncertainty in the line positioning and sometimes the difficulty in clearly localizing the Kelvin cusps generates a low accuracy in the velocity estimation because the waves distances change significantly with slightly different line directions. Moreover, the well assessed relationship among the vessel speed and the Kelvin wake is based on the measure of the Kelvin wavenumber or, as Zilman does, the cusps distance. The problem of Zilman’s approach is that in real SAR image the aim of measuring the cusps distance, since real SAR images are not always clear, leads to a general peaks distance. On the contrary the proposed algorithm relates the general peak distance with the real wavenumber, that is generally masked by the ship turbulent wake, by the application of a developed model that takes into account the shape of the Kelvin waves. The ongoing activity is the comparison with other existing algorithms that works in image domain (especially the ship-wake displacement) and with the ground truth (AIS data), possibly in different observation conditions (incidence, polarimetry, ship speed, sea state…). In particular, since each image is linked with the corresponding AIS datum, the position and the velocity are interpolated (with good agreement with real values especially when the time gap is little) and then compared with the outcome of different image analysis methods. Preliminary results show that incidence angle appears very critical for Kelvin arms detection, whereas very low vessel velocities implies the absence of turbulent wake, being critical for the application of the ship-wake displacement algorithm.
16:20 - 17:00
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09:00 - 10:20
09:00 -
Advances in Studies of Upper Ocean Mesoscale Processes and Dynamics from satellite sensor synergy: Benefitting from the GlobCurrent and Ocean Virtual Laboratory Projects.
Johannessen, Johnny A.
Johannessen, Johnny A. (1); Chapron, Bertrand (2); Collard, Fabrice (3); Kudryavtsev, Vladimir (4) - 1: Nansen Environmental and Remote Sensing Center, Norway; 2: Ifremer; 3: OceanDataLab; 4: Russian State Hydrometeorological University
The visualization platform developed in the ESA funded GlobCurrent Project (http://www.globcurrent.org) and the Ocean Virtual Laboratory Project (http://ovl.oceandatalab.com) are systematically explored to maximize the benefit of sensor synergy for advances in studies and understanding of upper ocean mesoscale processes and dynamics. The highly complex and dynamic greater Agulhas Current regime is chosen. Both radar altimetry, microwave and infrared radiometry, imaging spectrometer and synthetic aperture radar data are explored in combination with in-situ observations from Argo profiling floats and surface drifter data. Individually the multi-sensor satellite data have inherent capabilities and limitations with regards to spatial resolution, coverage and repeat sampling. The sensor synergy is a highly attractive approach to shortcut some of the limitations. As such, it advances the interpretation and application as previously clearly demonstrated by Kudryavtsev et al. (2012).
In this presentation, the focus is on detection and interpretation of ocean frontal boundaries. Under ideal imaging conditions (no clouds, weak to moderate winds, etc) essentially all optical and microwave remote sensing sensors manifest presence of ocean fronts such as related to gradients and changes in: (i) infrared- and microwave radiometer-based sea surface temperature; (ii) imaging spectrometer-based chlorophyll distribution and concentration; (iii) altimeter-based sea level anomaly; (iv) SAR-based surface roughness and (vi) SAR-based range Doppler shift. A common driver for the expression of these frontal boundaries is the surface currents and their spatial changes and meandering pattern including instabilities and formation of eddies. In particular, surface current changes and gradients related to shear, vorticity and strain can lead to rotation, divergence and convergence with corresponding upwelling and downwelling in the upper ocean. These distinct surface current changes therefore establish a spatial correlation in gradients of sea surface temperature, sea level anomaly and sea surface roughness. Capitalizing on this correlation it is demonstrated that satellite sensor synergy clearly advances the routine monitoring of the two-dimensional surface expression of mesoscale upper ocean processes and dynamics. This approach, here demonstrated for meanders and eddies in the Agulhas Current core and in the Agulhas Return Current, largely shortcut the limitation with individual sensor imaging capabilities. Jointly with coincident in-situ observations, it evidently creates an important step toward advances in near real-time monitoring and quantitative interpretation of mesoscale upper ocean processes and dynamics. As such, it is also a promising approach in terms of assessment as well as systematic use of the detection capabilities provided by the approved NASA and CNES Surface Water and Ocean Topography (SWOT) mission expected to be launched in 2021. Arguably, the approach will also strengthen high resolution model validation and assimilation.
09:20 -
Multi-sensor investigation of upwelling caused by cyclonic eddies
Alpers, Werner
Alpers, Werner - University of Hamburg, Germany
In studies of upwelling usually data from infrared and optical sensors are used which provide information on the sea surface temperature (SST) and the chlorophyll-a (Chl-a) concentration. In this paper it is shown that also synthetic aperture radars (SAR) images can also give valuable contribution to such studies. Upwelling regions become detectable by SAR because they are associated with a reduction of the radar backscatter due to 1) a change of the stability of the air-sea interface or/and 2) the presence of biogenic slicks. While the change of the stability of the air-sea interface due to the presence of cold surface water in the upwelling region causes only a small reduction of the radar backscatter, biogenic cause a very strong reduction, usually of more than 10 dB. In areas of strong upwelling, the biological productivity is high due to increased nutrient supply from lower water levels. The biota living in this area secrete surface active material that ascends to the sea surface and forms there biogenic slicks, which often cover large areas in the form as monomolecular layers. The biogenic slicks damp the short-scale surface waves, which are responsible for the radar backscattering, as strongly as mineral oil films, and thus areas covered with the surface films become visible on SAR images as areas of strongly reduced radar backscatter (dark areas). Biogenic slicks often get entrained in the surface current field of eddies and thus render the structure of eddies visible on SAR images.
This paper focusses on upwelling caused by cyclonic eddies which are generated by different mechanisms: 1) detachment from a filament of an upwelling front (in the Benguela upwelling system), 2) interaction of a strong current with a headland (Cap-Vert at the east coast of Senegal), 3) intensification of a meandering current (the Agulhas Return Current off the coast of South Africa and the Atlantic Ionian Stream in the Mediterranean Sea off the south coast of Sicily). These upwelling events are studied by using Sentinel-1 SAR images, Modis SST and Chl-a maps and model data of geostrophic surface currents. It is shown that this synergism yields new insights into upwelling mechanisms, e. g., into upwelling associated with cyclonic meanders of the Agulhas Return Current. A Sentinel-1 SAR image is presented that shows that on the rim of a cyclonic meander strong surface convergence occurs, which causes increase in the short-scale sea surface roughness and thus of the radar backscatter. At the same time, the Chl-a map shows on the rim increased the Chl-a concentration indicating increased of biogenic productivity, while the SST map shows a double-sign structure of the rim (a warm outer ring and a cool inner ring). A physical explanation of this phenomenon is presented.
09:40 -
Observing Small Eddies with Multiple Sensors
Holt, Benjamin
Holt, Benjamin - Jet Propulsion Laboratory, United States of America
Making use of the fine resolution and repeat capabilities of two Sentinel-1 SARs, optical imagery from Sentinel-2, and optical/thermal IR imagery Landsat8, multiple acquisitions over a few days period that provide fascinating and enticing repeat observations of rapidly evolving small eddies. Often these sequences require coincident SST and ocean color imagery to help clarify the signatures. The study will show recent observations over Southern California and Lake Superior that illustrate the value of rapid sequences using multiple sensors. Also shown will be sequences of small eddies in the marginal ice zone obtained from multiple SAR sensors. From such sequences, useful derivations of eddy rotational velocity may be obtained.
10:00 -
Synergistic use of Sentinel-1 SAR, Sentinel-2 MSI and Sentinel-3 OLCI to detect and monitor the January 2018 Sanchi oil spill incident in the China Sea.
Blondeau-Patissier, David Michel
Blondeau-Patissier, David Michel (1); Schroeder, Thomas (1); Irving, Paul (2); Witte, Christian (3) - 1: CSIRO Oceans and Atmosphere, Brisbane, Australia; 2: Australian Maritime Safety Authority, Canberra, Australia; 3: Department of Science, Information Technology and Innovation, Brisbane, Australia
The latest European Space Agency’s (ESA) Sentinel missions support emergency response such as oil spill monitoring by providing timely and cost-free data at high temporal, spectral and spatial resolution.
With the launch of Sentinel-3A early 2016, and the previously launched Sentinel-1A/B and Sentinel-2A/B satellite series, the three different missions now offer the potential for the synergistic use of optical, thermal and SAR measurements for the detection and mapping of oil spills anywhere in the world’s ocean. Scanning the Earth’s surface with two satellites significantly increases revisit capability, estimated to be approximately 5 days for Sentinel-1 and Sentinel-2, and 1 day for Sentinel-3 at the equator. This significant improvement in data acquisition frequency, in comparison to the previous ESA Envisat 2002-2012 mission, allows for the synergetic use of three Sentinel satellites for near-real time oil spill monitoring.
The recent incident that occurred 400 km offshore in the China Sea with the Sanchi Iranian tanker on 14th January 2018 is of a sufficiently large size (i.e., several oil slicks of light crude oil covering a surface area > 300 km2) that it allows for this synergistic mapping to be tested.
We investigate the combined use of Sentinel-1A/B SAR, Sentinel-2A/B Multi-Spectral Instrument (MSI) and Sentinel-3A Ocean and Land Colour Instrument (OLCI) imagery acquired from the 17th January 2018 onwards for improved discrimination of this oil slick from the various sensors. Interferometric Wide swath (IW; 250 km) and Extra Wide swath (EW; 400 km wide) modes acquisitions from Sentinel-1, specifically used for wide area coastal monitoring including ship traffic and oil spill monitoring, and full resolution (300 m) scenes from Sentinel-3 at 865 nm in particular, proved very useful for the oil slick mapping and the evaluation of its environmental impact by tracking its dispersion over time and space.
We will show that the synergetic use of ESA Sentinel satellites offers an enormous potential for innovation in the context of oil spill monitoring, and how this combined approach can be successfully transferred to our new exciting project on oil spills detection along the Great Barrier Reef in Australia.
10:50 - 11:50
10:50 -
The Copernicus Maritime Surveillance service:Quasi real time SAR-based applications in the service of Member States
Vicente, Ricardo
Vicente, Ricardo; Lourenço, Pedro - European Maritime Safety Agency, Portugal
The Copernicus Maritime Surveillance (CMS) service provides monitoring of human activity at sea based on satellite Earth Observation data, namely SAR and optical imagery. The European Maritime Safety Agency (EMSA) is entrusted by the European Commission (EC) to implement CMS as a component of the Copernicus Programme[1].
The objective of CMS is to support monitoring activities of European coastal States National Administrations and relevant EU bodies, with responsibilities in the maritime domain, in the fields of: fisheries control, maritime safety and security, law enforcement, customs, pollution monitoring and other activities which affect EU maritime interests (e.g. defence).
In CMS, Earth Observation (EO) products are combined with other maritime related information to provide end users a tailored made, near real time (NRT) service in support to a wide range of maritime functions. The service is delivered to authorized users only, according to precise data access police rules and aims enhancing the maritime surface picture. CMS products include: EO SAR and Optical images, EO value added products and fusion products, with spatial resolutions spanning from 30 cm to 100 m. Products include: ship detection, oil spill detection and activity detection. The service level is focused in NRT product delivery (less than 30 min for SAR and 45 min for Optical products).
User satisfaction feedbacks from operational usage, as well as a yearly comprehensive evaluation on the service as a whole, have been crucial in identifying areas for improvement and in fulfilling operational user needs. Since the start, (September 2016) the following new CMS products were identified: Lost container detection, Iceberg detection, NRT ice monitoring, Wake detection, and Aircraft debris detection.
Besides new products, the evolution of the space-related infrastructures will bring new EO capabilities that will enable addressing use cases that are currently difficult to support using space based assets. Examples of these new capabilities include: vessel radar detection, high resolution thermal infrared imaging sensors, high resolution satellite video and satellite based GSM/satellite phone detection. Additionally the increase in the number of satellite constellations may enable a more continuous monitoring of the maritime domain, which will enable the tracking of non-cooperative targets, addressing a significant gap in existing EO based systems. Finally, with the expected increases in both volume and frequency of EO data, the potential for use of automatic behaviour detection algorithms and other spatial based analytics (relying on state of the art machine learning algorithms), for maritime surveillance applications, is greatly increased.
At this stage, considering the operational needs for new and improved EO products, as well as the new satellite based capabilities that are in the short-term horizon for deployment, it is clear that there is a strong opportunity for Research and Development institutions’ to improve and expand the existing EO product catalogues to better address the maritime surveillance operational needs of a wide range of users.
[1] Established by the Regulation (EU) No 377/2014 of the European Parliament and of the Council of 3 April 2014
11:10 -
The assimilation of Sentinel-1A and 1B SAR wave spectra in the global CMEMS wave system
Aouf, Lotfi
Aouf, Lotfi (1); Dalphinet, Alice (1); Husson, Romain (2); Law-Chune, Stephane (3) - 1: Meteo-France, France; 2: CLS; 3: Mercator-Ocean
Meteo-France has implemented the upgraded global Copernicus Marine Environement and Monitoring Service (CMEMS) wave system in March 2018 with major achievements. This consists in first forcing the wave model MFWAM by the surface currents provided by the CMEMS ocean system. Secondly the wave system assimilates operationnally the L2 SAR directional wave spectra provided by the satellite Sentinel-1A and 1B in the frame of copernicus programme. The validation of the upgraded system has been performed over a period of two years with buoys and significant wave heights from altimetry. This paper aims to draw up the remarkable improvement of swell waves parameters induced by the assimilation of SAR wave spectra. For instance the peak wave period of long waves (greater than 200 m of wavelength) is improved by roughly 20%. On going studies related to accounting a variable correlation length in the assimilation scheme has been also investigated. Assimilation experiments have been also conducted to evaluate the consistency quality flag implemented for the Level 3 swell partitions provided by ESA in the frame of CMEMS.
In this study the impact of the assimilation of SAR wave spectra in regional wave model dedicated to the french overseas such La Réunions and Polynesia, is also examined. A better swell directional properties forecast has a relevant consequence on key parameters such as surface stress and Stokes drift for the ocean/waves coupled system. The impact of the assimilation of SAR wave spectra has been evaluated in a coupling experiment between NEMO and MFWAM models. Preliminary results on key parameters such surface currents and sea surface temperature will be discussed in this paper. Further comments and conclusions will be presented in the final paper.
11:30 -
Meteo-marine parameters from Sentinel-1 SAR imagery: towards near real-time services for the Baltic Sea
Rikka, Sander
Rikka, Sander (1); Pleskachevsky, Andrey (2); Jacobsen, Sven (2); Alari, Victor (1); Uiboupin, Rivo (1) - 1: Tallinn University of Technology, Estonia; 2: German Aerospace Center (DLR)
Method
for estimating meteo-marine parameters from satellite Synthetic Aperture Radar
(SAR) data, with a purpose of near-real-time (NRT) service over the Baltic Sea,
is presented and validated. The empirical function CWAVE_S1-IW which is based
on the spectral analysis of radar image subscenes is used for retrieving total
significant wave height from Interferometric Wide swath (IW) Sentinel-1A/B imagery.
CMOD algorithm was used for wind speed estimation from SAR images. In total, 15
Sentinel-1A/B scenes (116 acquisitions) over the Baltic Sea were processed for
comparison with off-shore sea state measurements (52 collocations) and coastal wind
measurements (357 colocations). Sentinel-1 wave height was spatially compared
with WAM wave model results (Copernicus Marine Environment Monitoring
Service-CMEMS). The comparison of SAR-derived wave heights shows good agreement
with measured wave heights (r = 0.88) and with WAM model (r = 0.85). The wind speed,
estimated from SAR images yields to agreement with in-situ data (r = 0.91). The study demonstrates that the wave retrievals from
Sentinel-1 IW data provide valuable information for operational and statistical
monitoring of wave conditions in the Baltic Sea. The Sentinel-1A/B wave data in
general agrees with the results retrieved from altimetry, however SAR provides
additional value when monitoring coastal region where altimetry data is
missing. Sentinel-1A/B data is valuable for model data validation and
interpretation in the regions where and during periods when in situ measurements are lacking. The
Sentinel-1 A/B wave retrievals provide more detailed information about spatial
variability of the wave field in the coastal zone compared to in situ measurements, altimetry wave
products and model forecast. Thus, SAR data enables estimation of storm
locations and areal coverage. Methods shown in the study are implemented in NRT
service in German Aerospace Center’s (DLR) ground station Neustrelitz.
11:50 - 12:20
13:20 - 15:00
13:20 -
CleanSeaNet: 10 years monitoring pollution in Europe
Antunes, Sónia
Antunes, Sónia; Santos, Sónia; Lourenço, Pedro - EMSA, Portugal
CleanSeaNet (CSN) is the European satellite-based Oil Spill Monitoring and Vessel Detection service, developed and operated by the European Maritime Safety Agency (EMSA). The service provides analysis of images, mainly from SAR but also from optical missions, to detect possible oil spills on the sea surface, and identify potential polluters. The service was launched in April 2007 and is now operational for a decade, supporting Member States’ actions to combat deliberate or accidental pollution in the marine environment.
CSN currently relies on data from five different synthetic aperture radar (SAR) satellites: Sentinel-1A, Sentinel-1B, Radarsat-2, TerraSAR-X and TANDEM-X. Data from these satellites is acquired and processed in near real time (NRT) by a set of contracted ground-stations and then transferred to EMSA for further value adding and distribution to end users. Information extracted from these satellite images is combined with EMSA specific datasets (i.e. vessel reporting information) and includes: oil spills location and relevant parameters (area, length and confidence level of the detection), potential polluter identification, vessel detections (including estimates of width, length, heading and confidence level of the detection) and wind and swell obtained from the SAR data. The analysis of the SAR images is semi-automatic, with trained operators that are able to distinguish between natural phenomena and discharges from vessels. Moreover, as from the phasing-in of Sentinel-1 in CSN service in the second half of 2015, the percentage of data coming from Sentinel has increased steadily, achieving 75% of the total satellite data input of CSN in 2017.
With the continuous monitoring of the European and neighbouring waters, a substantial amount of data could be gathered so far, enabling trend analysis over time. Over the past decade, almost 25,000 images have been delivered by the CSN service, providing coverage of 4,300 million km2 of sea surface. The number of possible spills detected in European waters has dropped by half during this period, from an average of 11 possible spills per km2 monitored in 2007 to five possible spills per km2 monitored in 2017. In this period, CSN also provided support to European coastal States in responding to 31 large accidental spills and oil related emergencies.
This paper provides an overview of the service, describing the coverage and technical features, and giving relevant statistics on the detected spills during the last decade. The trend of the results is analysed and the impact of Sentinel-1 data usage is also addressed. In fact, the spatial resolution and quality of Sentinel-1 made it possible to detect smaller spills than before: The average size of spills detected in 2017 and 2016 was 25% smaller than in 2015. Additionally the optimisation of CleanSeaNet planning, with the use of new tools, increased the ratio of sea to land surface captured on the images in 2017.
Results demonstrate the overall the long-term deterrent effect of CleanSeaNet, which together with the national and regional actions of the coastal states clearly contributed to a significant reduction in the total number of spills.
13:40 -
Remote sensing of film slicks with co-polarized X-C-S-band scatterometer and TerraSAR-X
Ermakov, Stanislav
Ermakov, Stanislav (1); Sergievskaya, Irina (1); da Silva, Jose (2); Kapustin, Ivan (1); Molkov, Alexander (1); Shomina, Olga (1) - 1: Institute of Applied Physics RAS, Russian Federation; 2: Department of Geoscience, Environment & Spatial Planning, University of Porto
Microwave radar is a very perspective tool for all-weather monitoring of organic films (slicks) which appear in radar imagery of the surface of water bodies as areas of reduced backscattering due to damping of short wind waves. Single band/one polarized radar seems to be insufficient for detailed film characterization, hence, new capabilities of the use of multi-polarization and multiband radar for monitoring of film slicks have been actively discussed in the literature.
In this paper results of recent field experiments on remote sensing of film slicks using dual co-polarized radars: a satellite X-band radar (TerraSAR-X) and a new X-/C-/S-band radar of the Institute of Applied Physics RAS mounted onboard a research vessel are presented. Assuming that the radar backscatter is a sum of Bragg (polarized) and non Bragg (non polarized) components each of the two can be eliminated from the total intensity of a radar signal at VV and HH polarizations. The Bragg component (BC) is due to radar scattering from resonance cm-scale wind waves, while the non Bragg component (NBC) is hypothesized to be associated with scattering from wave breakers. Therefore, one can analyze separately variations of BC and NBC due to films, and contrasts for both the components in slicks can be estimated. It has been obtained in experiment that contrast (damping ratio) demonstrates similar dependences on radar wave number for both BC and NBC. The contrast values for all the components of radar return depend on azimuth angle (an angle between radar look and wind velocity) being minimal and weakly dependent on radar wavenumber for cross-wind observations; for upwind or downwind observations the contrast grows monotonically with radar wavenumber. In most cases the contrast values for NBC are found to be nearly the same or a little less than contrasts for BC. Supposing that NBC is associated with wave breaking, one can conclude that apart from enhanced damping of cm-scale Bragg waves due to film, the latter also influences processes of wave breaking for longer, dm- and m-scale wind waves. Results of preliminary wave tank observations of steep gravity-capillary waves are presented demonstrating strong influence of films on micro breaking wave features, such as toe- and bulge structures, which presumable play dominant role in radar non Bragg scattering. To understand better suppression of radar backscatter in slicks further studies of the action of films on wave breaking processes is needed. This work was supported by RSF (Project 18-17-00224).
14:00 -
Oil Slick Characterization Using C-band and X-band Spaceborn SAR Images and Model Based Polarimetric Technique
Ivonin, Dmitry
Ivonin, Dmitry (1); Ivanov, Andrey (1); Brekke, Camilla (2); Skrunes, Stine (2) - 1: Shirshov Institute of Oceanology RAS, Russian Federation; 2: University of Tromso - The Arctic University of Norway, Tromso, Norway
The possibilities of the polarimetric technique proposed in (Ivonin et al. 2016) to characterize the oil slicks visible on C-band (Radarsat-2) and X-band (TerraSAR-X) synthetic aperture radar (SAR) images are studied. The technique is based on the model of the electromagnetic scattering on the sea surface developed in (Kudryavtsev et al., 2003), and is related to the ratio between the damping within the slick of the short capillary-gravity waves and damping of wave breakings. This ratio is indicating by a parameter called RND (ratio of Resonant to Nonresonant signal Damping).
The technique is valid for oil slicks with thicknesses less than the radar wave skin depth, which excluded the dependence of the technique on the dielectric constant of oil. Moreover, the technique in the current version can work with the SAR images acquired at the incidence angle greater than 27- 29° when the Kirchhoff reflections from the slopes of long wind waves can be neglected.
For testing we used six SAR scenes, four acquired by Radarsat-2 and two by TerraSAR-X, containing spills of crude oil, oil emulsion, and plant (vegetable) oil (which was used to simulate simulating natural biogenic slicks), which were released in the North Sea during the Oil-on-Water Exercises in 2011 and 2012. Some of them had a high signal-to-noise ratio (SNR) in comparison to the minimal level of the signal in slicks; therefore the noise subtraction was applied before the image processing.
By processing it is shown that the mineral oils (crude oil and emulsions) considering in terms of the parameter RND and the Bragg wave number correlate well between themselves locating on the plot along a single line, which covers both C- and X- frequency bands as well as a range of incidence angles 27-49°. This fact supports the assumption that the RND parameter can describe in a general way the properties of oil slicks even sensing in different frequency bands and at different incidence angles.
Due to such coherent behavior of the RND parameter it was possible to determine the zone of RND, within which the mineral oils (crude oil and its emulsion) should lay with a high probability. The plant oil should lay outside of this zone. The width of the zone was defined by the RND standard deviation obtained during calculations. The measure of the data reliability to belong to or to be out of the zones is governed by the RND standard deviation also.
This work was supported by the Russian Foundation for Basic Research (grant no. 18-55-20010) and by the Research Council of Norway through the NORRUS project (grant no. 233896).
1. Ivonin et al. (2016). Interpreting sea surface slicks on the basis of the normalized radar cross-section model using RADARSAT-2 copolarization dual-channel SAR images. Geophysical Research Letters, 43(6), 2748-2757.
2. Kudryavtsev et al. (2003). A semiempirical model of the normalized radar cross-section of the sea surface 1. Background model. Journal of Geophysical Research: Oceans, 108(C3).
14:20 -
Algae blooms in the Baltic Sea observed in overlapping synthetic aperture radar and optical images combined with in-situ data
Johansson, A. Malin
Johansson, A. Malin (1); Skrunes, Stine (1); Brekke, Camilla (1); Johansson, Johannes (2); Karlson, Bengt (2) - 1: UiT The Arctic University of Norway, Norway; 2: Swedish Meteorological and Hydrological Institute, Sweden
During the summer months, from June to September, the Baltic Sea experiences blooms of cyanobacteria (blue green algae) that have several different effects of the ecosystem. Some of the cyanobacteria are toxic. During calm conditions surface accumulations of the cyanobacteria are formed, these are a nuisance, e.g. to tourism. The Swedish Meteorological and Hydrological Institute, SMHI, operates a service named the Baltic Algae Watch System with the aim to observe the spatial extent of blooms and to predict drift patterns. Optical satellite images are primarily used for these products but optical sensors cannot penetrate clouds, and during cloudy conditions gaps are left in the products. Synthetic Aperture Radar (SAR) images are not impeded by clouds, though the radar signal cannot penetrate the ocean surface and therefore only the surface blooms may be observed in these images. Algae blooms observed in SAR images are seen as low backscatter areas and are known to be an oil spill look-alike. Hence, improved information about algae blooms in SAR images will not only be useful in separating them from oil slicks but may also be used as a complementary data source for operational algae watch products.
Here we compare algae blooms observed in spatially and temporally overlapping SAR images and optical images and validate the results with in-situ observations. In July and August 2017 algae blooms in the Baltic Sea were recorded in SAR images from Sentinel-1 and Radarsat-2 as well as optical Sentinel-3 OLCI and Aqua-MODIS images. The Sentinel-1 data is ScanSAR IW images and the Radarsat-2 images are fully polarimetric images. The Radarsat-2 images were collected to overlap in time and space with in-situ measurements. The biomass of algae was estimated as chlorophyll a and also based on microscopy. Algorithms developed by Kahru for the identification of cyanobacteria blooms from optical images were used to separate the blooms into surface and subsurface blooms. In the Sentinel-3 OLCI images and Aqua-MODIS images, we separate the surface and subsurface slicks and compare them with the corresponding Sentinel-1 SAR backscatter values and the damping ratio. The high temporal resolution of both Sentinel-1 and -3 enables us to observe the seasonal evolution of the algae bloom and to investigate the type of algae bloom captured in the SAR images. The high resolution fully polarimetric Radarsat-2 images are used to study the algae blooms. We investigate how the outline of the algae blooms in the SAR images overlap with those observed in the optical images using polarimetric parameters such as the polarization difference and the co-polarized cross-product.
14:40 -
How to identify rain events over the ocean on C-band synthetic aperture radar images?
Alpers, Werner
Alpers, Werner (1); Zhang, Biao (2); Mouche, Alexis (3); Chan, Pak Wai (4) - 1: University of Hamburg, Germany; 2: Nanjing University of Information Science and Technology, Nanjing, China; 3: Laboratoire d'Océanographie Spatiale, IFREMER, Plouzané, France; 4: Hong Kong Observatory, Hong Kong
It is well known that rain events leave fingerprints on synthetic aperture radar (SAR) images acquired over the ocean, but it is not always easy to identify them unambiguously, especially not on C-band SAR images. Rain becomes visible on SAR images acquired over the ocean via several mechanisms: 1) by variations of the sea surface roughness caused by downdraft winds associated with rain cells and by rain drops impinging onto the sea surface (surface scattering) and 2) by scattering and attenuation of the radar beam by rain drops in the atmosphere (volume scattering). Surface scattering is particularly intricate at C-band because the Bragg waves responsible for the radar backscattering at this radar frequency lie in the transition region, where the impinging raindrops can increase (usually) or decrease the backscattered radar power, and also because volume scattering and attenuation can contribute significantly to the radar signature when the rain rate is high.
In this paper we report about progress that has been made in our study of C-band radar signatures of rain over the ocean. Such studies are relevant also for retrieving sea surface wind fields from C-band scatterometer data. Rain is a main source of error in wind retrieval algorithms, especially when co- and cross-polarized scatterometer data are used, which will be the case in the future. In this study we have analyzed mainly Sentinel-1 SAR images acquired over the South China Sea and have compared them with rain data from the weather radar of the Hong Kong Observatory and from the Global Precipitation Measurement (GPM) mission. In contrast to previously analyzed ERS and Envisat data, the Sentinel-1 data are acquired at VV and VH polarization which allows us to better discriminate between surface and volume scattering. The results of this investigation will be presented, which include also statements on how the radar signatures depend on rain rate, rain type, and wind speed, and how to discriminate radar signatures of rain from radar signatures of “look-alikes”.
15:30 - 16:30
15:30 -
Long Oblique Internal Hydraulic Jumps at the Penghu Channel of the South China Sea
Ferreira, Adriana M. dos Santos
Ferreira, Adriana M. dos Santos; da Silva, José Carlos - University of Porto, Portugal
It is well known that stratified flows through a channel with a lateral contraction can generate Internal Solitary Waves (ISWs) when the flow speed is nearly critical (or transcritical) with respect to the linear long internal wave phase speed. These waves propagate in the upstream direction and may be arrested (or trapped) during the early stages of their lives (da Silva & Helfrich, 2008). Just as a compressible flow suddenly changes from a supersonic to subsonic state by going through a shock wave, a supercritical flow in a shallow canal can change into a subcritical state by going through a hydraulic jump. In supercritical (barotropic) flow conditions, obliquely oriented hydraulic jumps occur (e.g., Ippen and Harleman 1956), in analogy to shock waves in supersonic flows. Since disturbances generated in the supercritical flow cannot propagate directly upstream, they instead accumulate along a hydraulic jump oriented at an angle (known as the Mach angle) that is oblique to the flow direction. There is an equivalent phenomenon in stratified flows, which theory treats oblique internal hydraulic jumps as arrested, long, interfacial waves. However, such internal hydraulic features have not been previously reported in the oceanography literature, at least not as a long and recurring phenomenon (though the general concept has been applied to describe observed obliquity of tidal, internal hydraulic jumps at the Columbia River mouth; Honegger et al., 2017). Here we present, for the first time, Synthetic Aperture Radar (SAR) observations and analyses of recurring, oblique, internal hydraulic jumps at the Penghu Channel of the South China Sea. The Penghu Channel can be characterized as a relatively shallow channel (average depth 150m) with a lateral constriction, into which the combined steady and tidal currents may easily reach supercritical speeds at the minimum breath of the channel. The jumps were identified in the SAR image data archives from various missions (e.g. ERS, ENVISAT, TerraSAR-X, Sentinel-1) as recurring phenomenon. Jump occurrence is revealed by (i) SAR signatures of a sharp gradient in the surface currents and it is corroborated by (ii) the transition from supercritical to subcritical flow in the cross-jump direction via our knowledge of velocity and density measurements. Using a two-layer approximation, observed jump (Mach) angles are shown to lie within the theoretical bounds given by the critical internal long-wave (Froude) angle and the arrested maximum-amplitude internal bore angle, respectively. The jump’s orientation and associated uncertainty are estimated with a Radon transform technique. Variability of the jump angles is shown to be consistent with that expected from the two-layer model, applied to varying stratification and current speed over a range of tidal conditions and seasonal steady currents. Correspondence between the SAR observations and the remarkable simple theory demonstrates its utility in characterizing internal hydraulic phenomena. The crestlength extent and spatial coverage of the phenomenon (tens of crests up to 25km long) implies significant energy dissipation through mixing in the Channel. Energy budget considerations and implications to mixing are briefly discussed based on the SAR observations and our knowledge of the along-channel stratification.
15:50 -
Radar signatures of internal solitary waves – revisited
da Silva, Jose C. B.
da Silva, Jose C. B. (1); Alpers, Werner (2); Magalhães, Jorge.M. (3) - 1: University of Porto, Porto, Portugal; 2: University of Hamburg, Hamburg, Germany; 3: University of Porto, Porto, Portugal
The theory of radar imaging of internal waves developed by Alpers (1985) is based on weak hydrodynamic interaction theory and Bragg scattering theory. It is able to describe quite well the observed radar signatures of internal waves at L-band, but underestimates the strength of the modulation (modulation depth) at C-and X-band. Although this theory has also been applied for describing radar signatures of internal solitary waves (ISWs) at X-band (Romeiser and Graber, 2015), one has to use an unrealistic value of the relaxation rate, which is not supported by theory. The physical mechanism responsible for the enhanced modulation at C- and X-band is scattering at breaking surface wave, which has not been included in the conventional imaging theory. It is well known that ISWs often manifest themselves on the sea surface as bands of very strong roughness, which include breaking waves with wavelengths of the order of meters. Such bands are observed even when there is no wind and the sea is glassy. Obviously, the generation of roughness bands associated with breaking surface waves cannot be explained by weak hydrodynamic interaction theory, where the variable surface current associated with ISWs modulates “gently” the Bragg waves responsible for the radar backscattering, Furthermore, the radar backscattering at C-band and X-band from these roughness bands cannot be explained by Bragg scattering theory alone; it requires taking into account also radar backscattering from breaking waves, which, in contrast to Bragg scattering, is non-polarized scattering (Kudryavtsev et al., 2005). In this paper we analyze the relative contribution of scattering from breaking surface waves to the total radar backscattering (Bragg and non-polarized scattering). To this end, we have analyzed TerraSAR-X images acquired at HH and VV polarizations over the Strait of Gibraltar and Sentinel-1A images acquired at VV and VH polarizations over the South China Sea at low wind speeds. We show that backscattering from the leading ISW fronts is dominated by non-polarized scattering, while backscattering from the rear is a mixture of Bragg and non-polarized scattering. Furthermore, we have analyzed a TerraSAR-X image acquired at HH polarization in the Stripmap Mode with a resolution of 3 m over the Amazone Shelf showing sea surface signatures of small-scale, but strong ISWs propagating upstream. Due the narrow roughness bands and the high resolution of this SAR image, we were able to determine the azimuthal displacement of “sea spikes” (elements of very strongly enhanced radar backscatter) due to the motion of these scatter elements. We have estimated the velocity of these scatter elements associated with wave breaking from the azimuthal shifts in the SAR image and found that they result from breaking of waves with wavelengths of the order of 1 m.
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Automatic Bathymetry Retrieval from Sentinel-1 and TerraSAR-X data
Wiehle, Stefan
Wiehle, Stefan; Pleskachevsky, Andrey; Jacobsen, Sven - German Aerospace Center (DLR), Germany
We have developed an algorithm for automatic derivation of the bathymetry in coastal seas using Sentinel-1 and TerraSAR-X satellite data.
Recently, the TanDEM-X mission successfully finished generating a new high-resolution topography worldwide. However, about 70% of the Earth is covered by oceans, where the sea floor topography cannot be measured by a single spaceborne Earth Observation technology today. With growing efforts in global shipping and offshore constructions like wind parks, the knowledge of bathymetry becomes increasingly important. Our algorithm can retrieve the bathymetry in continental shelf areas with water depths from about 100 to 10 meters. In these depths, the shoaling effect leads to swell waves becoming shorter when reaching shallower waters.
The developed algorithm is part of a maritime SAR processing chain designed to derive multiple L2 products like wind, sea state, ship detection or sea ice automatically and in Near Real Time (NRT). For the bathymetry retrieval, the land is masked out and the image is subdivided into a grid of square subscenes. In preparation for the FFT analysis, spectral contaminations like ships, wind parks, current boundaries or wind streaks are filtered out for an accurate sea state and wave length retrieval. Then we can retrieve the peak wave length for each subscene from the FFT spectra.
To calculate the water depth, the dispersion relation in intermediate depth waters must be solved, which requires the peak wave length and peak wave period as parameters. However, the wave period cannot be retrieved from SAR images. Measurement buoys for wave period data are very sparse, but can be used as input when available. We therefore use existing coarse datasets like General Bathymetric Charts of the Oceans (GEBCO) as first guess data on the water depth. The optimal peak wave period is determined with a root mean square deviation (RMSD) analysis comparing the depths at every grid point of the scene for a range of possible wave periods and using the one resulting in the smallest deviation. With this, the water depth can then be calculated at every grid point. Comparisons to existing bathymetry data yield an unbiased RMSD of about 12 meters for the full 10m to 100m range of depths or about 15% of the depth.
The presented method requires the presence of long swell waves and light to medium wind, hence, not every SAR acquisition of coastal waters is suitable for bathymetry derivation. For this application, TerraSAR-X is technically preferable due to its ability to detect wavelengths down to 30m, where Sentinel-1 requires longer wavelengths of 120m for bathymetry retrieval. However, TerraSAR-X data are only available when previously ordered while Sentinel-1 data are acquired constantly and freely accessible from the Copernicus data hub, which facilitates the retrieval of suitable scenes worldwide.
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Exploring the use of SAR remote sensing to detect microplastics pollution in the oceans
Davaasuren, Narangerel (1); Marino, Armando (1); Boardman, Carl (1); Ackermann, Nicolas (2); Alparone, Matteo (3); Nunziata, Ferdinando (3) - 1: The Open University, United Kingdom; 2: Swiss Federal Railways SBB, Luzern, Switzerland; 3: The Parthenope University of Naples, Italy
The increase in plastic pollution is advancing micro level pollution and the total weight of microplastics (<0.33mm-1.00 mm) in the ocean column. The amount of plastic in the North Pacific is estimated as 21 x 108 tons and in the North Atlantic as 10.4 x 108 tons. The plastic in marine environment will eventually degrade and it will be promptly colonized by bacteria releasing surfactants. Such surfactants will have the effect of damping the capillary and small gravitational waves on the ocean surface. Since SAR is sensitive to roughness induced by capillary waves, it may be exploited to detect bacterial activities related to plastic pollution.
In this work we used Sentinel-1A and COSMO SkyMed radar images acquired in the Atlantic and Pacific gyres to detect surfactants that may be associated to plastic pollution. We are using SAR, because the damping properties of surfactants produce dark areas in images. Since area of low backscattering in SAR images could also be produced by other oceanographic/meteorological event, we exploited geophysical remote sensing products associated to time and locations synchronised to SAR acquisitions. Among other products we considered sea surface temperature, surface wind, chlorophyll, surface reflectance, turbidity and wave heights. Additionally we made sure that the areas were not within busy shipping routes. The result of the analysis is that, including effects due to colocation errors of SAR and meteorological data, we could identify a large amount of linear slicks in SAR images that were not directly related to apparent meteorological conditions. Such slicks in the gyres have the appearance of oil slicks, however in some areas they are in large amount and they are not connected to large ship traffic. At the moment these slicks seems to only be visible when the wind conditions are moderate (e.g. 6m/s) as it happen for ordinary oil slicks.
Besides the work on radar data, we are making controlled experiments with micro-plastic pollution in sea water, to understand the amount and type of surfactants produced by microbes colonising plastics.
The conclusion of our study is that radar remote sensing has the potential to detect plastic pollution areas under sorter meteorological conditions.
Demonstration of Spatially Coincident and Quasi-Simultaneously Collected Sentinel-1 and RADARSAT-2 Imagery for Enhanced Ocean Surveillance
Van Wychen, Wesley; Vachon, Paris - Defence Research and Development Canada, Canada
Using a case study approach, the utility of spatially coincident (overlapping and/or adjacent) and quasi-simultaneously collected (within ~25 minutes of each other) Sentinel-1 and RADARSAT-2 imagery is demonstrated for enhanced ocean surveillance off of Canada’s east coast. We show how ship detection and characterization is improved by using spatially coincident RADARSAT-2 OSVN and Sentinel-1 IW imagery. Specifically, the higher resolution of the IW imagery allows small ships to be identified with higher certainty and allows for more accurate extraction of vessel characteristics (such as length and width estimates). Further, the displacement of the ship targets between image acquisitions allows for the direction and speed to be estimated, providing additional information that is not (necessarily) available when utilizing a single image. From an environmental monitoring standpoint, these quasi-simultaneous images allow for high confidence delineation of ocean surface features associated with thermal fronts, eddies, water mass boundaries, and atmospheric fronts. For example, we demonstrate the ability to map the movement of a strong storm system as it tracks between temporally separated, but spatially coincident, SAR images. Further, we show how Sentinel-1 Level-2 datasets, such as the Radial Velocity Product (RVL), provides additional contextual information that aids in the interpretation of RADARSAT-2 image products (such as for identification of the Gulf Stream North Wall). Finally, we show how spatially adjacent and simultaneously collected Sentinel-1 and RADARSAT-2 images can be used to expand coverage over which ocean surveillance can be undertaken by providing the equivalent of an ~800 km swath. This work provides practical examples of the benefits of collecting these types of SAR datasets for ocean surveillance, and suggests that planned observation scenarios could, in the future, be developed to fully maximize these opportunities. Looking further into the future, these examples demonstrate opportunities that may become routinely feasible after the launch of the RADARSAT Constellation Mission.
Iceberg detection extending the Dual-pol Ratio Anomaly Detector (DPolRAD) to ALOS-2 L-band quad-polarimetric data
Marino, Armando - The Open University, United Kingdom
Icebergs represent a danger to navigation in cold waters. Detection and tracking of large icebergs using space-borne scatterometers, altimeters and synthetic aperture radar (SAR) systems have seen a large amount of work in the last decades. However, the identification of small icebergs is still challenging especially when these are embedded in sea ice.
In this work, a recently proposed iceberg detector the intensity Dual-Pol Ratio Anomaly Detector (iDPolRAD) is tested using ALOS-2 quad-polarimetric L-band data. The detector was originally designed for dual-polarized incoherent SAR images and it was optimised and validated for the use with Ground Detected Sentinel-1 data [1].
This work with quad-pol L-band has two main purposes: 1) we want to investigate the capability of the same detection formalism (the DPolRAD) when this is adapted to use quad-polarimetric data (for possible future mission); 2) we want to understand how the original DPolRAD detector (i.e. using the same typology of dual-pol data) comperes when we use L-band instead than C-band.
The ALOS-2 data were acquired on the East Coast of Greenland, around the Kangerlussuaq glacier where we expect a large number of grounded icebergs visible in the images.
The results show that different polarisations could bring different information. In particular, the use of two coherent co-channels (HH and VV) could allow to set up the detector to identify anomalies in the horizontal reflections, rather than oriented reflection and volume as can be done with the incoherent HH and HV. Therefore the availability of quad-pol would allow the possibility to have a bank of DPolRAD detectors, each one focused on a different scattering mechanism. This would improve the detection performance. However, a quad-pol algorithm would not be operational at the moment due to the limited size of the quad-pol swath. We also observed that L-band data allows to better see some scattering mechanisms associated to reflections. This is probably due to the lower backscattering from the ice body compared to C-band which reduces the volume scattering at advantage of reflections from iceberg surfaces and ice-water interfaces.
Reference:
[1] A. Marino, W Dierking, and C. Wesche, “A depolarization ratio anomaly detector to identify icebergs in sea ice using dual-polarization SAR images,” vol. 54, no. 9, pp. 5602–5615, 2016.
Spatial Scale Effect on Wind Speed Retrieval from SAR Images
Zhang, Kangyu; Huang, Jingfeng; Guo, Qiaoying; Mansaray, Lamin R - Zhejiang University, China, People's Republic of
Synthetic aperture radar (SAR) instruments on board satellites are valuable for high-resolution wind field mapping. However, there is still no consensus on the optimal resolution for wind speed retrieval from either co-polarized or cross-polarized SAR images. This paper presents a comparison strategy for investigating the influence of spatial resolutions on sea surface wind speed retrieval accuracy from SAR images.
In order to evaluate the spatial scale effect on wind speed retrieval from co-polarized SAR images, we makes a quality assessment of wind speed retrieval accuracy based on four commonly used C-band geophysical mode functions (CMOD4, CMOD-IFR2, CMOD5 and CMOD5.N) at spatial resolutions ranging from 50 m to 50 km using Sentinel-1 Interference Wide Swath (IW) mode images. In comparison with the buoy wind measurements, our results show that the CMOD5 function is the most effective wind speed retrieval algorithm, whose retrieved wind speeds have a low RMSE of 1.17 m/s and a bias of -0.28 m/s at a spatial resolution of 500 m. What’s more, a phenomenon has been reveal that the variance of wind speeds retrieved from co-polarized SAR images decrease exponentially with the decrease of spatial resolutions. For Sentinel-1 IW mode images, the variance of wind speeds retrieved with CMOD5 decrease rapidly from 50 m to 500 m, with a drop in RMSE of 40%, and thereafter levels off. Thus, a spatial resolution of 500 m, in concert with CMOD5 function, could be recommended for wind speed retrieval using Sentinnel-1 IW mode data.
In terms of assessing the effect of spatial resolution on the accuracy of wind speeds retrieved from cross-polarization images, due to the lack of in situ measurements, the wind speeds retrieved from VH-polarization images were compared with those from VV-polarization images at different spatial resolutions using RADARSAT-2 fine quad polarization images. Firstly, for wind speeds retrieved from VV-polarized images, the optimal geophysical C-band model (CMOD) function was selected among the four CMOD functions. Secondly, the most suitable C-band cross-polarized ocean (C-2PO) model was selected between two C-2POs for the VH-polarized image dataset. Then, the VH-wind speeds retrieved by the selected C-2PO were compared with the VV-polarized sea surface wind speeds retrieved using the optimal CMOD, which served as reference, at different spatial resolutions. Results show a similar accuracy trend as the co-polarized SAR data that the variance of VH-polarized wind speed decreased rapidly with the decrease in spatial resolutions from 100 m to 1000 m, with a drop in RMSE of 42%. However, the improvement in wind speed retrieval accuracy levels off with spatial resolutions decreasing from 1000 m to 5000 m. This demonstrates that the pixel spacing of 1 km may be the compromising choice for the tradeoff between the spatial resolution and wind speed retrieval accuracy with cross-polarized images obtained from RADASAT-2 fine quad polarization mode.
SAR Monitoring of the A-68 Iceberg Drifting Displacements
Parmiggiani, Flavio (1); Moctezuma-Flores, Miguel (2); Guerrieri, Lorenzo (1); Battagliere, Maria Libera (3) - 1: ISAC-CNR, Bologna, Italy; 2: Dept. Telecomm., UNAM, Mexico City, Mexico; 3: Italian Space Agency (ASI), Rome, Italy
The fracture of the Larsen C ice shelf, which had been regularly observed in the last months of 2016, started to rapidly grow in 2017 so that, in February, only an ice strip, 20 km long, kept attached a huge section of the shelf to the Antarctic Peninsula. The final collapse, expected in 2017, occurred indeed between July 10 and July 12, with a loss of an area of some 6,000 km2, corresponding to about 9-12% of the entire shelf. According to the US National Ice Center (NIC) criteria, the calved iceberg was named A-68. In the frame of the ASI "COSMO-SkyMed Open Call for Science Initiative", we followed the initial phase of the A-68 iceberg drifting trajectory and melting process. The analysis covers a period of 6 months, with the database composed of a set of COSMO-SkyMed ScanSAR Huge images.
Coastal bathymetry estimation from Sentinel – 1 data.
Lamas, Luisa (1); Pinto, José Paulo (1); Sancho, Francisco (2); Azevedo, Alberto (2); Birrien, Florent (2); Vilar, Pedro (1); Moura, Ana (1) - 1: Instituto Hidrográfico, Lisboa, Portugal; 2: Laboratório Nacional de Engenharia Civil, Lisboa, Portugal
The global bathymetry at 1 km resolution is already known and available through multiple different data sets (e.g., GEBCO [1]). However, small-scale shallow water topographic features, like sand banks, reefs and bars, can change due to storms, and may not be correctly marked in the official charts. These changes can be relatively fast and are not easily measured by traditional sound surveying methodologies and, therefore, methods that rely on Earth Observation from space can be valuable for the monitoring of the coastal bathymetry.
In this study, coastal bathymetry is derived with a wave-tracking algorithm through wave parameters retrieved from SAR images from the Sentinel-1 satellites. The wave tracking algorithm starts by applying the Fast Fourier transformation algorithm (FFT) at an offshore squared area with 1 km length to estimate the mean wavelength and wave direction. The mean depth of this offshore area is obtained by an independent source (GEBCO), to mark the limit of the intermediate waters. The algorithm then tracks down the shoaling waves until the wave breaking zone, using the wave direction retrieved from the 2D spectrum centered at each tracking point. Then, through the linear wave dispersion relationship, changes in depth are calculated based on changes in the wavelength between each position of the wave track. The output of the algorithm is a smoothed 2D bathymetry field that results from the interpolation of the depth computed at each tracking point.
This work is a part of a research application developed within the EU H2020 Coastal waters Research Synergy Framework (Co-ReSyF) project, to demonstrate the operational capabilities of the platform.
Several case studies at different locations off the Portuguese Coast will be presented, using high resolution SAR images from Sentinel-1. Several issues regarding the synergy between SAR and Optical data to retrieve high-resolution bathymetry in shallower waters will be briefly discussed.
References:
[1] https://www.gebco.net/
Using SAR for Characterization of Offshore Wind Farm Wakes and Coastal Wind Speed Gradients
Hasager, Charlotte; Ahsbahs, Tobias; Badger, Merete; Hansen, Kurt S.; Volker, Patrick - Technical University of Denmark, Denmark
Offshore wind farms influence the atmosphere downwind for considerable distances and SAR has been used to demonstrate this effect in earlier studies. SAR has also been used to characterize winds in the coastal zone influenced by the proximity to the land. The novelty of the present study — focusing on the Anholt wind farm in the Kattegat Strait between Denmark and Sweden — is the interplay between the wind farm wake effect and a strong horizontal wind speed gradient from the coastline and further offshore. The wind farm was constructed after the Envisat ASAR mission ended and prior to the Sentinel-1 mission.
The study is based on the SAR wind archive available at https://satwinds.windenergy.dtu.dk . The SAR data are processed at DTU using CMOD5.N and wind directions from the Climate Forecast System Reanalysis (CFSR) from 2002 to 2010 and the Global Forecasting System (GFS) from 2011 to 2017. The pixel size is 500 m.
The offshore winds observed by Envisat are compared to Weather and Research and Forecasting (WRF) model results. More specifically, the variation in wind speed along the first (western) row of wind turbines (stretching 20 km from north to south located closest to the Danish coastline) for winds coming from 265° (±15°) show good agreement between SAR and WRF. The wind speed variation along this row of turbines is also assessed based on the Supervisory Control And Data Acquisition (SCADA) data from the wind turbines kindly provided by Ørsted and Partners. The data are 10 minute values from January 2013 to June 2015. Interestingly, the average variation in wind speed along the western row for winds coming from 265° (±15°) show around 1 m/s higher winds at the northernmost turbines compared to the southernmost turbines. This difference is most likely caused by the varying distance to the coast. The fetch is up to 50 km in the north and down to 16 km in the south. Wind speeds relative to the center turbine from SAR and SCADA agree within 0.1% while WRF over-predicts around 1% as compared to SCADA. All data sets quantify a significant coastal wind speed variation.
The comparison of winds from SAR and SCADA (extrapolated from 81.6 m hub-height with logarithmic profile to 10 m) in non-waked and waked conditions give results of R2 of 0.97 with RMSE of 1.80 and 1.70 m/s and bias -0.12 and -0.52 m/s, respectively.
An investigation of the wind farm wake effects is completed using the Envisat data versus the Sentinel-1 data to provide the difference between free-stream and wind farm wake conditions. Various horizontal transects aligned with the wind direction and perpendicular to the wind direction are analysed. The uncertainty of the average wind speed is also assessed and significant variations are mapped (Ahsbahs et al. in review, Wind Energy Science).
The conclusions are that SAR enables quantification of spatial horizontal wind speed gradients and wind farm wakes. The large SAR wind archive can be used to explore complex cases providing a measurement independent of modelling results.
SAR Remote Sensing of Internal Waves in the UK Continental Shelf. Automatic detection and impact estimation.
Kurekin, Andrey; Miller, Peter; Land, Peter - Plymouth Marine Laboratory, United Kingdom
The importance of monitoring internal waves (IWs) for oceanographic studies is hard to overestimate. IWs are responsible for transferring energy between large-scale tides and small-scale mixing. On the continental shelf IWs affect coastal areas through nutrient mixing in the euphotic zone, transfer of phytoplankton rich waters, coupling of benthic and pelagic systems, sediment re-suspension and cross-shore pollutant transportation. Observation of IWs provides insight into the mechanisms of their generation, propagation and dissipation. It helps to evaluate and predict the impact that IWs may have on the environment and ecology in UK shelf seas.
Traditionally, measurements of IW fields have been carried out using instruments deployed in the ocean. They are time consuming, expensive and unsuitable for large-scale observations. Synthetic Aperture Radar (SAR) remote sensing method provides an alternative to in-situ observations of IWs. SAR is insensitive to cloud cover and solar illumination and provides excellent spatial coverage and consistency of observations. SAR sensors produce maps of sea-surface roughness, within which the presence of underlying IW crests and troughs are revealed due to their effect on small capillary waves, seen as alternating light or dark lines in the radar backscatter. While SAR imagery has been frequently used to study particular IWs, usually the linear features are detected manually due to their indistinctness within the speckled noise inherent in such images. Manual analysis would not be practical or appropriate for objectively inferring the long-term distribution of IWs across a large sea area.
We evaluated many potential methods for automated IW detection from SAR data, reviewed methods for generating distribution maps of IWs, and established the existence and feasibility of detecting IWs within the UK continental shelf. We have identified methods from the literature that can be used to identify regions where IWs are likely to occur, and reviewed ways in which IWs can affect the seabed. We implemented and applied the new methods for automated detection of IW features to a 7-year time-series of ENVISAT ASAR sensor data acquired in 2006-2012 by the European Space Agency, to build detailed maps of IW occurrence and climatology for the UKCS. Up to a hundred SAR scenes per month covered the region of interest, over 3,400 in total. Regions with high likelihood of seabed disturbance were identified by combining the IW occurrence data with mixed layer depth and bathymetry. Monthly and annual climatology maps of the UKCS have been produced showing the spatial and temporal variability of IW interaction with the seabed.
Evaluation of Altimeter-based sea ice Lead detectors with Sentinel-1 data
Longépé, Nicolas (1); Thibaut, Pierre (1); Vadaine, Rodolphe (1); Poisson, Jean-Chistophe (1); Guillot, Amandine (2); Boy, Francois (2); Picot, Nicolas (2) - 1: CLS, France; 2: CNES, France
Altimeters measure the topography of the surfaces they fly over, but in the Arctic context where sea ice can be found, it is important to determine whether the measurement corresponds to open ocean, ice floe or leads within the sea-ice. The knowledge of the surface type is important for data interpretation and the computation of the freeboard obtained by difference between the open water level and the floe surface level. Ice thickness estimate is then derived from the freeboard estimate. The assessment and then the optimization of altimeter-based sea ice lead detectors is thus a prerequisite to further improve the quality of ice thickness maps. This can be carried out using a set of references cases provided by Earth Observation images, such as the ones provided by Copernicus Sentinel-1 (S-1) images
In this study, a dedicated S-1 lead detector has been implemented. Its general principle is to detect local minima thanks to morphological reconstruction filter, followed by watershed algorithm using the sigma0 image gradient.
For the first time, a robust and consistent methodology for the joint assessment of SAR and Altimeter lead detector has been developed. Compared to the existing state of the art, it is proposed to fully account the 2-D geometric problem when comparing 1-D altimeter track and 2-D SAR image. The distance from nadir and the surface of leads within the altimeter footprint are especially considered when building the SAR-based reference database.
We collocated S-1 / SRAL Sentinel-3 data with time lag less than 8 minutes, corresponding to 169 S-1 images. The overall accuracy between the S-3 and S-1 lead maps is good with more than 90 % of good agreement depending upon how the SAR-based reference dataset is built. The False Positive Rate is very low with less than 3-6 %. A new criterion, the so called Matthews coefficient, is introduced, and provide new parametrization for the SRAL S-3 lead detector. A tradeoff between false alarms and good detection is numerically found.
A comparison with collocated French-Indian SARAL AltiKa altimeter and S-1 data is also provided. A similar analysis is performed using AltiKa SARAL altimeter data, showing enhanced performance for SRAL S-3 data acquired in Delay-Doppler mode with reduced off-nadir returns.
This study has been partly funded by the European Space Agency (ESTEC/ESA) via the S3-CD studies, by the PEACHI SARAL contract and the French Space Agency CNES.
Automatic Iceberg Detection in the Barents Sea for Icebergs in Open Water, Fast Ice and Pack Ice
Soldal, Ingri Halland (1,2); Korosov, Anton (1); Marino, Armando (3) - 1: Nansen Environmental and Remote Sensing Center, Norway; 2: University of Tromsø, Norway; 3: The Open University, United Kingdom
The Barents Sea is a region where maritime operations will be more frequent in the coming years. Icebergs can be a threat to these operations which makes it crucial to have knowledge about their position at any time. Most methods for automatic iceberg detection are focused on the detection of icebergs much larger than the typical size found in the Barents Sea. Therefore they are not applicable to this area. They also tend to cover only one specific environment of the iceberg.
Marino et al (2016) proposed a method called DPolRAD that utilizes the polarization properties of the SAR signal to distinguish icebergs from sea ice. It uses the different polarimetric behaviour in cross- versus co-polarized channels to reduce surface scattering (i.e. sea and sea ice) and enhance volume scattering (i.e. icebergs). Here this method has been applied to Sentinel-1 EW SAR images over Svalbard and Franz Josef Land to detect icebergs in the Barents Sea, with the modification of applying a gaussian filter instead of a boxcar filter in the calculations. Together with a statistical detector called the constant false alarm rate (CFAR), automatic iceberg detection is performed in three cases; open water, fast ice, and pack ice.
Four probability density functions (PDFs) are tested as input to the CFAR detector; the gamma-, generalized gamma-, normalized inverse gamma-, and the gaussian distribution. The parameters for each PDF are calculated using the background data surrounding the pixel under test, and the choice of variables for the DPolRAD and CFAR methods are based on consistency, scatter plots, and confusion matrices. The Google Earth Engine is used to colocate SAR and optical images for validation of iceberg pixels and for manual detection. This is useful to verify the detection results.
The results show that the normalized inverse gamma-PDF is most representative for the data calculated by the DPolRAD method. This is also the PDF that most easily can distinguish iceberg pixels from non-iceberg pixels, and by applying the PDF to the DPolRAD and CFAR methods we obtain high detection rates.
With this detector it may be possible to do systematic surveillance of icebergs in the Barents Sea, both using past and future data, which can give information about their distribution and path.
Reference:
A. Marino, W Dierking, and C. Wesche, “A depolarization ratio anomaly detector to identify icebergs in sea ice using dual-polarization SAR images,” vol. 54, no. 9, pp. 5602–5615, 2016.
Pancake ice thickness mapping from wave dispersion observed in SAR imagery
Aulicino, Giuseppe (1); Wadhams, Peter (1); Parmiggiani, Flavio (2) - 1: Università Politecnica delle Marche, Ancona, Italy; 2: ISAC-CNR, Bologna, Italy
The polar regions are distinctly different from other geographic areas and share a common set of characteristics, including remoteness; limited accessibility; low population densities; sensitive ecosystems of global importance; and increasing economic activity. In recent years, the uniqueness of these polar regions and their importance to the world has been recognized. This is made more urgent by the fact that the Arctic is warming much more rapidly than other regions of the world, and the summer sea ice is thinning and shrinking significantly. Such physical change can best be monitored by remote sensing from space. Although a number of satellite derived approaches exist, they need to be adapted to the new generation of sensors and the data provided need to be further validated by ground truth.
The early autumn voyage of RV Sikuliaq to the southern Beaufort Sea (Oct-Nov 2015) and the PIPERS winter cruise in the Ross Sea (May-Jun 2017), offered very favorable opportunities for observing the properties and thicknesses of frazil-pancake ice types. The operational regions were overlaid by a dense network of retrieved satellite imagery, including SAR (synthetic aperture radar) imagery from Sentinel-1 and COSMO-SkyMed (CSK). This enabled us to fully test and apply the SAR- waves technique, first developed by Wadhams and Holt (1991), and experimentally verified by Wadhams et al. (2004) for deriving the thickness of frazil-pancake icefields from changed wave dispersion. A line of sub-images from a main SAR image is analysed running into the ice along the main wave direction. Each sub-image is spectrally analysed to yield a wave number spectrum, and the change in the shape of the spectrum between open water and ice, is used to retrieve frazil-pancake ice thickness. For each of the case studies considered here, there was good or acceptable agreement on thickness between the extensive in situ observations and the SAR-wave calculation.
Analysis of the Oil Seep-affected Sea Surface Microwave Backscattering
Khenouchi, Horiya (1); Buono, Andrea (2); Nunziata, Ferdinando (2); Velotto, Domenico (3); Migliaccio, Maurizio (2); Smara, Youcef (1) - 1: University of Sciences and Technology Houari Boumediene, Algiers, Algeria; 2: Università degli Studi di Napoli Parthenope, Italy; 3: Remote Sensing Technology Institute, German Aerospace Center (DLR), Germany
In this study, the sensitivity of environmental and sensor parameters that rule microwave backscattering of sea oil seeps is investigated by contrasting analytical models with actual satellite synthetic aperture radar (SAR) measurements.
The topic that will be addressed is of paramount importance in many marine and maritime applications as remotely sensed oil seep observation, that is very interesting not only from a scientific point of view, but also from an economical and environmental perspectives being related to oil and gas exploration and extraction activities. Moreover, the problem of sea oil seep backscattering, i. e., the backscattering from reserves of hydrocarbons naturally coming up from the bottom of the ocean to sea surface, is rather challenging and needs further investigation since the processes that lie at the basis of the escape, the development and the lifespan of oil seeps are extremely variable and irregular. The latter result in a quite different backscattering modeling approach if compared to conventional scattering models that deal with sea surface backscattering with and without oil slicks.
Hence, in this study, the two scale Boundary Perturbation Method (BPM) is adopted as a reference scattering model to predict sea surface backscattering and, then, the effects of the presence of the oil seep, that include damping properties, reduction of friction velocity and changes in the dielectric permittivity are taken into account. The firsts are considered including the rheological parameters of different kinds of surfactants, i. e., weak-damping biogenic films and strong-damping crude oil; the second is included by applying a penalty factor to slick-free sea surface friction velocity and the third is accounted for by modeling the heterogeneous oil seep according to the effective medium approximation theory, i. e., different kinds of mixture of seawater and oil droplets are considered.
Then, the influence of all of those parameters to sea oil seep backscattering is evaluated for different SAR acquisition parameters, i. e., incident wavelength and angle of incidence, sea state conditions, i. e., wind speed, and surfactants’ properties, i. e., damping and concentration. Preliminary results, obtained by contrasting model’s predictions and actual backscattering SAR measurements collected at L-, C- and X-band over well-known sea oil seeps, demonstrated that wind speed, damping properties and incidence angles most affected the backscattering from sea oil seeps, while incident wavelength and the adopted oil/water mixture have a negligible impact on their backscattering.
Determining the dielectric constant of oil slick using Synthetic Aperture Radar
Quigley, Cornelius Patrick - UiT The Arctic University of Norway,Tromsø, Norway
Oil spills and discharges represent a significant ecological risk to the marine environment. As a result, having timely and accurate information on the location and evolving state of slicks can aid in clean-up efforts as well as to inform future preventive measures. For this purpose, Synthetic Aperture Radar (SAR) imagery has proven to be an indispensable tool given its insensitivity to overlying cloud cover. A notable feature of oil in SAR images of the ocean surface is that oil will appear as a dark patch in relation to the surrounding sea surface if wind speed is sufficiently high. This is due to the damping of the wind induced capillary waves which causes the oil infested region to become smoother than the surrounding sea surface. By applying theoretical scattering models to SAR imagery of oil infested waters it is investigated in this paper, if, and under which conditions, variations in the estimated dielectric constant can be detected over an oil slick. The dielectric constant is a quantity that is intrinsically related to the material in question and has a larger value for sea water than mineral oil. By inverting for the dielectric constant, it is believed that specific internal zones within slick will be observed that correspond to regions where emulsions of various oil/water ratios are present. Inverting for the dielectric constant in this way can yield information on the quantity of oil present within each resolution cell.
A series of images of oil spills of various types and emulsions were taken during oil-on-water exercises for the years 2011-2016 off the Norwegian coast using a variety of sensors such as Radarsat-2 (RS-2), TerraSAR-X (TSX) and UAVSAR. These sensors correspond to the X, C and L bands of the electromagnetic spectrum. For the derivation of the dielectric constant the Small Perturbation Model (SPM), the Extended Bragg Model (X-Bragg) and the Composite model are inverted using a look-up table approach. The linear mixture model, which describes the variation in the complex dielectric constant as the oil emulsifies is used to invert for the volumetric content of oil within slick.
Current work done on quad polarimetric RS-2 observations for the oil-on-water exercises for the year 2011 using the SPM for a period of time when the wind speed was within the validity range of this model suggests that an inversion for the dielectric constant may be possible over the oil infested areas but that the dielectric constant may be underestimated over ocean regions. This is likely due to the insensitivity of the model for high values of the dielectric constant. Current work is being conducted to reliably remove the effects of thermal noise which is induced by the operation of the sensor and manifests itself as an additive signal in the form of radar cross section.
Future work involves inverting for the dielectric constant using the other two methods cited above and using acquisitions that were taken for different wind speeds and incidence angles.
The challenges of Automatic Oil Spill Detection and Polluter Identification Services
Hajduch, Guillaume; Vadaine, Rodolphe; Nhun Fat, Béatrice; Maaref, Nadia - C.L.S, France
The routine monitoring of oil spill discharges from spaceborne SAR observation was demonstrated in the early 2000’s and is since an operational service managed by the European Maritime Safety Agency (EMSA) as part of the CleanSeaNet service. The CleanSeaNet service is based on radar satellite images, covering all European sea areas, which are analyzed to detect possible oil spills on the sea surface. When a possible oil spill is detected in national waters, an alert message is delivered to the relevant country. Analyzed images are available to national contact points within 30 minutes of the satellite passing overhead [1].
The definition and evolutions of this operational service were supported by the realization of a set of R&D studies and projects ranging (ESA / Roses, FP7 Marcoast [2], FP7 Sea-U [3]). The initial challenge was first to demonstrate and improve the responsiveness of such service for the oil spill detection. It then evolved to a more complete service supporting the polluter identification as well, by applying drift models to the detected spills and by correlating with AIS information.
For now, approximately 2,000 images are ordered and analyzed per year [1] on behalf of EMSA. This does not include all the potential images to be acquired over the European areas of interest. The service is performed by several Service Providers applying a panel of automatic processing required to collect the appropriate knowledge of the maritime situation over the area of interest for this service (vessel detection, wind measurement, swell measurement) and supervised oil spill detection and polluter identification. This latest supervised step implies that each single image under analysis needs to be inspected by a specially trained SAR image analyst, which then create constraints and costs on the operations. This supervised step is required for ensuring the quality of service in terms of detection and false alarm rates compatible that cannot be achieved by operational method so far.
This paper will present the challenges of full automatic oil spill detection including: the need of a reference data base of pollutions, a stringent requirement on computation time for near real time operational service, and the various ways to tackle the problem (ad hoc techniques, machine learning, deep learning…). Some preliminary results and the program of work to achieve a complete automatic service will be exposed as well.
[1] Ten year of CleanSeaNet service: http://emsa.europa.eu/csn-menu.html
[2] The Marcoast project: http://www.copernicus.eu/projects/marcoast
[3] The Sea-U project http://www.copernicus.eu/projects/seau
A statistical approach of wake detection based on a priori information provided by AIS data
Hajduch, Guillaume; Vadaine, Rodolphe - C.L.S, France
Vessel detection and route characterization is of interest for various services ranging from fishing monitoring, border control and traffic monitoring. The ship wake detection is usually applied as a post processing to the vessel detection itself and is a mean to estimate the route of the vessel together with its speed by measuring the Doppler shift between the turbulent and the vessel itself. Various wake detection techniques are proposed usually based on Radon transform [1,2,3].
We previously defined and developped a detection of wake signature in the Hough transform domain. The Hough transform aims to accumulate evidences of presence of the object of interest. The Hough transform for the detection of linear features is very similar to the Radon transform. In fact, both the Radon transform, and the Hough transform correspond to the same transformation.
In this paper we present an optimized Hough/Radon wake detection algorithm based on practical considerations of the SAR imaging mechanism. More specifically, the Doppler shift between the vessel and its wake is driven by the radial speed of the vessel, that is itself strongly correlated to the orientation of the wake. It is then possible to strongly reduce the domain of detection of the wake signature in the Hough/Radon transform domain to reject wakes in the proximity of the vessel of interest but that cannot physically relate to it. This is of typical interest for dense traffic areas, low incidence angle and high resolution, were multiple wakes may be observed.
In addition to this considerations on radial velocity, we introduce the use of statistics on AIS messages (speeds, heading) as well to further constrain the detection of the wake in the Hough/Radon domain. Indeed, in some regions of the maritime domain the maritime traffic is strongly driven by a set of Traffic Separation Schemes (TSS) imposing specific vessels heading. For such areas, it is then possible to precompute histograms of speeds and heading from databases of AIS data, that can be used as a priory probability density functions to be considered in the wake detection scheme.
In this paper, both classical Hough/Radon and improved (using statistical AIS information) are evaluated in terms of probability of detection and false alarms and compared to a recent paper dealing of wake detection on Sentinel-1 images [3].
[1] Rey MT et al, Application of Radon Transform Techniques to Wake Detection in Seasat-A SAR Images. IEEE Trans. Geosci. Remote Sensing 1990, 28, 553-560
[2] Copeland A.C et al, Localized Radon Transform-Based Detection of Ship Wakes in SAR Images. IEEE Trans. Geosci. Remote Sensing. 1995, 33, 35-45
[3] Graziano MD, et al, Performance Analysis of Ship Wake Detection on Sentinel-1 SAR Images, Remote Sensing, MDPI, 2017
Detection of range/azimuth ambiguities and permanent objects in the maritime domain by multi temporal stacking of SAR images
Hajduch, Guillaume; Vadaine, Rodolphe; Vincent, Pauline - C.L.S, France
Vessel detection and route characterization is of interest for various services ranging from fishing monitoring, border control, traffic monitoring [2] and oil spill monitoring [1]. The first step of vessel detection is to detect the bright echoes in the SAR images [3]. However, it is then of interest to reject the bright points that are not directly related to a vessel. Such kind of points can be related to various phenomena or object. For instance, cities may produce range or nadir ambiguities in the maritime domain observed by SAR that may impeach the detection of small vessels in the impacted areas. Such artefacts are more frequently observed on very high-resolution products, but we also characterized this on Sentinel-1 Interferometric Wide Swath products. In addition, some objects like oil rigs, or fish farms shall not be confused to vessels by automatic processing.
In this paper we present a method of detection and discrimination of both range and azimuth ambiguities of coastal objects and permanent echoes. This method is based on the multi temporal stacking of SAR images acquired on the same area of interest. Some methods were previously proposed to detect such specific points, but they are usually applied as a post processing of vessel detection. Our method applies object detection after a full multi temporal stacking (not before), then enabling to enhance the contrast over the permanent points by taking advantages of wind variability with the time in the maritime domain. It then allows to characterize smaller objects than the one spotted by standard techniques based on post-processing of large number of vessel detection reports.
The main result of this paper is a set of maps illustrating the permanent artefacts and objects over the Mediterranean seas for various Sentinel-1 configurations. Those maps can be used to filter out permanent echoes from standard vessel detection techniques. Furthermore, the method allows to produce maps of range/azimuth and permanent points that can be used as denoising information before application of vessel detection algorithm.
[1] Ten year of CleanSeaNet service: http://emsa.europa.eu/csn-menu.html consulted on 26 January 2018
[2] The FP7 Dolphin project: http://gmes-dolphin.eu/
[3] G.Hajduch et al, Ship Detection: from processing to instrument characterization, SeaSAR 2008
SAR Observations of Ocean-Atmosphere Coupling over the Somali Current
Caruso, Michael; Graber, Hans - University of Miami, United States of America
Accurate high resolution measurements of ocean surface stress are required to improve predictions in areas of strong atmosphere-ocean coupling. Scatterometer observations have shown that there is significant covariability between wind stress and sea surface temperatures and that 10-m winds are affected by ocean currents. These studies used relatively low resolution satellite scatterometer and microwave radiometer data along with temporal averaging and high pass filtering to resolve the atmosphere-ocean coupling. The reduced wind speed over cooler side of a front produces increased curl for wind parallel to the front and increased divergence for wind perpendicular to the front. Errors are introduced since the 10 m derived winds are relative to the moving ocean and the local sea state and surface currents are neglected.
The Western Arabian Sea is characterized by a seasonally reversing circulation influenced by strong monsoon winds. During the winter monsoon, the southward flowing Somali Current merges with the northward flowing East African Coastal Current to form the eastward flowing South Equatorial Counter Current. This pattern shifts during the summer monsoon as the East African Coastal Current feeds the Southern Wedge, Somali Current and Great Whirl. The summer monsoon winds are influenced by the ocean currents and coastal upwelling over the Western Arabian Sea. This covariability influences the moisture and heat transfer into the troposphere where it can affect the rainfall over India.
Sentinel-1 synthetic aperture radar data were acquired in early June over the Somali Current during the 2016 and 2017 summer southwest monsoons. The upwelling of cold water along the Somali coast, the Southern Wedge, and Great Whirl are typically well developed by June. Wind stress curl computed from the SAR derived wind field clearly show sub-mesoscale features along the Somali Current and the Great Whirl that are correlated with patterns in the SST data. The SAR data shows that during the 2017 Summer Monsoon, the Great Whirl and Southern Wedge developed later than in 2016. In 2017, the Somali Current and the Southern Wedge were broader and the Great Whirl was shifted significantly to the southwest. The result was that the covariability was less distinct in early June 2017 than it was in 2016. The wind stress curl also shows a bifurcation in the Somali Current in 2017, where part of the current forms the Great Whirl. Sentinel-1 data for the 2017 monsoon season were supplemented with Radarsat-2 and TerraSAR-X data to characterize the intraseasonal variability of the ocean-atmosphere coupling.
Application of the Sentinel satellites for tracing surface slicks along the Irish Western Shelf
Mullins, Monica Mary; Croot, Peter L. - National University of Ireland, Galway, Ireland
Application of the Sentinel satellites for tracing surface slicks along the Irish Western Shelf
Mullins, M.1and Croot. P.L1
1 Earth and Ocean Sciences, School of Natural Sciences, National University of Ireland Galway
A key challenge facing the maritime industry at present is developing the capability to distinguish in real time, maritime accidents from naturally occurring phenomena such as phytoplankton blooms and natural oil seeps. The ability to correctly determine the nature of the observed surface slick will facilitate applications in maritime safety, oil exploration, harmful algal blooms/ecosystem services and other marine based activities. This project aims to link detection and monitoring of these natural and manmade surface slicks using satellite based observations incorporating the new ESA sentinel series of satellites. For example, the Sentinel 1 satellites, incorporating SAR bands, allow for the determination of surface slicks even under the cloudy conditions typically found along the Irish west coast. However SAR by itself is unlikely to accurately predict the nature of the observed slick and so predictive assessments will be made by combining SAR with ocean colour (Sentinel 3) and scatterometer winds (Sentinel 1). Overall our work has 4 main objectives:
This publication/presentation* has emanated from research supported in part by a research grant from Science Foundation Ireland (SFI) under Grant Number 13/RC/2092 and is co-funded under the European Regional Development Fund and by PIPCO RSG and its member companies.
An Empirical Algorithm for GF-3 SAR Wave Height Retrieval from Quad-Polaried Wave Mode Images
Wang, He (1); Yang, Jingsong (2); Wang, Jing (1) - 1: National Ocean Technology Center, China, People's Republic of; 2: Second Institute of Oceanography, China, People's Republic of
Gaofen-3 (GF-3), the first Chinese civil C-band synthetic aperture radar (SAR), has been successfully launched by the China Academy of Space Technology (CAST) on 10 August 2016. Among its 12 imaging modes, wave mode is designed to monitor the ocean surface wave over the open ocean. An empirical retrieval algorithm of significant wave height (SWH), termed QPCWAVE_GF3, is developed for quad-polarized SAR measurements from GF-3 in wave mode. QPCWAVE_GF3 model is built using six SAR image and spectrum related parameters. Based on a total of 2,576 WaveWatch III (WW3) and GF-3 wave mode match-ups, 12 empirical coefficients of the model are determined for 6 incidence angle modes. The validation of QPCWAVE_GF3 model is performed through comparisons against independent WW3 modelling hindcast, and observations from altimeters and buoys from January to October in 2017. The assessment shows a good agreement with RMSE between 0.5 m to 0.6 m, and SI around 20%. In particular, application of QPCWAVE_GF3 model on SWH estimation for two storm cases from GF-3 data in wave mode and Quad-Polarization Strip I (QPSI)mode are presented respectively. Results indicate that the proposed algorithm is suitable for SWH estimation from GF-3 wave mode, and promising for other similar data.
Improving Sentinel-1 Ocean Wind Field Consistency and Accuracy by Minimizing Antenna Beam Signatures
Jacobsen, Sven; Velotto, Domenico; Pleskachevsky, Andrey; Wiehle, Stefan - German Aerospace Center (DLR), Germany
Wind information retrieval over the sea surface from microwave sensor data has a long and successful history. The accuracy of geophysical model functions (GMFs) applied to relate the microwave backscatter to wind speed and direction in 10m height has been continuously improved from early scatterometer missions to contemporary spaceborne SAR sensors like the current Sentinel-1 (S-1) pair. The statistical accuracy of SAR-based wind speeds is nowadays better than 2m/s and therefore the data is appreciated by met-ocean modellers to validate and improve their models.
State of the art high resolution SAR systems can provide wind fields with a resolution of less than 100 m which have been successfully validated against co-located LiDAR data. SAR-based data is particularly interesting for the offshore wind industry as it combines a resolution near to LiDAR capabilities and a large cross-track coverage of up to 250km for e.g. the two S-1 satellites. The S-1 systems work in TOPS SAR imaging mode, which has been designed to reduce the scalloping effect in burst mode SAR imaging, e.g. ScanSAR. The large spatial coverage of these systems allows wind park operators to get regular snapshots of entire marine regions such as the German Bight and cross-validate model data especially with focus on the impact of wind shadows of adjacent turbine clusters.
A close inspection of the Sentinel-1 Interferometric Wide Swath (IW) mode data over the ocean reveals antenna beam patterns remaining in the calibrated and noise-corrected data, resulting in discontinuous wind speed values across the beam boundaries. While of minor importance when calculating coarse wind fields of several kilometres resolution, high resolution wind fields as desired by offshore wind operators suffer from this uncertainty. A compensation for these errors contained in S-1 IW wind fields is believed to further increase the acceptance of SAR-bases wind data in both, industrial users of the data and the Numerical Weather Prediction (NWP) community for assimilation in met-ocean models.
We present an analysis of the impact of this effect on estimated wind fields based on more than 8000 Sentinel acquisitions with level-2 OCN data available and compare to high resolution ECMWF model data. We propose a correction scheme to minimize the beam pattern impact. The method can be applied to both, level-2 OCN data products and level-1 original IW image data.
Multi–polarization methods to detect mud flat areas using C- and X-band spaceborne SAR data.
Ferrentino, Emanuele (1); Marino, Armando (2); Nunziata, Ferdinando (1); Migliaccio, Maurizio (1); Li, Xiaoming (3) - 1: Università degli studi di Napoli Parthenope, Engineering Department, Italy; 2: The Open University, Engineering and Innovation, Milton Keynes, United Kingdom; 3: Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing, China
The study of intertidal zone close to coastal areas is of paramount importance due to both anthropomorphic activities and natural phenomena, which threaten the stability of land and safety of the people. However, the monitoring of intertidal zone is not trivial due to the presence of mud flats and aquacultures. In fact, field surveys are difficult to perform in mud flat areas because of weather, tidal conditions and other natural factors.
Within this context, remote sensing plays an important role for intertidal zone monitoring. Optical images have the great advantage of being simple to interpret and they are easily obtainable. However, optical radiation is severely affected by cloud cover, solar illumination, and other adverse meteorological conditions. These problems can be solved using radar sensors, which guarantee all-day and almost all-weather acquisitions, together with a wide area coverage. In particular, the Synthetic Aperture Radar (SAR) can be very useful for intertidal zone monitoring purposes, because of its fine spatial resolution.
The main goals of this study are to develop multi-polarimetric methods to analyze scattering mechanisms that characterize mud flat area using full- and dual-polarimetric SAR data.
The first part of this study is based on the detection of the mud flat area undertaken according to the Polarimetric Notch Filter and the change detection approach proposed in [1] and [2], respectively. The extended Bragg model [3] is adopted as a reference to model sea surface polarimetric covariance / coherence matrix.
Experiments, undertaken on actual SAR data collected over the intertidal zone near Jiangsu, China, by the C-band RadarSAT-2 and X-band TerraSAR-X missions show that the proposed methodologies, well detect the mud flat area and the hard objects deployed on them. Furthermore, a detailed analysis shows that full-polarimetric channels performs best in terms of land / water discrimination when compared to the dual-polarization channels.
[1] A. Marino, (2013), “A Notch Filter for Ship Detection With Polarimetric SAR Data", IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 6(3), 1219-1232.
[2] A. Marino; S.R. Cloude, and J. M. Lopez-Sanchez, (2013), “A New Polarimetric Change Detector in Radar Imagery”, IEEE Transactions on Geoscience and Remote Sensing, 51(5), 2986 -3000.
[3] I. Hajnsek, E. Pottier, and S. R. Cloude (2003), “Inversion of surface parameters from polarimetric SAR”, IEEE Transaction on Geoscienze and Remote Sensing, 41(4), 727–744.
Classification of Maritime Objects in TerraSAR-X Imagery
Velotto, Domenico (1); Tings, Björn (1); Bentes, Carlos (2) - 1: German Aerospace Center (DLR), Germany; 2: Technical University of Munich, Germany
For an adequate security of the maritime domain is necessary to be aware of the objects location and their activities, e.g. ship traffic, in relevant areas of the sea. TerraSAR-X, a satellite radar imager operating at X-band, is a powerful tool to detect maritime objects, e.g. ships, oil platforms, icebergs etc., which can raise potential risk for maritime traffic and environment. The use of satellite radar to create such awareness has the benefits, among many, that can detect also no self-reporting objects; it operates almost independently of cloud cover and beyond coastal ranges. However, radar images are less easy to interpret and carry no direct objects identification information, like it happens in optical images or messaging report systems such as Automatic Identification System (AIS) and Long Range Identification and Tracking (LRIT). Therefore, the challenge is to classify the different types of maritime objects based on the radar signal only. Different machine learning techniques are here investigated to this end.
In this paper the problem at hand is restricted to the classification of 5 common type of maritime object in Synthetic Aperture Radar (SAR) images, i.e. tanker ship, cargo ship, windmill turbine, offshore platform and harbour structure. Cargo and tanker represent are the most correlated in terms of size and mechanical structure, so the most challenging to discriminate. Indeed windmill and platform are yet static object but with the increasing offshore energy production the amount and geographical location is not always available, i.e. new structure being built, removed or moved. Land masking process in high resolution SAR ship detection might suffer from false alarms of harbour structure not included in medium resolution landmask database and therefore being able to identify this class gives the opportunity to reduce this risk.
The Multi Layer Perceptron (MLP) and Convolutional Neural Network (CNN) models are the two feature extractors and classifiers used to accomplish this task. The performance of the two models are evaluated depending on the depth of the network layers and resolution of the input image using a relatively large classification dataset composed of real SAR samples extracted from TerraSA-X images.
Ocean surface current retrievals from satellite and coastal remote sensing: A comparison of Envisat ASAR and HF-radar observations from the west coast of Norway.
Moiseev, Artem (1); Hansen, Morten W. (1); Johannessen, Johnny A. (1,2) - 1: Nansen Environmental and Remote Sensing Center, Bergen, Norway; 2: Geophysical Institute, University of Bergen, Bergen, Norway
The ocean surface circulation is responsible for significant transport of heat, salt, passive tracers and ocean pollutants. On basin scales, surface currents and their variations are major players in climate and weather fluctuations. Surface currents impact the steepness of surface waves and are thus important for generating reliable marine sea state forecasts for maritime operations including fisheries and ship routing. Because of their significance in advecting passive particles, knowledge of surface currents is also important for oil spill and marine debris response actions, as well as search and rescue operations. Assimilation of surface current data in ocean circulation models is a common practice today, thanks mostly to the regular availability of satellite altimetry data and surface drifter data.
During the last decades surface current observations have also emerged from coastal high-frequency (HF) radar and satellite synthetic aperture radar (SAR) observations, thanks to the Doppler shift measurements. HF-radar systems derive hourly surface current observations in the near-coastal regions at a radial distance of up to 200 km offshore with a spatial resolution from 500 m to 6 km. Between 2005 and 2010, an HF-radar (CODAR) was operating at the Fedje island on the west coast of Norway. The range coverage of this system was nearly 50 km.
In comparison, the range Doppler shift obtained with SAR can be used to derive a signal related to the range directed surface current across the swath (~500 km) at a spatial resolution of 1-10 km. According to previous theoretical analyses, the velocity associated to the geophysical Doppler shift can be approximated as a linear sum of wind-wave induced sea surface velocities and the sea surface current in the range direction. The wave-state contribution can be estimated using an empirical relationship between the range Doppler velocity and the near surface wind field. When the wave-state contribution is then subtracted from the observed geophysical Doppler shift, a measure of the surface current in the range direction can be obtained.
In this presentation, the surface current retrieved from Envisat ASAR data is assessed in comparison to the CODAR SeaSonde measurements at Fedje.
Identifying potentially harmful jellyfish blooms using SAR Sentinel 1 images
Orasi, Arianna (1); De Angelis, Roberta (1); Bruschi, Antonello (1); Drago, Aldo (2); Gaucci, Adam (2); Deidun, Alan (2); De Leo, Francesco (3); Zampardi, Serena (3); Ingrosso, Gianmarco (3); Boero, Ferdinando (3) - 1: ISPRA, Italy; 2: University of Malta, Physical Oceanography Research Group, Dept. of Geosciences, Msida MSD 2080, Malta; 3: University of Salento DiSTeBA Centro Ecotekne, Lecce 73100, Italy
In the Mediterranean Sea, the mauve stinger Pelagia noctiluca is probably the most frequent jellyfish species, which appears to be blooming with increased frequency, and many others jellyfish species appear to be thriving, with significant impacts on fishing and other human activities.
The main current practical way of assessing the presence and the abundance of gelatinous macrozooplankton is through visual detection, in particular through skin diving, boats, airplanes, beaches, video cameras and citizen science campaigns. It is also possible to detect jellyfish blooms through indirect methods such as echosounding records and radio tracking.
The large spatial extent of jellyfish blooms suggests that some form of remote sensing might be an efficient way to provide an early warning system for such phenomena. A recent study has been performed using cloud-free EO ocean colour scenes.
This study aims to explore the potential of Synthetic Aperture Radar for detecting jellyfish blooms. The advantages of SAR are that it obtains high-resolution remote sensing synoptic images covering broad marine domains, and it can be used day and night under all weather conditions. The detection is based on the damped radar return produced by biogenic surface slicks such as those produced by the presence of plants and animals in the ocean.
Firstly, jellyfish blooms datasets in the Mediterranean Sea have been acquired using information collected during EU-funded projects such as the MEDJELLYRISK one (www.jellyrisk.eu) and also through ad hoc citizen science campaigns as the Spot the Jellyfish one in Malta (www.ioikids.net/jellyfish). The sighting datasets are well-supplemented with metadata, including information about geographic position, species and abundance.
When available, SAR Sentinel 1 images corresponding to ongoing jellyfish bloom events are acquired and pre-processed in order to remove speckle noise that constrains image interpretation and further processing of the image. Finally the potential identification of jellyfish blooms is attempted through an adaptive thresholding and a clustering through the closest pixels method, similar to the one used to detect oil spills. A case study is presented, relating to a Pelagia noctiluca bloom identified along the south-eastern coast of the Maltese Islands in June 2016.
Sea Ice Drift Retrieval From Sentinel-1 Data
Demchev, Denis (1,2); Khmeleva, Viktoriya (2) - 1: Arctic and Antarctis Research Intitute, Russian Federation; 2: NANSEN INTERNATIONAL ENVIRONMENTAL AND REMOTE SENSING CENTRE, Russian Federation
We propose a feature-tracking algorithm for sea ice drift retrieval from a pair of sequential Sentinel-1 wide-swath images. The method is based on feature tracking comprising feature detection, description, and matching steps. The approach exploits the benefits of nonlinear multiscale image representations. We evaluated several state-of-the-art feature-based algorithms, including A-KAZE, Scale Invariant Feature Transform (SIFT), and a very fast feature extractor that computes binary descriptors known as Oriented FAST and Rotated BRIEF (ORB) on dual polarized Sentinel-1A C-SAR extra wide swath mode data over the Arctic. The A-KAZE approach outperforms both ORB and SIFT up to an order of magnitude in ice drift. The experimental results showed high relevance of the proposed algorithm for retrieval of ice drift at subkilometre resolution from a pair of SAR images with 100-m pixel size. We found that feature tracking using nonlinear scale-spaces is preferable due to its high efficiency against noise with respect to image features compared with other existing feature tracking alternatives that make use of Gaussian or linear scale spaces.
The developed ice tracking system perfomance is demonstrated by an operational ice drift and deformation analysis over Ob's Bay (SW part of Kara Sea) for winter season of 2016-2017 using Sentinel-1A data to support navigation in sea ice. Some new ice regime characteristics on ice kinematics is obtained.
Accuracy Assessment of Sentinel-1A/B Wind Speed Errors in the Korean Coast
Jang, Jae-Cheol; Park, Kyung-Ae; Park, Jae-Jin - Seoul National University, Korea, Republic of (South Korea)
Sea surface wind is one of the key components in the study of waves, currents, ocean circulation, and atmospheric-ocean interactions, providing us with a overall understanding of complex marine phenomena. As interest in climate change has increased, the importance of observing the global wind field has been emphasized and researchers have been producing global wind fields data using scatterometer. These scatterometer data are accurate to around ±2 m/s for wind speed and 20˚ for wind direction, but they have disadvantages that are deficiency of coastal wind field data and impossibility of analysis of small scale marine phenomena due to low spatial resolution. In this study, the sea surface wind in the coastal region of Korea was calculated from Sentinel-1A/B Interferometric Wide swath (IW) mode VV polarization data and the accuracy of each algorithm was assessed. From May 1, 2015 to September 30, 2017, 395 Sentinel-1A/B data were processed, land masking was performed using Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) data, ship was detected and removed by applying the adaptive threshold method, and in order to remove speckle noise of the Sentinel-1A/B image itself, an ensemble average was calculated by setting a moving window. CMOD4, CMOD_IFR2, CMOD5, CMOD5.N, and CMOD5.Na algorithms were applied to the preprocessed Sentinel-1A/B data. The results show that the root mean square error (RMSE) and bias are 1.83 m/s and –0.64 m/s for CMOD4, 1.82 m/s and –0.59 m/s for CMOD_IFR2, 1.69 m/s and –0.38 m/s for CMOD5, 1.68 m/s and 0.31 m/s for CMOD5.N, and 1.65 m/s and 0.14 m/s for CMOD5.Na, and CMOD5.Na algorithm showed the best simulation of sea surface wind in the seas around the Korean Peninsula. By region, the Yellow Sea showed an increase in bias compared to other region, which is interpreted as a result of the change in sea surface roughness due to low water depths.
Impact of the SAR wind in regional wave model on the french Mediterranean sea
Dalphinet, Alice (1); Aouf, Lotfi (1); Husson, Romain (2); Michaud, Héloïse (3) - 1: Météo-France, France; 2: Collecte Localisation Satellites, France; 3: Service Hydrographique et Océanographique de la Marine, France
The Mediterranean sea is known for the high spatial variability of its surface wind. The fluxes are constrained by the numerous reliefs very near the coasts and the effect of the islands, creating sustained local winds. Moreover autumn and winter seasons bring heavy storms and Medicane (Mediterranean hurricane) that enhance the surface fluxes and make even more complex wind fields.
The atmospheric mesoscale models are more and more able to simulate a variable surface wind by a fine representation of the relief and a capacity to simulate marine breeze due to diurnal cycle. The non-hydrostatic model of Meteo-France, Arome, run for instance operationally at a resolution of 1,3 km. Its surface fields are very realistic but can suffer of an incorrect localisation of the processes with a mismatch greater than its fine scale. In particular during a convective event, the very fine stormy cells can be shifted of some kilometers compared to reality.
The hig resolution SAR winds present the advantage to give a very precise measurement of surface wind intensity. The use of such satellite wind observation has a very important advantage for waves forecast in coastal areas in the Mediterranean sea. In this work the quality of the SAR winds are evaluated by comparison of Arome wind as input in a wave model. For that purpose, we use a MFWAM configuration at 2,5 km and a configuration of Wavewatch III around the french Mediterranean coasts. The WW3 configuration has been developped and implemented in operations at Météo-France in 2015 in the framework of the HOMONIM project, by Météo-France and Shom. It uses an unstructured grids up to a resolution of 200 meters.
The SAR winds are integrated in the Arome wind fields. Both simulations with Arome and merged wind forcing are conducted on particular events that happened in the french Mediterranean coast in 2016 and 2017. The sea state outputs are compared with altimeter and buoy observations. The comparison shows the good skill of SAR winds and the benefit of using more accurate winds than 1,3 km for a high resolution wave model like the WW3 configuration.
Further conclusions and results will be discussed in the final paper.
Observing Oil Releases from Platforms Using Synthetic Aperture Radar
Skrunes, Stine; Johansson, Malin; Brekke, Camilla - UiT The Arctic University of Norway, Norway
Space borne Synthetic Aperture Radar (SAR) is used for operational surveillance of ocean areas and oil spill detection [1]. Oil spills are frequently detected around oil platforms due to the releases of so-called produced water (see, e.g., [2]). This is water from the reservoir that has been separated from the oil and gas at the platform, and which is in part released into the sea. Release of produced water is legal, within given limits. The water still contains oil and can form surface slicks, similar to other oil spills.
Understanding the signatures of produced water and how they are related to, e.g., the relative oil volume and/or concentration, and to environmental conditions, can be helpful for the operational services. For example, distinguishing a “normal” release of produced water from an “abnormal” release (elevated amounts) in a SAR image is currently an unsolved problem. Very little research on these topics have been done before.
In this study, we investigate SAR data acquired over the oil platform Brage, located in the North Sea. In addition to the SAR data, in situ information on wind conditions and oil releases have been obtained from the platform operator. This type of data allows for comparison of SAR signatures of produced water of the same oil type, acquired under varying conditions. The aim of the study is to evaluate how the SAR signatures vary with, e.g., the amount of oil in the release and wind speed. Both wide swath low resolution ScanSAR imagery (Radarsat-2 and Sentinel-1) and high-resolution quad-polarization data (Radarsat-2) are available. A comparison between the two data types, in order to evaluate any potential advantage of the high-resolution data in characterization of the releases, is also of interest.
[1] Solberg, A. H. S. (2012), Remote Sensing of Ocean Oil-Spill Pollution, Proc. of the IEEE, 100(10), 2931-2945, doi: 10.1109/JPROC.2012.2196250.
[2] Espedal, H. A., and Johannessen, O. M. (2000), Detection of oil spills near of shore installations using synthetic aperture radar (SAR), Int. J. Rem. Sens., 21(11), 2141-2144, doi: 10.1080/01431160050029468.
Sargassum Monitoring in the Caribbean Sea with Sentinel-1 and Landsat-8 data
Hajduch, Guillaume; Husson, Romain; Nhun Fat, Béatrice; Longépé, Nicolas; Sutton, Marion - C.L.S, France
The grounding of sargassum rafts is affecting multiple regions in the Caribbean Sea and in the Gulf of Mexico. Large sargassum rafts are drifting over long distance, probably from the Bresilian coast of Gulf and Guinea. The precise cause of this phenomena is not well known, even if anthropic sources (pollution from the Amazon) are suspected. The impact of this grounding on the coastal area is extremely bad as the decomposition of the algae produces hydrogen sulfide: a nauseous gaz that can be toxic at high concentration. The grounding of sargassum rafts induces significant decrease of touristic activities in the impacted area, with a noticeable impact on the local economy. Monitoring the sargassum rafts before their grounding is of interest in order to deploy preventing actions (catching the rafts at sea before a predicted grounding).
In this paper we present an operational demonstration of sargassum monitoring deployed in April 2016. This service is based on:
(1) observation and detection of the rafts on both Sentinel-1 SAR data and Landsat-8 optical data. The signature of Sargassum in SAR images is an increase of the local backscattering due to an increase of the surface roughness. The spectral signature of the sargassum in the optical domain was studied and panchromatic images were produced considering a dedicated composition of blue, near infra red (NIR) and short wave infra red (SWIR). Both types of sensors (SAR and Optical) were used as complementing means of observation considering the capacity of the SAR to observe under the clouds and by night, while the optical sensor provide opportunities to capture signatures at higher resolution and different mean solar time.
(2) prediction of their drift: A drift model considering lagrangian advection under the effect of wind and current was tuned and used in order to predict the drift
(3) diffusion of the observed and predicted drift over a period of 72h ahead on web portal. This portal allows displaying the various satellite images, the delineation of sargassum rafts, the predicted drift, the generation of formal reports, etc
This demonstration allowed to evaluate the capacities of sargassum detection using SAR and optical imagery. More specifically, the detection performances with respect to incidence angle and polarization were assessed, the requirement on revisit frequency was refined so as the requirement on delay of delivery of analysis of each satellite image. This kind of service can benefit from additional medium resolution observation for instance from Modis and Sentinel-3 OLCI.
Aknowledgement: this demonstration service was partially funded by CNES.
Ocean Surface Wind Speed Retrieval from Sentinel-1 dual-polarization imagery
Ye, Yufang; Eriksson, Leif E. B. - Department of Space, Earth and Environment, Chalmers University of Technology, Sweden
Ocean surface wind is an essential parameter for ship navigation and the study of ocean. Satellite sensors such as synthetic aperture radar (SAR), microwave radiometer, radar scatterometer and radar altimeter can be used to retrieve information of ocean surface winds. SAR sensors are the ones that achieve finest spatial resolution independently of weather and environmental conditions. The availability of C-band SAR data from satellite, and the direct interactions of C-band wavelengths with ocean surfaces has made C-band the favorite frequency for SAR wind estimation.
In this study, results from two regression models [1,2] for retrieval of ocean surface wind speed from C-band dual-polarization SAR images are compared with wind speeds from the commonly used C-band geophysical model function CMOD5.N [3]. The first model uses co-polarization (VV) normalized radar cross-sections (NRCS) and antenna beam incidence angle as independent variables. The second one uses VV NRCS, cross-polarization (VH) NRCS, instrument noise floor, and incidence angle as predictors. To train and evaluate the models, a database containing wind measurements from 107 NDBC/NOAA buoys along the east and west US coast collocated and coinciding with Sentinel-1 IW mode images throughout the year of 2017 was constructed. In total, there are over 3000 samples in the database, among which 75% are used for training and 25% for evalution. Results of the latter model proved to give a higher accuracy than the former due to the use of additional VH variables. Furthermore, the two regression models (without wind direction models) demonstrate better accuracies in the retrieved wind speed than what is achieved with CMOD5.N, which uses wind direction information.
References:
[1] Komarov, S., Komarov, A. and Zabeline, V., 2012. Marine wind speed retrieval from RADARSAT-2 dual-polarization imagery. Canadian Journal of Remote Sensing, 37(5), pp.520-528.
[2] Komarov, A.S., Zabeline, V. and Barber, D.G., 2014. Ocean surface wind speed retrieval from C-band SAR images without wind direction input. IEEE Transactions on Geoscience and Remote Sensing, 52(2), pp.980-990.
[3] H. Hersbach, 2010. Comparison of C-band scatterometer CMOD5. N equivalent neutral winds with ECMWF. Journal of Atmospheric and Oceanic Technology, 27(4), pp.721-736.
Precise SAR Doppler calibration and application for sea ice drift estimation
Park, Jeong-Won; Hansen, Morten; Korosov, Anton - Nansen Environmental and Remote Sensing Center, Norway
The range Doppler method is a unique application of SAR, that can extract instantaneous velocity field at low extra cost in all kinds of existing SAR system. The main potential applications of this method are within near-surface wind and sea surface current retrieval, and identification and speed measurement of moving targets, including sea ice. As such, this study applies the range Doppler method on sea ice drift retrieval. The main challenges in applying the method to sea ice drift are (1) the relatively low speed of general sea ice drift, ranging from, e.g., 2 to 23 cm/s in the Fram Strait, and (2) uncertainties in the estimation of the geophysical contribution to the range Doppler shift. Regarding the first challenge, the measurement uncertainty must be lower than the actual speed of the object at hand for the method to be useful in drift retrieval from a single scene. In terms of Doppler frequency, an error of 1 Hz would translate to an error in the ground velocity of 6.7 cm/s at 25° look angle using the Envisat ASAR system. Thus, the accuracy of the geophysical Doppler shift should be better than 2-3 Hz for detecting moderate ice drift. The second challenge is related to the correction of non-geophysical contributions to the range Doppler centroid shift. The main sources of uncertainties are, here, the estimation of the geometric Doppler shift and the antenna electronic mispointing. Previous studies were only able to suppress the measurement uncertainty down to 5 Hz for single scenes. This is not good enough for application of the SAR range Doppler method in sea ice drift retrieval.
We therefore examine an extensive set of Envisat ASAR ScanSAR data from January 2010 to improve the Doppler calibration. Following this, we evaluate the feasibility of using Doppler measurements for sea ice drift retrieval. This is done by comparing the Doppler measurements with results from the conventional cross-correlation (CC) based sea ice drift estimation method. In the Doppler calibration, we propose a comprehensive global data analysis rather than focusing on individual scenes only. For estimation of the geometric Doppler shift, the antenna misalignment along the axes of the satellite body and incomplete attitude knowledge are examined. Corresponding calibration procedures are then suggested. This correction results in an RMSE is 2.8 Hz for non-moving land pixels. Following the geometric calibration, electronic antenna mispointing is estimated. This is done by finding an empirical relationship between land covering range Doppler centroid shift retrievals corrected for geometric contributions, and the antenna look-angle. This relationship is then used to compensate the antenna mispointing in image, at any location. Following the signal corrections, a preliminary comparison of SAR Doppler-based instantaneous velocity and the CC-based mean velocity shows an offset of 0.09 m/s between the two. In general, the SAR-Doppler based velocity estimates are about 1.2 times higher than the ones resulting from the CC-method. This can be understood by considering the rotating nature of sea ice motion.
New and Improved Geophysical Doppler Shift from Envisat ASAR - Impact on Near-Surface Wind Retrieval
Hansen, Morten Wergeland; Park, Jeong-Won; Moiseev, Artem - Nansen Center, Norway
The synthetic aperture radar (SAR) Doppler centroid shift has been demonstrated to contain geophysical information about sea surface wind, waves and current at an accuracy of 5 Hz and pixel spacing of 3.5-9x8 km2. This corresponds to a horizontal surface velocity of about 20 cm/s at 35 degrees incidence angle. The ESA Prodex ISAR project aimed to implement new and improved SAR Doppler shift processing routines to enable reprocessing of the wide swath acquisitions available from the Envisat ASAR archive (2002-2012) at higher resolution and better accuracy than previously obtained, allowing combined use with Sentinel-1 and Radarsat-2 retrievals to build time-series of the sea surface velocity in the Nordic Seas.
New geophysical Doppler shift retrievals from Envisat ASAR acquisitions in January-March 2010 are used together with normalized radar cross section (NRCS) measurements and model wind forecasts to provide high resolution near-surface wind in a Bayesian scheme, following Mouche et al. (2012). In contrast to previous analyses, the new method takes into account spatial variability of the wind field. The results are assessed in comparison to buoy measurements from NOAA NDBC standard meteorological buoys, scatterometry data, and the results obtained by Mouche et al., 2012.
The retrieval of Doppler shift information from satellites, and subsequent signal partitioning into wind, wave, and current information, has potential for application within both operational oceanography (e.g., ship routing, offshore operations) and climate research.
Case Study of Oil Pollution from Ship in the Gulf of Lion Using Sentinel Data
Lavrova, Olga; Bocharova, Tatiana; Nazirova, Ksenia - Space Research Institute RAS, Russian Federation
The new quality of observations of marine processes ensured by the Sentinel sensors was demonstrated by an analysis of three consecutive satellite images captured over the Gulf of Lion in the Mediterranean Sea on the 19th of June 2017 with time intervals of 5 and 7 hours. Sentinel-1A,B SAR-C and Sentinel-2A MSI images were used to investigate in detail the evolution of the oil slick occurred as a result of ship discharge of waters containing petroleum products. Joint processing and analysis of the satellite data was performed using the tools of the See the Sea portal developed at the Space Research Institute RAS. Wind parameters were provided by the Meteoblue meteorological service.
The evolution of the slick was not the same along the ship path. Different parts of it were most likely influenced by different hydrometeorological conditions along the distance of about 35 km. This was confirmed by current modeling and analysis of the wind field. South-southeast wind varied from 4,39 to 8,16 m/sec. However, in the immediate region of initial discharge, near the coast, no considerable displacements of the slick was observed, only spreading. It was suggested that here the effects of wind were offset by the coastal current with opposite direction shown by the modeling. Or, this portion of the discharge could be too massive to be influenced either by wind, which was probably not so strong at the coast, or current. Further out from the coast, the other parts of the slick, as expected, drifted northwest at an average speed of 15–19 cm/sec, under the south-southeast wind.
This and many other examples show, that today, with the instruments onboard the Sentinel satellite family, we can examine sea surface phenomena at scales down to a few meters, compare radar and optical data obtained almost simultaneously over the same region at high and comparable spatial resolutions; observe drift and evolution of sea surface pollution and water dynamics due to smaller time intervals between data acquisitions.
Surface waves damping due to oil and oil derivatives film in application to ocean remote sensing
Sergievskaya, Irina; Ermakov, Stanislav; Lazareva, Tatyana - Institute of applied physics, Russian Federation
Applications of different radar and optical methods for detection of oil pollutions based on the effect of damping of short wind waves by surface films have been extensively studied last decades. The main problem here is poor knowledge of physical characteristics of oil and oil derivatives films which are responsible for wave damping and respectively for possibilities of their remote sensing. In this paper results of laboratory studies of damping of gravity-capillary waves on the water surface covered by crude oil and oil emulsion, diesel fuel and kerosene films are presented. A laboratory method based on measuring the damping coefficient and wavelength of parametrically generated standing waves has been applied for determination of the film characteristics. The investigations were carried out in a wide range values of film thicknesses (from some hundreds millimetres to a few millimetres) and in a wide range of surface wave frequencies (from 10 to 27 Hz). The selected frequency range corresponds to the operating wavelengths of microwave, Х - to Ka -band radars typically used for the ocean remote sensing. The studied range of film thickness covers typical thicknesses of routine spills in the ocean. To retrieve parameters of the oil films from the experimental data the surface wave damping was analyzed theoretically in the frame of a model of two-layer fluid with viscous-elastic properties of the upper and lower boundaries of the upper layer. Physical parameters of oil derivative films were estimated when tuning the film parameters to fit theory and experiment. It is obtained that characteristics of waves, measured in the presence of different oil films are different, in particular, because their volume viscosity is strongly different and because in some case (oil emulsion, kerosene, diesel fuel) it is necessary to take into account the film solubility. Comparison between wave damping due to crude oil, kerosene and diesel fuel films have shown some capabilities of distinguishing of oil films from remote sensing of short surface waves. The work was supported by the Russian Science Founfdation (Project 18-17-00224).
Air-Sea Interaction over Mesoscale Eddy through Sentinel-1A/B Wind Field
Park, Jae Jin; Park, Kyung Ae; Jang, Jae Cheol - Seoul National University, Korea, Republic of (South Korea)
Sea surface wind is one of the most important variables of atmospheric-ocean interaction, and the exchange of heat, momentum, and materials is performed through the sea surface wind. To understand the global climate system, which is large in spatial and temporal scale, it is essential to study complex relationship between the two. Winds in coastal area cause a variety of marine phenomena, especially Eddy, which frequently occurs in the spring on the Korean peninsula East sea, and has physical and biological variability. Using the Sentinel-1A/B SAR data, we calculated high-resolution sea surface winds in the East Sea and simultaneously collected NOAA AVHRR sea surface temperature and COMS/GOCI chlorophyll-a data around Eddy. We also confirmed the presence of eddy in the East Sea using satellite altimetry data. As a result, lower sst was appeared in the eddy than in the surrounding area, and the SAR image in the same area showed low sigma nought value, that is, weak sea surface wind. In addition, the magnitude of the wind speed varies according to the stability of marine-atmospheric boundary layer. The steady state shows low wind speed, while the unstable state shows strong wind speed. When the wind was blowing over Eddie, the bloom position of Eddie was changed according to the wind direction. The bloom of the eddy occurred on the left side in the wind direction, which is the influence of the rotation direction inside eddy and the wind stress curl on the eddie boundary. This study analyzed the spatial variation of chl-a around eddy when the winds were blowing on coastal area, suggesting the impact of air-sea impacts caused by wind fields.
Island-Induced Modification of Sentinel-1A/B Wind Forcing and Impact on Ecosystem
Park, Kyung-Ae; Jang, Jae Cheol; Park, Jae Jin - Seoul National University, Korea, Republic of (South Korea)
Sea surface wind is one of the most important variables in the ocean to understand physical processes in the ocean associated with various marine phenomena such as surface waves, currents, ocean circulation, air-sea heat transfer, biological features, and so on. Satellite scatterometers have long provided us with a good quality of sea surface wind field. Due to the limitations of a satellite scatterometer near the coast, however, it has not been possible to obtain the surface wind field essential for understanding coastal features. Recently, more frequent observations of Synthetic Aperture Radar (SAR) has made it possible to obtain high-resolution wind field near the coastal area including numerous islands. In particular, such a high-resolution wind field can be used to understand spatial distinction of differential wind forcings and its impact on air-sea-land interactions near the complicated coastlines and many of islands around Korean peninsula. Various oceanographic phenomena have been affected by coastal topography and the wind forcings due to air-sea interaction in marine atmospheric boundary layer. In this study, we showed the dominant change of wind field due to the orographic effect of island and the effect of wind field change on oceanic ecosystem using Sentinel-1A/B Interferometric Wide swath (IW) mode data. Land masking was performed using the Shuttle Radar Topography Mission Digital Elevation Model data, the adaptive threshold method was applied to detect and remove the ship in the ocean as one of error sources, and to remove the speckle noises by applying a moving window and an ensemble average. The sea surface winds were retrieved by applying the CMOD5.N algorithm to the preprocessed Sentinel-1A/B IW mode data, and then wind stress curl and Ekman pumping were also calculated to understand wind forcings on sea water. In order to investigate a relationship between wind speed and sea surface temperature (SST), sea surface temperatures (SST) are estimated based on National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) with 1.1 km resolution prior to and post Sentinel-1A/B observations. As a result, the wind field was revealed to be modified by SST change as well as the island effect. Time series of Geostationary Ocean Color Imager (GOCI) chlorophyll-a (chl-a) concentrations confirmed important biological impacts on the sea surface by the change in EKP field. Utimately, this study addresses the role of air-sea-land interaction on physical process as well as biological process of low-level oceanic ecosystem.
NEW WIND INVERSION METHODS USING SENTINEL-1 WIDE SWATH PRODUCTS
Husson, Romain (1); Mouche, Alexis (2); Berger, Henrick (1); Archer, Olivier (2); Vincent, Pauline (1); Longepe, Nicolas (1); Grouazel, Antoine (2) - 1: CLS, France; 2: IFREMER, France
Wind measurements derived from SAR acquisitions provide a unique high resolution and wide view of coastal regions at day and night time, independently from the cloud cover. Classical estimation methods rely on Bayesian inversion using ancillary wind outputs from model together with the SAR co-polarized channel, as implemented for the Level-2 Wide Swath OCN products delivered by Sentinel-1 ESA ground segment.
To improve this wind inversion, a pre-processing step is introduced. Its objective is twofold:
- First, better qualify and flag the SAR image region where the sea surface backscattering processes are not related to the wind stress and potentially impact by other processes such as rain impacts, ships or low winds. This qualification is based on the analysis of the SAR image roughness heterogeneity at multiple resolutions.
- Second, based on this quality mask, the wind direction is estimated using gradient methods in presence of wind streaks and an associated confidence index. Integrating this estimation in the overall wind inversion is particularly motivated by the absence of good quality Doppler information that could help further constrain the wind direction.
This additional wind direction information is derived from co- and cross-polarized channel, when available, and integrated in the overall wind inversion using a new Bayesian scheme. Using a large SAR/buoy co-located dataset, it is validated and compared to the performances of the ESA processor. Applying the pre-processing step and using the heterogeneity mask shows a clear performances improvement.
Currently, the ESA ground segment inversion is based on CMOD-IFR2 Geophysical Model Function (GMF). The performances of this GMF in the wind inversion are compared to other up-to-date GMFs.
Finally, the NRCS cross-pol channel is integrated in the overall inversion scheme together with a dedicated cross-pol GMF ina two step Bayesian scheme. This inversion method is specifically designed for extreme winds assessed using a SAR dataset of hurricane SAR observations acquired during past SHOC campaign. In the end, the capability to retrieve good quality wind maps remains partly limited by the quality of the Sigma0 noise correction, particularly affecting the cross-pol channel and the availability of good quality Doppler measurements.
This study has been partly funded by the ESA SEOM Ocean studies, and the Mission Performance Center (MPC) activities.
The NextGEOSS project and its user-driven approach to build a Earth Observations Data Hub : example of marine Pilots
Voidrot, Marie-Francoise (1); Sutton, Marion (2); Hamre, Torill (3); Bye, Bente Lilja (4); Catarino, Nuno (5); De Lathouwer, Bart (6); Concalves, Pedro (7); Ksoll, Wolfgang (8); Grosso, Nuno (5); Meyer-Arnek, Julian (9); Mueller, Andreas (9); Goor, Erwin (10) - 1: Open Geospatial Consortium Europe, France; 2: CLS, France; 3: NERSC, Norway; 4: BLB, Hønefoss, Norway; 5: Deimos, Portugal; 6: Open Geospatial Consortium Europe, Belgium; 7: Terradue, Portugal; 8: Viderum, Germany,; 9: German Aerospace Center DLR Oberpfaffenhofen, Germany; 10: VITO, Belgium
Several initiatives and projects contribute to support Group on Earth Observation's (GEO) global priorities including support to the UN 2030 Agenda for sustainable development, the Paris Agreement on climate change, and the Sendai Framework for Disaster Risk Reduction. Running until 2020, the NextGEOSS project evolves the European vision of a user driven GEOSS data exploitation for innovation and business, relying on the three main pillars:
These 3 pillars support the creation and deployment of Earth observation based innovative research activities and commercial services.
This presentation will emphasize how the NextGEOSS project uses a pilot-driven approach to consolidate the system in a pragmatique way, integrating the complexity of the existing global ecosystem, leveraging previous investments, adding new cloud technologies and resources and engaging the diverse communities to address all types of Sustainable Development Goals (SDGs).
A set of 10 initial pilots have been defined by the project partners to address the main challenges and include as soon as possible contributions to SDGs associated with Food Sustainability, Bio Diversity, Space and Security, Cold Regions, Air Pollutions, Disaster Risk Reduction, Territorial Planning, Energy.
This presentation will emphasize the working process and NextGEOSS assets developing marine pilots using Sentinel-1 data.
The first pilot aims at showcasing examples of products for selected areas in Cold Regions, combining remote sensing data, in situ data and model products from relevant European infrastructures and international frameworks. The pilot focuses on three areas: (1) the Arctic/Svalbard region, (2) Antarctica, and (3) the Himalayan glaciers. Within each region, products will be developed for users and stakeholders in the GEO community, polar research and education. An initial list of products includes, among others, sea ice type and drift maps for the Fram Strait, based on Sentinel-1 SAR data; sea ice concentration for the Arctic, based on satellite altimeter and passive microwave data; near-surface atmospheric aerosol properties (near-real-time) at Zeppelin station, Svalbard and at Troll station, Queen Maud Land, Antarctica.
The second pilot implements a sargassum identification and tracking demonstration. The detection is based on Sentinel-1 and Landsat data. Coupled with a proprietary drift model, the sargassum can be then tracked to identify precisely the impacted shorelines. From a technical point of view, this pilot shows an example of integration between S1 data and optical data. This activity is expected to be of high interest for institutional user communities working on the preservation of beach areas, several industrial actors interested in the processing of algae as well as the scientific community who is considering the sargassum as a possible indicator for climate change.
All initiatives with an interest in and need of Earth Observations (data, processes, models, ...) are welcome to candidate to become a new pilot initiatives.
NextGEOSS is a H2020 Research and Development Project from the European Community under grant agreement 730329.
Analysing Oceanographic Products from the Sentinel Satellites to validate Modeled Sea Surface Wind Speed and Sea State Parameters from DWD and ESA
Hashemi, Mohammad (1); Rabus, Bernhard (1); Lehner, Susanne (2) - 1: Simon Fraser Unversity, Canada; 2: DLR, Oberpfaffenhofen, Münchner Strasse 20, D 82234 Wessling
Studies showed that changes in ocean state parameters affect the roughness of the ocean surface and scatterometry techniques like SAR can measure these changes in the sea surface roughness and thus the sea surface wind speed effectively. Changes in sea surfaces features like Lee waves, wind rolls due to atmospheric rolls, wind shadows, swell waves are associated with change in sea surface roughness. Synthetic Aperture Radar (SAR) is sensitive to ocean surface roughness because it can observes the changes in Bragg backscatter, tilt and sea surface movement of the radar signal as the resonant of the ocean capillary waves changes. Sentinel-1 Wave Mode data is regularly acquired over open oceans in order to produce oceanographic products (level-2 OCN) giving information on sea surface wind speed and 2D ocean wave information.. Sentinel satellites data products are available free of charge to all users and can be analyzed with the SNAP toolbox. We tested the Sentinel-1 A and B products for use at a meteorological service. We acquired the wave mode images from the Sentinel-1 data hub over 2017. We analyzed its performance over NOAA bouys and compare to the results of the German wea ther Service (DWD) global ocean wave model. We improve the performance of the wave mode product by filtering wave mode images showing sea surface features such as sea ice, land, ships and dark surface patterns. We further tested a deep learning model, which connects the imagery space directly to the meteorological information.
Oil Spill Mixing Ratio Study Using Compact Polarimetry SAR
Li, Hai Yan (1,3); Wu, Jin (1,2); Perrie, William (3) - 1: University of Chines of Academic of Sciences, China, People's Republic of; 2: Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences; 3: Fisheries and Oceans Canada, Bedford Institute of Oceanography
Synthetic Aperture Radar (SAR) is a key sensor for detection of oil spills by virtue of its ability to perform in almost all-weather, in day or night conditions. As SAR progressed from single-polarization (single-pol) to fully (or multi-) polarimetric SAR (pol-SAR) imagery, oil spill analysis and detection methodologies also progressed from conventional single-pol to pol-SAR images. A new architecture is compact polarimetry (CP) SAR, which is coherent dual-pol SAR, whereby one polarization is transmitted, and two orthogonal polarizations are received, each with relative phase, with respect to the other [Raney, 2007]. The CP SAR can obtain as much information as possible with the wider swaths and overcome the limitation of quad-polarimetric (QP) SAR with narrow swath. The potential of CP SAR makes it urgent to study oil spill monitoring with this new SAR mode.
There are essentially two ways to study oil spill detection and monitoring with the CP data. One method is related to the reconstruct a pseudo QP signature from CP SAR [Shirvany et al., 2012]. And the other approach is through the Stokes scattering matrix or the sample coherence matrix [Li et al., 2016; Espeseth et al., 2017]. It is shown that the CP mode is comparable with the QP mode in its ability to distinguish the various slicks from open water [Espeseth et al., 2017].
All these study concentrated on detection and classification of oil spills, and paid less attention to oil-water mixing ratio. It is an important parameter for volume estimation of oil spills and effectively emergency response to oil spills. Collins et al. [2013] investigated the reconstruction of pseudo QP covariance matrix from simulated HP data and computed the oil-water mixing index suggested in [Minchew, 2012]. However, these algorithms do not contribute additional information to the original CP data [Raney, 2016]. Moreover, these algorithms need prior assumptions, which cannot be satisfied in the real remotely sensed scenes, thereby limiting potential applications [Raney, 2016].
Here, we will study oil-water mixing ratio of oil spills on the ocean surface with respect to the CP mode, as simulated from existing QP UAVSAR data. Our approach is focused on using the backscatter parameters to obtain polarimetric information from the CP coherence matrix directly. The relationship between the elements of CP coherent matrix and QP scattering matrix will be established based on tilted-Bragg scattering model. The elements ratio of CP coherent matrix is only related to the incidence angle and the dielectric constant, independent of the surface roughness or ocean waves. This characteristic is similar to that of the co-pol ratio [Minchew, 2012], and suggests that the parameter is a potential candidate for measurement of the oil-water mixing ratio with CP data. The nonlinear and non-constrained optimization and look up table methods are employed to calculate the oil-water mixing ratio from CP data. And the results will be compared with those from QP observation.
Integrating incidence-angle dependence in the segmentation of UAVSAR images
Cristea, Anca; Doulgeris, Anthony Paul - UiT The Arctic University of Norway, Norway
SAR images display a characteristic intensity variation along the range dimension, caused by the side-looking geometry and the fact that the backscattered intensities depend on the incidence angle.
In the case of wide-swath images, where the incidence angle range is large, this intensity variation from near range to far range is significant enough to affect image segmentation performed on absolute intensity values. The effects are an over-segmentation which creates banding in the range direction, as well as the dillution of the real class distinction. In addition, it has been shown that the decay rates vary for different classes, thus reducing the efficiency of previously proposed global-correction methods.
As a solution, we propose an unsupervised general-purpose segmentation (clustering) algorithm that incorporates the incidence-angle variation into the standard mixture modeling. We consider the simplified case, where, per class, the intensity variation in the log-domain is assumed to be Gaussian, and the incidence angle variation is considered to be log-linear.
We demonstrate the algorithm’s efficiency with UAVSAR images containing oil spills and ships on an open-water background, acquired in the North Sea during 2015 (the NORSE2015 experiment). By considering the intensities of the HH channel and an incidence angle range spanning from 30 to 60 degrees, we obtain a clear improvement over the non-correcting segmentation algorithm. The banding effect induced by intensity variations within the water class is removed, and the main image structures (water, oil slicks and ships) are grouped into distinct classes.
The success of our preliminary modeling is encouraging for further development. The simple, approximative incidence-angle relation can be improved, in order to account for the near-range behavior of wide-swath instruments such as the UAVSAR. The Gaussian distribution can be replaced with a more appropriate model that accounts for heavy tails or texture. At last, we are considering extensions for polarimetry, provided that the different incidence-angle dependent behaviors can be accurately modeled for each channel.
Sea Ice Classification with Sentinel-1 using Multi-Block Classification
Zohary, Moein (1); Fabry, Pierre Laurent (1); Bercher, Nicolas (1); Femenias, Pierre (2); Bouffard, Jérôme (2) - 1: ALONG-TRACK SAS, France; 2: ESA/ESRIN, Italy
This work deals with the classification of sea-ice in Sentinel-1 images. One of the main issues of using SAR images for sea ice classification is backscattering variation due to the local incidence angle and different normalization methods have been proposed including linear, iterative and class based normalization. In this research we propose a multi-scale classification independently applied in different incidence angle blocks of 5 degrees. Support Vector Machine (SVM) is used as a classifier of Sentinel-1 SAR images and is tuned to be relaxed on noise and misclassified training data. This classifier is applied iteratively at different scales and the output of each step is used as prior knowledge for the next classification iteration. Operational egg shape ice charts are used to validate the performance of this method. This work is being performed in the frame of the Cryo-SEANICE project funded by ESA and it complements another communication where we show an example of synergistic use of altimetry data and SAR imagery through a dedicated tool.
A Study of Ice Edge Detection Based on a Sub-Aperture Coherence Method
Olsen, Ole Morten - Norwegian Space Centre, Norway
There has been done several studies using sub-aperture coherence methods (i.e. IHP) for ship detection. The method is utilizing that the sea surface have shorter de-correlation time than solid targets. In this study such a method has been tested for detecting the edge between sea ice and sea surface in SAR images. The study has looked in detail hoe this method works also in the marginal ice zone.
A data-driven calibration method for ocean surface current retrieval from Sentinel-1 Doppler centroid.
Collard, Fabrice (1); Johnsen, Harald (2); Engen, Geir (2); Recchia, Andrea (3); Chapron, Bertrand (4); Bras, Sergio (5) - 1: Ocean Datalab, France; 2: Norut, Norway; 3: Aresys srl; 4: IFREMER, France; 5: ESA, ESTEC
Combining restituted Sentinel-1 attitude processed on ground from telemetry measurements and Doppler centroid estimates, a data-driven approach is proposed to reconstruct the geometric Doppler along Sentinel-1 orbit. Together with a land scene based measurement of the electronic miss-pointing induced Doppler shift as function of elevation angle, the geophysical Doppler (LEVEL2 RVL product) can be calibrated to an accuracy compatible to the requirements to observe ocean surface currents. A new innovative method is used to estimate the sea state contribution to the geophysical Doppler based on an Intregral Spectral Value from the image cross spectra imaginary part. Preliminary results will be shown of Sentinel1 derived surface current in the line of sight direction for both WV and IW mode. Resulting performance is estimated by comparison with in-situ surface current measurements.
Spiral Eddy Topology in the Mediterranean Sea from SAR images
Redondo, Jose Manuel (1); Jorge, Jose (1); Karimova, Svetlana (2) - 1: UPC Barcelona Tech., Spain; 2: Space Research Institute, Russian Academy of Sciences
09:00 - 10:50
09:00 -
Preliminary study of atmospheric boundary layer rolls with Sentinel-1 wave mode SAR vignettes
Wang, Chen
Wang, Chen (1,2); Mouche, Alexis (1); Foster, Ralph (3); Vandemark, Douglas (4); Chapron, Bertrand (1); Stopa, Justin (1) - 1: Laboratoire d’Oceanographie Physique et Spatiale, Ifremer, Brest, France; 2: UMR LabSTICC, Institut Mines-Telecom Atlantique, Brest, France; 3: Applied Physics Laboratory, 1013 NE 40th St, Seattle, Washington, USA; 4: Ocean Processes Analysis Laboratory, University of New Hampshire, New Hampshire, USA
Roll vortices that are approximately aligned with the mean wind are a common feature of the turbulent atmospheric boundary layer (ABL) in near-neutral to moderately unstable stratification. This organized secondary circulation usually spans the depth of the ABL and forms bands of overturning circulations with alternating linear regions of enhanced (narrower and stronger) up-drafts and (weaker and broader) down-drafts forming between the counter-rotating roll circulations. Surface wind convergence is enhanced near the base of the updrafts and reduced at the bottom of the downdrafts. Similarly, the near-surface wind is reduced near the base of the updrafts and enhanced below the downdrafts. The net effect is an enhancement of the fluxes across the boundary layer that does not depend on the local mean vertical gradients. This non-local contribution to the turbulent fluxes is believed to play an important role in the overall air-sea exchange of momentum, heat and water vapor. Although the physics of rolls are becoming better understood through numerical and theoretical modeling, at present there is no global roll “climatology”. Parameterizing the contribution of rolls to the ABL fluxes requires numerous observations in order to characterize the structure of these rolls and the conditions under which they form and for which they do not. These are not easily obtained, especially over the world’s oceans.
The Sentinel-1 constellation (S-1A & S-1B) launched by European Space Agency (ESA) in April of 2014 and 2016 routinely collects ∼120,000 (20 x 20 km, 5 m pixel) Synthetic Aperture Radar (SAR) wave mode vignettes in each month over open ocean. These SAR images provide routine and detailed sea surface imprints of many geophysical phenomena in nearly all weather conditions. When present, rolls usually induce a quasi-periodic pattern in the sea surface roughness that can be detected by SAR. The broad coverage of the S1 WV data provides a new and unique opportunity to characterize wind rolls at global scale.
As a pilot study for a larger effort to characterize the atmospheric conditions associated with roll formation and consequent structure, a dataset consisting of 4800 S-1A SAR wave mode vignettes is manually created based on visual interpretation. These selected 4800 images with pure and clear patterns of wind rolls are uniformly distributed in 12 months of 2016. The horizontal wavelength and orientation of wind rolls of each vignette are extracted through the standard SAR image spectral analysis. Considering the vignette image size and the spectral characteristics of ocean swell, we restrict the horizontal wavelengths to the window 0.6-8 km. In addition, we collocated in time and space the SAR dataset with characteristic variables from ECMWF surface analyses. We seek the relationships between the near-surface stratifications (such as characterized by a bulk RIchardson number) and the roll characteristics. These results will be used to develop a strategy for determining an over-ocean roll climatology using routine S-1 observations with an eye toward improving PBL parameterization.
09:20 -
Determination and use of the Wind field from SENTINEL-1 SAR in Coastal Areas
Zecchetto, Stefano
Zecchetto, Stefano - National Research Council of Italy, Istituto Scienze Atmosfera e Clima, Italy
The possibility to get the wind field from the Synthetic Aperture Radar (SAR) images is extremely important in regional seas and coastal areas, only partially covered at present by satellite wind data. Here, the wind fields are often not well reproduced by both global and regional atmospheric models (Accadia and Zecchetto, 2007, Zecchetto and Accadia, 2014), probably because of the interaction between the wind flow and the orography and effects of the local sea water temperature. Furthermore, in small basins the atmospheric model winds are seldom validated, due to the lack of in-situ winds observations.
To retrieve the wind field from SAR two things are mandatory: a reliable radar-backscatter versus wind speed algorithm and a suitable estimate of the wind direction.
A method based on the 2D Continuous Wavelet Transform has been developed and applied to Sentinel-1 SAR images to retrieve the wind direction. The SAR derived wind directions have been compared with in-situ data the variability of which, expressed as the wind direction variance, has been found similar to that obtained from SAR (Zecchetto et al., 2016) and dissimilar to that from the WRF regional atmospheric model. Thus, it may be taken as a proxy for the reliability of the SAR wind directions.
The problem of the wind direction determination from SAR without any external information is thus virtually solved by the 2D-CWT method , and the resulting wind fields have been analysed and compared with those derived using WRF wind directions and with the OWI ESA products.
Possible uses of the SAR derived wind fields (wind vorticity and energy spectra) are also introduced and commented.
09:40 -
SAR Wind Maps and Derived Products: New Possibilities for Offshore Wind Energy Exploitation
Hasager, Charlotte
Badger, Merete; Ahsbahs, Tobias; Karagali, Ioanna; Hasager, Charlotte - Technical University of Denmark, Denmark
Unlimited access to L2 ocean wind fields from SAR opens up new possibilities for application of such wind products in connection with offshore wind energy exploitation. The European Space Agency (ESA) provides an Ocean Wind field component (OWI) retrieved from Sentinel-1 (S-1) observations. The Technical University of Denmark holds a comprehensive archive of SAR wind products for the European Seas generated in a systematic manner for both the Envisat and S-1 missions (see https://satwinds.windenergy.dtu.dk/). Institutions in the United States and Canada provide similar data offerings. End user’s access to wind maps and derived products tailored to wind energy applications has thus eased significantly.
This presentation shall focus on the value of a long-lasting collection of SAR observations for mapping of wind resources offshore. We first investigate the compatibility of wind fields retrieved from different European SAR sensors (Envisat, S-1A, S-1B) through comparisons with in situ observations of the wind speed. Any wind speed biases must be eliminated before the SAR wind data sets can be merged to a single time series.
Based on the merged SAR wind time series, statistical analyses are performed to estimate the mean wind speed, the Weibull distribution, and the energy density within grid cells of the dimension 0.02° latitude and longitude. This leads to detailed wind resource maps over the European seas including coastal waters. The advantage of SAR winds for offshore wind energy exploitation lies partially in the resolved coastal wind speed gradient. Gradients calculated from SAR winds compare well with ground based remote sensing wind measurements from the shoreline up to 3 km offshore. Wind speed gradients from SAR in coastal areas may be influenced by an increased uncertainty in the modelled wind direction, which we use as input for the SAR wind speed retrieval. Preliminary results suggest that small-scale local changes in the wind direction can introduce changing biases with the distance to shore.
Wind speeds retrieved from SAR are valid at the standard height 10 m above sea level. Wind resource maps for higher levels in the atmosphere, where wind turbines operate, are calculated on the basis of long-term average wind profiles given by a Numerical Weather Prediction (NWP) model. Wind resource maps based on SAR and modeling are made available as part of a larger mapping effort called the Global Wind Atlas (http://science.globalwindatlas.info/science.html) for the heights 10, 50, 100 and 150 m. The maps are updated as new S-1 SAR data is collected.
Altogether, synergies between SAR winds and outputs from NWP models are promising for wind resource mapping and wind farm planning in coastal areas where other types of observations are difficult and costly to gather. The open access to SAR wind maps and derived products represents an attractive new opportunity for wind energy developers and the use of SAR winds for planning of offshore wind farms could become standard practice in the near future.
This work is partially funded by the H2020 project CEASELESS (https://ceaseless.barcelonatech-upc.eu/en).
10:00 -
A Novel Approach to SAR wind Retrieval
Nilsen, Vegard
Nilsen, Vegard; Engen, Geir; Johnsen, Harald - Norut, Norway
Abstract—A novel approach to ocean wind retrieval from high range bandwidth Synthetic Aperture Radar (SAR) data is demonstrated and validated using global Sentinel-1 (S1) a and b WV data acquired in October 2016 and January 2017. The first parameter, an integral spectral value (ISV), is defined from the high wavenumber area of the full resolution ocean image cross spectra. The second paramter is the slope of the cross spectra phase plane (CSAPP). Together with the normalized radar cross section (NRCS) constitute the input to a data driven model for ocean wind speed and ocean wind direction retrieval system. The model is trained on S1B data and validated with S1A and S1B data co-located against ECMWF and Ascat data. The cross-spectral azimuth phase plane slope follows two sinusoidal functions, one symmetric and one anti-symmetric, with respect to the wind direction. The anti symmetric part is in direct relation to the azimuth wind direction, while the symmetric part indicates quadratic phase term related to surface acceleration effects. For wind speed standard deviation we achieve 1.54 m/s for S1A, with a bias of 0.18m/s.
10:20 -
The Azimuth Cut-Off Method to Estimate Wind Speed under Extreme Weather Conditions
Corcione, Valeria (1); Nunziata, Ferdinando (1); Portabella, Marcos (2); Grieco, Giuseppe (3); Migliaccio, Maurizio (1) - 1: Università degli Studi di Napoli Parthenope, Dipartimento di Ingegneria, Naples, Italy; 2: The institute of Marine Sciences (ICM-CSIC), Spain; 3: Koninklijk Nederlands Meterologisch Instituut (KNMI), De Bilt, The Netherlands
Tropical cyclone is a generic term that designs a rapidly rotating storm system characterized by a low pressure center that produces strong winds and heavy rainfall. Cyclones are differently named according to the ocean basin where they develop: “Hurricanes” in the Northern Atlantic and Northeast Pacific Ocean, “Typhoons” in the Northwest Pacific Ocean, and “Cyclones” in the Indian Ocean and Southwest Pacific Ocean. Although tropical cyclones are among the most dangerous and destructive natural disasters, current models are still not able to give an accurate forecast of their intensity and track. Hence, within this context, spaceborne active microwave sensors, like the Synthetic Aperture Radar (SAR), are of paramount importance because of their all-weather and all-day capabilities together with a fine spatial resolution. In particular, SAR data can be used to directly monitor position and intensity of the storms, to analyze their structure or the rainfall-rate but also to estimate wind speed and direction. The purpose of this study is to apply the azimuth cut-off approach under extreme weather conditions.
In literature, the azimuth cut-off method is used to retrieve wind speed and several studies have been carried out to analyze the dependence of λc on sea surface parameters. In particular, there is a linear relationship between λc values and geophysical parameters, like wind speed under low-to-moderate regimes and significant wave height Recently, in [1] the ACF-based λc approach has been improved to deal with high wind speed regimes, e.g.; extreme weather conditions. The key issues that allow to extend the method to high wind regimes concern the tuning of the method with respect to pixel spacing, box size and the homogeneity of the SAR imagery. In particular, the box size is set at about 1 km × 1 km and an adaptive window size is selected for the median filter to account for the pixel spacing and first results on wind speed estimation under tropical cyclone conditions are presented.
In this study, an actual SAR dataset collected under tropical cyclones is used to discuss the soundness of this improved azimuth-cut-off method under extreme weather conditions .
[1] M. Portabella, V. Corcione, X. Yang, Z. Jelenak, P. Chang, G. Grieco, A. Mouche, F. Nunziata, W. Li, “Analysis of the SAR-derived wind signatures over extra-tropical storm conditions”, Dragon 4 Symposium, Copenhagen, Denmark, 26-30 June.
10:40 -
Sentinel-1 for hurricane monitoring
Mouche, Alexis
Mouche, Alexis (1); Chapron, bertrand (1); Zhao, Yili (1); Zhang, Biao (5); Johnsen, harald (2); Husson, romain (3); Longépé, nicolas (3); Collard, Fabrice (4) - 1: Ifremer, France; 2: Norut, Norway; 3: Collecte Localisation Satellite, France; 4: Ocean Data Lab, France; 5: NUIST, China
Remote sensing techniques appear to be the most efficient way to monitor Tropical Cyclones (TC), as these low pressure systems evolve in data-sparse oceanic regions. In particular, C-Band SAR sensors are the only active microwave sensors able to observe ocean surface night and day and through clouds at high resolution (50 m) with a wide coverage (400 km swath). During summer 2016 (and in 2017), a campaign (SHOC) dedicated to the observation of hurricanes with Sentinel-1 was performed in close collaboration with ESA Mission Planning team in the frame of the ESA Scientific Exploitation of Operational Missions program.
First, this study presents the strategy adopted to adapt the acquisition planning to TC observations, and provide an overview of the hurricane database built with Sentinel-1 observations. Sentinel-1 is the first European SAR mission able to acquire data in wide swath modes with both co- and cross- polarizations. The quality and the potential of the signal acquired in cross-polarization are thus discussed. As observed, the signal in cross-polarization is found to more than 10 times sensitive than in co-polarization. Then, a new algorithm to combine co- and cross-polarized channels for ocean surface wind measurements over extremes is proposed and validated.
This study also shows how mandatory is the multi-sensors approach for such events. In particular, the benefit of having SMAP and SMOS radiometers for algorithm development is demonstrated. Co-located over extreme events, C-band co- and cross-polarized NRCS and L-band ocean surface roughness brightness temperature (TB,rough) are directly compared to analyze the similarities and differences between these two parameters at medium resolution (about 25 km). NRCS in VH and VV polarization (σ0,VH, σ0,VV) were acquired by Sentinel-1 C-band synthetic aperture radar. TB,rough is estimated from Brightness Temperatures (TB) measured by the L-band radiometer on-board Soil Moisture Active Passive (SMAP) mission. In situation of rain rate less than 20 mm/hr, a striking linear relationship is found between active C-Band cross-polarized NRCS and passive L-Band TB,rough. Compared to both high TB,rough and σ0,VH, co-polarized σ0,VV measurements saturate. As interpreted, this can correspond to a regime change of the air-sea interactions during extreme events.
11:10 - 12:30
11:10 -
Using co-cross polarization coherence from TOPS SLC S-1 data for wind field retrieval
Longépé, Nicolas
Longépé, Nicolas (1); Husson, Romain (1); Mouche, Alexis (2); Pottier, Eric (3); Hajduch, Guillaume (1); Collard, Fabrice (4); Archer, Olivier (2) - 1: CLS, France; 2: IFREMER, France; 3: University of Rennes I, France; 4: Ocean Data Lab, France
Following the development of the scatterometer community, SAR-based wind retrieval has been originally based on a single observed quantity; the co-polarized Normalized Radar Cross Section (NRCS) in the VV channel. The use of HH polarization has been integrated later using the co-pol ratio at first. Some developments on the use of dual-pol (co- and cross-pol) wind field retrieval applied to high wind situation have been carried out lately using Sentinel-1 (see Mouche et al 2017). With S-1, the SAR community has now access to a overwhelmed amount of dual-pol SAR data over the ocean sea surface. However, the capacity for TOPS EW/IW S-1 mode to preserve polarization phase is often neglected. This talk/paper aims at studying the benefit of the dual-polarimetric information provided by SLC TOPS-mode S-1 products (including phase).
Ocean sea surface is often used to calibrate the polarimetric information via the reflection symmetry approach. The real and the imaginary parts of the co-cross polarization coherence should be equal to 0 from sea surface. However, some studies (e.g. Tsai et al 2000) have shown this condition is theoretically met at specific conditions such up or down wind situation. (Zhang et al. 2012) showed first analysis with Radarsat-2 full polarimetric data from a limited sea of wind condition and incidence angle range.
Here, we have adopted a massive approach with more than 3800 SLC IW products processed (about 27 TB). The co-cross polarization coherence (magnitude and phase) has been computed together with collocated ECMWF wind speed/direction.
In this talk/paper, we study this massive processing and provide some analysis beyond the-state-of-the-art:
This study has been partly funded by the ESA SEOM S-1 Ocean studies, and the Mission Performance Center (MPC) S-1 activities.
11:30 -
Analysisof the 2017 Atlantic Hurricane season Hurricane Irma derived from SARSentinel-1, RADARSAT-2 and TerraSAR-X data
Lehner, Susanne
Lehner, Susanne (1); Alexander, Soloviev (2); Cayla, Dean (2); Bernhardt, Rabus (3) - 1: Germen Aerospace Center, Oberpfaffenhofen, Germany; 2: Nova Southeastern University; 3: Simon Fraser University
The 2017 Atlantic hurricane season was with three major hurricanes a particular active one. The Category 4 hurricane Irma made landfall on the Florida Keys on September 10th 2017 and was imaged several times by the ESAs Sentinel-1A and B satellites as well as RADARSAT-2 in C-band, and the TerraSAR-X satellite in X-band.In the following as well images of hurricanes Jose and Maria were acquired.
Imaging the hurricanes by space radar gives the opportunity to observe the sea surface and thus measure the wind field and the sea state and sea surface motion under hurricane conditions through the clouds even in this severe weather, although rain features, which are usually not observed in SAR become visible due to damping effects.
The high resolution TerraSAR-X imagery showed sea state under the hurricane as well as the footprint of individual tornadoes on the sea surface together with their turbulent wake imaged as a dark line due to increased turbulence. The water-cloud structures of the tornadoes are analyzed and their sea surface structure is compared to optical and IR cloud imagery. An estimate of the wind field using standard XMOD algorithms is provided, although saturating under the strong rain and high wind speed conditions.
The Copernicus Sentinel-1 A and B satellites, which are operating in C-band provided several images of the sea surface under hurricane Irma, Jose and Maria. The data were acquired daily and converted into measurements of sea surface wind field u10 and significant wave height Hs over a swath width of 280km about 1000 km along the orbit using as well ESA´s SNAP toolbox..
The wind field of the 2017 hurricanes as derived from Sentinel VV or HH imagery by CMOD is as well provided by NOAA on their web server. In the hurricane cases though the wind speed saturates at 20 m/sec and is thus too low in the area of hurricane wind speed. The HV imagery now permits to retrieve hurricane wind speed above 60m/sec by simple linear algorithms.HV hurricane imagery as well shows a different eye size and is compared to cloud imagery.
As Sentinel imagery is as well provided as complex imagery, sea surface movement can be analyzed. Sea surface motion is derived from Doppler centroid measurements showing a strong westward motion in the northern part of the hurricane and an eastward one in the southern part.
A new theory on hurricane intensification based on Kelvin-Helmholtz instability is discussed and a first comparison to the SAR data is given.
11:50 -
Effects of Asymmetric Secondary Eyewall on Tropical Cyclone Evolution in Hurricane Ike (2008)
Zhang, Guosheng
Zhang, Guosheng; Perrie, Will - Bedford Institute of Oceanography, Canada
In hurricane studies, the secondary eyewall is important in tropical cyclone (TC) evolution and intensification and is routinely assumed to be axisymmetric. A unique opportunity to investigate the secondary eyewall in two-dimensions is provided by the high spatial resolution (about 1 km) sea surface winds observed by spaceborne synthetic aperture radar (SAR) over hurricane Ike (2008). Hurricane Ike contains an asymmetrical secondary eyewall which is different from classical Eyewall Replacement Cycle (ERC) theory. Here, we extract the asymmetric characteristics using our SHEW (Symmetric Hurricane Estimates for Winds) model, which is based SAR-derived wind fields capturing the eyes of hurricanes. Thus, we analyze the related hurricane evolution by comparisons of SAR imagery with SFMR (stepped frequency microwave radar) aircraft measurements. We find the following characteristics for the asymmetric secondary eyewall: the primary eyewall does not weaken as long as the secondary eyewall exists, the asymmetric secondary eyewall does not contract, and the concentric eyewalls endure for a long period, more than 30 hours. We suggest that, as observed by SAR images, this persistence results from the low wind area in the secondary eyewall supplying a pathway for the boundary layer inflow to maintain the primary eyewall intensity, and reduce the energy needed for secondary eyewall contraction.
TC intensity may change due to the oceanic, environmental and internal dynamic processes. In the axisymmetric framework typical of hurricane processes, earlier studies demonstrated that heating in the secondary eyewall induces a negative tendency for the tangential winds inside the heating annulus. This can cut off the boundary layer inflow to the primary eyewall resulting in weakening of hurricane intensity. However, as an internal process, ERC theory does not eliminate other contributions such as those due to warm oceanic water. In fact, the convective rings defined as annular regions of active convective heat release result from evaporation latent heat from the warm ocean surface. Large-scale vertical wind shear has been proposed as a driver for hurricane asymmetry and is supposed to be the most important environmental contributor for TC intensity evolution. Therefore, if the asymmetric secondary eyewall of hurricane Ike (2008) is driven by vertical wind shear, our suggestion that the boundary layer inflow pathway may act as a supplementary internal process to the ERC theory provides the connection between the large-scale atmospheric environment and TC intensity changes. Additional studies should evaluate mechanisms related to the boundary layer inflow pathways in the hurricane asymmetric secondary eyewall, which can potentially improve intensity forecasts.
References:
Zhang and Perrie, (2018): Effects of Asymmetric Secondary Eyewall on Tropical Cyclone Evolution in Hurricane Ike (2008). In press GRL.
Zhang, G., B. Zhang, Perrie….. (2014), A hurricane tangential wind profile estimation method for C-band cross-polarization SAR, IEEE TGRS, doi:10.1109/TGRS.2014.2308839.
Zhang, Perrie,…. (2017a). A Hurricane Morphology and Sea Surface Wind Vector Estimation Model Based on C-Band Cross-Polarization SAR Imagery. IEEE TGRS, 55(3), 1743-1751.
Zhang, G., Li, X., Perrie,….. (2017b). A Hurricane Wind Speed Retrieval Model for C-Band RADARSAT-2 Cross-Polarization ScanSAR Images. IEEE TGRS.
12:10 -
Signature of the Agulhas Current in Synthetic Aperture Radar derived winds
Krug, Marjolaine
Krug, Marjolaine (1,2,3); Schilperoort, Daniel (2,3); Collard, Fabrice (4); Hansen, Morten (5); Rouault, Mathieu (2,3) - 1: CSIR, South Africa; 2: University gf Cape Town; 3: Nansen-Tutu Centre for Marine Environmental Research; 4: OceanDataLab; 5: Nansen Environmental and Remote Sensing Centre Bergen
Systematic Envisat Advanced Synthetic Aperture Radar (ASAR) acquisitions over the Agulhas Current between 2007 and 2012 are used to investigate the signature on high resolution (1km) satellite-derived winds of this intense and warm current, which velocity field is provided by Globcurrent products. SAR winds show an increase in magnitude over the Agulhas Current up-, down- and cross-current wind conditions with contributions from both the ocean surface current and Sea Surface Temperature (SST). In down-current conditions, when relative wind is expected to be reduced, SAR wind speeds typically increase by 1 to 2 m/s over the Agulhas Current consistent with the effect of SST perturbation on surface wind speed. In up-current wind regimes, intense spurious increases of up to 20 m/s in the retrieved SAR wind speeds are observed at the inshore edge of the Agulhas Current. On average, SAR winds speeds at the northern Agulhas Current wall increase by 4 to 6 m/s when the winds are against the current and 1 to 2 m/s when the winds are with the current. Similar but less intense wind accelerations are observed in the Jason-2 dataset in up-current wind conditions. Using co-located ocean current, SST and significant wave height information, we argue that the strong acceleration in SAR ocean winds observed at the inshore Agulhas Current front cannot be explained only by the presence of SST gradients or localised current convergence / divergence at the front. Our analysis rather suggests that wave-current interactions in regions of strong current shear produce enhanced sea surface roughness signatures and lead to artificially high estimates of SAR-derived wind speeds. There is future potential in using high resolution SAR imagery to map strong ocean current fronts and thus improve our ability to monitor ocean surface currents from space.
12:30 - 13:00
14:00 - 15:00
14:00 -
Retrieval of sea ice deformation from Sentinel-1 and assimilation into the Lagrangian sea ice model
Korosov, Anton
Korosov, Anton; Rampal, Pierre - Nansen Environmental and Remote Sensing Center, Norway
Shipping and commercial activities in the Arctic are inevitably increasing as a results of the climate change, opening of the Northern sea route and oil exploitation. As a consequence, demand for better operational sea ice forecast in the coastal areas becomes crucial both for the climate/marine researchers and for the commercial users.
A new general circulation sea ice model neXtSIM (Rampal et al, 2016) has been developed at NERSC on the basis of the Maxwell elasto-brittle rheology framework (Dansereau et al, 2016) in order to simulate realistic sea ice conditions for regions as large as the whole Arctic and over time scales up to several years. The model equations are discretized with the finite-element method and a pure Lagrangian scheme has been implemented to preserve the discontinuities (highly localized features like cracks, leads and ridges) simulated by the model. Sea ice deformation simulated with this model spontaneously localizes along linear-like faults separating essentially undamaged ice plates/floes. The new sea ice model accounts for the local level of mechanically-induced damage of sea ice. Initialization of sea ice damage using observations would likely increase the model skill in terms of both sea ice drift and deformation forecasting. Satellite derived ice deformation can be used to estimate the local damage of the ice. The main idea is that ice damage in a model using the elasto-brittle rheology could be initialized with a higher value at locations where the sea ice cover has deformed during the previous couple of days.
Sea ice drift is derived from sequences of Sentinel-1 SAR images using the combined feature tracking and pattern matching algorithm (Korosov and Rampal, 2017) at unprecedented 5 km spatial resolution. For the first time drift is derived not on a regular grid but on a Lagrangian mesh of the neXtSIM model. Anisotropic filtering is applied to sea ice drift (Bouillon & Rampal, 2015) for suppressing small scale noise. Deformation is derived from the spatial derivatives of the sea ice drift at high spatial resolution. The temporal scale of ice deformation (duration of ice drift integration) is varied for identification of persistent deformation zones. Ice damage is calculated as a function of sea ice deformation.
The neXtSIM model is initialized from the satellite derived sea ice concentration, thickness and sea ice damage and simulates sea ice thermodynamic and kinematic processes. Improvements of the model skills for 5 days forecasts are estimated by comparison with satellite observations. Preliminary results show that accounting for sea ice damage in the model initialization may significantly improve the skill of neXtSIM not only to predict ice damage, but also to improve prediction of sea ice drift and ice floe size distribution.
14:20 -
Accuracy of a Phase-Correlation Technique for Fully Automated Sea Ice Motion Retrieval based on Sequential SAR Images
Frost, Anja
Frost, Anja; Wiehle, Stefan; Jacobsen, Sven - DLR, Maritime Safety and Security Lab Bremen, Germany
In order to improve ship routing in polar waters, we present a software processor to retrieve high resolution sea ice motion fields from spaceborne Synthetic Aperture Radar (SAR) image sequences fully automatically.
Sea ice is almost continually in motion. Within hours, wind and ocean currents can cause significant changes within the sea ice. When the ice is pulled apart by winds or currents from opposite directions, the ice fractures, and open water leads appear. When ice is strongly pushed together by converging wind and currents, the ice sheet will break and either pile up randomly one piece over another, forming a thick, uneven surface, or be forced upwards, creating high walls called ridges. Such obstacles are difficult or impossible even for icebreakers to overcome.
SAR satellites such as TerraSAR-X or Sentinel-1 are well suitable to map different structures in the sea ice. Due to their near-polar orbit, spatially and temporally near coincident acquisitions in high latitudes are possible on a daily basis.
The core of the presented software processor for sea ice motion retrieval is the well-known phase correlation technique, executed within a hierarchical motion estimation framework presented in our previous work. The output of the processor is a vector field indicating the sea ice displacement, which can be converted into sea ice velocity. Now, we investigate the accuracy of the retrieved displacement.
Our test deals with a series of TerraSAR-X ScanSAR mode images acquired over drift buoys that are located in arctic waters, as well as with collocated Sentinel-1 acquisitions for comparison. We monitored the buoys during July 2017 and January 2018. In the winter sequences, an ice concentration of >90 % is predominant, while the summer acquisitions capture an ice concentration of 50 % - 80 %. Altogether, the accuracy of motion vectors estimated from TerraSAR-X image pairs amounts to 30 m (1σ-error). The motion field has a resolution of 150 m x 150 m, which gives a very detailed look into the local sea ice motion, detecting small variations.
The presented processor is intended to be part of the operational data processing chain at DLR Ground Station Network sites. In ongoing work, we implement parallel processing in order to reduce computing time so vessels in ice infested waters can receive information on local sea ice motion in near real-time.
14:40 -
Assessing Deformation, Drift, and Topography of Sea Ice Using InSAR to Support Use and Management of Coastal Ice
Aldenhoff, Wiebke
Aldenhoff, Wiebke (1); Dammann, Dyre Oliver (1); Eriksson, Leif (1); Eicken, Hajo (2); Mahoney, Andrew (3); Meyer, Franz (3) - 1: Chalmers University of Technology, Sweden; 2: International Arctic Research Center, University of Alaska Fairbanks, Alaska; 3: Geophysical Institute, University of Alaska Fairbanks, Alaska
Arctic sea ice provides important services to people, including coastal communities and industry, as well as marine wildlife. In many regions of the Arctic, operations on or near sea ice are increasingly limited by a shorter ice season, reduced ice extent, more prolific ice hazards, reduced stability of shorefast ice, and rougher, less trafficable ice surface conditions. We are exploring the use of Synthetic Aperture Radar Interferometry (InSAR) as a viable tool to address the recognized need for cost effective approaches to provide stakeholder-relevant data on key constraints for sea ice use, in particular pack ice drift, landfast ice stability and morphology.
Depending on the temporal baseline between satellite passes, InSAR has the capability to quantify sea ice drift as well as detect small-scale landfast ice displacements, which are linked to important coastal hazards, including the formation of cracks, ungrounding of ice pressure ridges, catastrophic breakout and complete removal of landfast ice. While InSAR has shown promise in a few key studies, it has yet to be thoroughly validated and its potential remains largely underutilized in sea ice science.
Results from recent studies demonstrate the potential use of InSAR for providing key information to stakeholders operating on or near sea ice:
There is great utility and potential demand for SAR products to support stakeholders near sea ice and the recent and expected increase in SAR satellites will likely increase the potential use of these techniques in operational settings.
15:30 - 16:30
15:30 -
Ice/water classification using dual-polarization C- and L-band synthetic aperture radar images over Fram Strait
Aldenhoff, Wiebke
Aldenhoff, Wiebke (1); Heuzé, Céline (2); Eriksson, Leif E. B. (1) - 1: Chalmers University of Technology, Sweden; 2: University of Gothenburg, Sweden
Accurate high resolution and near-real time determination of sea ice conditions is necessary for safe navigation in sea ice infested waters and refinements of climate models, but it has not been achieved to date. In particular, distinguishing ice from open water in low sea ice concentration areas is challenging and the result may depend on the frequency band of the sensor used for the retrieval. Here an algorithm for ice/water classification of C- and L-band dual polarization synthetic aperture radar (SAR) data is presented. The algorithm is based on backscatter intensities in co- and cross-polarization and autocorrelation as a texture feature and separates sea ice, open water and thin ice/calm water. The mapping between image features and ice/water classification is made with a neural network. The area of interest is Fram Strait located between Greenland and Svalbard, where most of the ice is exported from the Arctic.
A comparison of the two different frequencies is made in order to investigate the potential to improve classification results and to increase the temporal resolution of highly dynamic areas with multi frequency data.
Accurate ice/water maps for both frequencies with a resolution of about 200 m are produced by the algorithm and the results of the two frequencies agree in 86% of the cases. The major deviation is introduced by the different sensitivity for thin ice/calm water, which has been allocated a separate class. Thin ice is discriminated more easily in L-band due to its longer penetration depth and reduced influence of small scale surface roughness. Differences are also pronounced in the marginal ice zone, where sea ice motion occur between the acquisitions of the different frequencies. Furthermore a larger contrast between open water and sea ice in the cross-polarization channel of C-band facilitates discrimination compared to L-band.
C-band reliably reproduces the outline of the ice edge, while L-band has its strengths in detecting thin ice/calm water areas within the ice pack.
The classification results show good agreement with lower resolution ice/water maps derived from met.no ice-charts and radiometer data from AMSR-2. Variations are found in the marginal ice zone where generalization of the ice charts and lower accuracy of ice concentration from radiometer data introduce deviations. Differences are also found within the ice pack where the presented algorithm discriminates areas of thin ice while these features might be omitted in the creation of ice/water maps by thresholding.
Usage of dual frequency SAR data could thus be beneficial for improving spatial and temporal resolution of ice cover information for ship navigation and modelling of ice extent and ocean-atmosphere interactions.
15:50 -
Estimation of First-Year Sea Ice Melt Pond Fraction from Compact Polarization SAR
Li, Haiyan
Li, Haiyan (1,2); Perrie, Will (1) - 1: Bedford Institute of Oceanography, Canada; 2: Key Laboratory of Computational Geodynamics, Chinese Academy of Sciences / Earth College, University of Chinese Academy of Sciences, Beijing, China
Melt ponds are a common feature on Arctic sea ice. They are linked to the sea ice surface albedo and transmittance of energy to the ocean from the atmosphere and thus constitute an important process to parameterize in Arctic climate models and simulations. This paper presents a first attempt to retrieve the melt pond fraction from compact polarization (CP) SAR imagery, which has wider swath and shorter revisit time than the quad-polarization systems, e.g. RADARSAT-2. The co-polarization (co-pol) ratio has been verified to provide estimates of melt pond fractions. However, it is a challenge to link CP parameters and the co-pol ratio. The theoretical possibility of making this linkage with the CP parameter C22/C11(the ratio between the elements of the covariance matrix of CP SAR) for melt pond detection and monitoring is presented with the tilted-Bragg scattering model for the ocean surface [Valenzuela, 1978]. Our empirical transformed formulation denoted the ‘compact polarization and quad-pol’ (‘CPQD’) model, is proposed. Our methodology is based on the existing VV/HH melt pond fraction retrieval method by Scharien et al. [2014]. The tilted-Bragg scattering model suggests the theoretical potential of C22/C11 from CP mode data for melt pond observations. Thus, the linkage between C22/C11,from CP mode data, and VV/HH, from quad-pol data, can be established with our collocated number of quad-pol observations of open ocean water. We used two data sets. Our CPQD is firstly based on 2062 RS-2 quad-pol SAR images, collocated with in situ measurements. Secondly, we compared the retrieved melt pond fraction with CP parameters simulated from quad-pol SAR data with results retrieved from the co-pol ratio from quad-pol SAR observations acquired during the Arctic-Ice Covered Ecosystem in a Rapidly Changing Environment (Arctic-Ice) field project conducted from May to June 2012. The results are shown to be comparable for observed melt pond measurements in spatial and temporal distributions. Thus, the utility of CP mode SAR for melt pond fraction estimation on first year level ice is established.
References:
Li, H., Perrie, W., Li, Q., & Hou, Y. (2017) Estimation of melt pond fractions on first year sea ice using compact polarization SAR. Journal of Geophysical Research: Oceans, 122, 8145–8166. https://doi.org/10.1002/2017JC013248
Scharien, R. K., Landy, J., and Barber, D. G. (2014) First-year sea ice melt pond fraction estimation from dual polarisation C-band SAR – Part 1: In situ observations, The Cryosphere, 8, 2147–2162, doi:10.5194/tc-8-2147-2014.
Valenzuela,G. R. (1978) Theories for the interaction of electromagnetic and ocean waves- A review, Boundary-Layer Meteorol., vol. 13, no. 1–4, pp. 61–85
10:50 - 11:10
15:00 - 15:30
13:00 - 14:00
09:00 - 10:40
09:00 -
A new Approach to Ocean Wave Measurements by SAR, Using Spotlight-Mode Data and Multi-Subaperture Image Analysis
Romeiser, Roland
Romeiser, Roland; Graber, Hans Christian - University of Miami RSMAS, United States of America
The analysis of combinations of subaperture SAR images to resolve the propagation direction ambiguity of ocean wave signatures has been discussed in the literature since the early 1990s. We will demonstrate in this presentation how spotlight-mode products of contemporary satellite SAR systems such as TerraSAR-X and COSMO-SkyMed enable us to take this concept to a new level. The long SAR integration times and small pixel sizes of these products make it possible to generate a short "movie" of subaperture images, in which wave motions become clearly visible even to the naked eye. By applying a filter based on the theoretical dispersion relation of ocean waves, we can then separate ocean wave signatures from other signatures to obtain a very clean, virtually noise-free picture of the moving ocean wave signatures before inverting them into surface slope and elevation fields. This does not only make the inversion easier, but it also eliminates the usual need to average image power spectra over several subwindows for noise reduction. As a result, the phases of the wave signatures can be taken into account in the inversion process, such that the retrieved wave field can be expected to be correct in its spatio-temporal representation (showing wave crests and troughs and their evolution during the SAR integration time at the correct locations), not just in the spectral representation that would be obtained from conventional wave retrieval techniques for SAR images. We will present several examples of TerraSAR-X and COSMO-SkyMed images and the retrieved wave fields and discuss the quality of the results and the capabilities and limitations of the proposed technique. In addition to the use of subaperture images and the dispersion relation filter, a novel element of our wave retrieval algorithm is the processing of logarithmically-scaled intensity images (dB images) instead of linear images. This approach can be demonstrated to reduce contributions of higher harmonics in the Fourier analysis of the images, and it is expected to improve the inversion to surface slope and elevation fields as well, although this is still a matter of ongoing tests at the time of writing of this abstract.
09:20 -
A Multiscale Approach Analysis of Polarimetric SAR Images to study Multimodal Ocean Wave Systems
Lorenzzetti, Joao Antonio
Lorenzzetti, Joao Antonio; Ortiz, Gustavo Prouvot - National Institute for Space Research / INPE, Brazil
The capability of estimating ocean wave spectrum and some of its statistics using synthetic SAR images has been demonstrated since the 1970s. Later algorithm developments using polarimetric SAR (PolSAR) data have indicated that fairly good estimation of some ocean wave parameters is possible without the need of a first guess spectrum, normally obtained from numerical wave models, and of complex computational iterative schemes needed to converge to a best solution. More recently, a C-band PolSAR ocean wave algorithm was presented [1] and validated for Radarsat-2 (RSAT-2) polarimetric imagery [2]. This algorithm estimates the long-wave slopes (in range and azimuth directions) using a linear polarization orientation modulation transfer function (MTF) developed by [3] in addition to the tilt, hydrodynamics and velocity bunching MTFs. Despite the relative good results obtained, the authors [2] acknowledged that further research was needed to determine the extent to which the algorithm is capable of handling a more general wind‐driven wave spectrum, particularly for cases of multimodal wave systems.
Motivated by these initial results and with a goal of advancing the polarimetric algorithm for general wave systems, we propose a modification of the original algorithm which was tested using polarimetric Radarsat-2 imagery. To separate the wave systems in multimodal cases and evaluate their individual statistics, the new approach uses a multiscale processing scheme together with the Mean Square Slope statistics theory of ocean waves, which has been originally developed from optical and radar altimetry sensors.
The first results obtained for a multimodal wave system present off Southeastern Brazil on August 12th, 2016, and using a C-Band full polarimetric Radarsat-2 image in Wide Fine Quad-Pol beam mode (Single Look Complex product) show a reasonable agreement when compared to data from a nearby meteocean wave buoy.
More C-band PolSAR imagery together with well collocated wave buoys data is required to better evaluate under several environmental conditions and to validate the methodology. We reckon that this new approach, being a fast and independent satellite algorithm to assess multimodal ocean wave systems, could contribute to several coastal/oceanic engineering applications and monitoring systems.
References:
[1] He, Y.; Shen, H.; Perrie, W. Remote sensing of ocean waves by polarimetric SAR. Journal of Atmospheric and Oceanic Technology, v. 23, n. 12, p. 1768–1773, 2006.
[2] Zhang, B.; Perrie, W.; He, Y. Validation of RADARSAT-2 fully polarimetric SAR measurements of ocean surface waves. Journal of Geophysical Research: Oceans, v.115, n. 6, p.1–11, 2010.
[3] He, Y. et al. Ocean wave spectra from a linear polarimetric SAR. IEEE Transactions on Geoscience and Remote Sensing, v. 42, n. 11, p. 2623–2631, 2004.
09:40 -
COASTAL SWELL MEASUREMENTS FROM SENTINEL-1 TOPS MODE
Husson, Romain
Husson, Romain (1); Johnsen, Harald (2); Collard, Fabrice (3); Guitton, Gilles (3) - 1: CLS, France; 2: NORUT, Norway; 3: OCEANDATALAB, France
Since the launch of Sentinel-1A and -1B missions in April 2014 and April 2016, respectively, SAR images are acquired massively and increasing in worldwide coastal regions using the new TOPS (Terrain Observation by Progressive Scans) acquisition mode. Such wide swath SAR images are a crucial source of information for the observation of wave in coastal areas, where complex interactions with currents occur. Besides, these wide swath images are the only acquisition mode over European seas and the Eastern Atlantic Ocean. However, there is currently no swell spectra product available based on this acquisition mode.
TOPSAR mode is intended to replace the conventional ScanSAR mode, used before with ENVISAT/ASAR, as it achieves the same coverage and resolution as ScanSAR, but with a uniform SNR (Signal-to-Noise Ratio). In TOPS mode, the ocean surface is sampled via successive bursts during which the antenna is electronically steered in the azimuth direction. This reduces the dwell time which directly impacts the capability to extract the swell spectra: as classical wave-spectra estimation methods using SAR SLC images are based on cross-spectra estimation, reducing the time separation between cross-looks hampers the ability to remove the ambiguity on the swell propagation direction.
This paper presents an alternative technique, taking advantage of the TOPS mode scanning and exploiting two complementary approaches based on the spectral information available inside each burst and in the overlapping region between two successive bursts.
For each subswath, the overlapping areas located at burst’s edges spans over only a few kilometers in azimuth and the look separation time reaches ~2s which enables resolving the propagation direction ambiguity of the swell spectrum even over a small area (to be compared with the 0.4s of S1 Wave mode, 20x20km). The possibility to estimate the swell phase velocity is demonstrated and compares well with wave dispersion relation for noise-free images. However, the swell spectra energy is noisier than when using the overall burst area due to the small overlapping area. As a complementary information, the swell spectral energy is estimated over the overall coverage of each burst and the previously removed ambiguity information is used by continuity of the swell field over the SAR image. The impact of proposing a modified Level-0 to Level-1 processing with wider inter-burst coverage is also presented.
This study has been partly funded by the ESA SEOM Ocean studies
10:00 -
SAR Image Cross-spectral Analysis of Intermediate Waves: Directional Properties and Potential in Deriving Surface Velocity
Li, Huimin
Li, Huimin; Chapron, Bertrand; Mouche, Alexis; Nouguier, Fredéric; Stopa, Justin - LOPS,Ifremer, France
The SAR image cross-spectrum between a pair of sub-looks separated in time has been widely used to help remove 180° ambiguity of detected ocean swell systems. Ocean swells move along the waves' propagation direction during this offset time of the order of SAR integration time, therefore, co- and cross-spectra can help to consistently estimate velocity characteristics of the randomly moving sea surface scatterers related to scales larger than SAR spatial resolution. The intermediate radial waves longer than SAR spatial resolution but shorter than ocean swells are, to first order, driven by surface winds. Thus, a new parameter MACS, averaged complex cross-spectra over intermediate radial waves domain, is proposed to manifest impacts of wind speed and direction on these moving scatterers.
Sentinel-1 wave mode operates in novel ‘leap frog’ acquisitionmode. A vignette is acquired every 100 km at two alternate incidence angles (23° and 36.5° respectively ), withtwo images at the same incidence 200km apart. Large data sets have been constituted systematically by co-locating Sentinel-1A/Bwave mode acquisitions and ECMWF forecast winds. Efficiently, distinctive features are revealedfor two incidence angles as well as dual-polarizations.
Phase of MACS consistently show temporal evolution of sea surface scatterers under various wind speeds. This relationship is distinct from incidence angle of 23° to that of 36.5°. In specific, variations of phase at 23° agrees well with the present nonlinear SAR imaging mechanism, being a approximate constant relative to wind speed. On the contrary, phase of 36.5° shows increasing trend with wind speed with different slopes at upwind and downwind. This may lead to a revisit on applicability of the present SAR imaging mechanism in reproducing the phase shift at incidence angle of 36.5°.
More encouragingly, imaginary component of MACS, to first order, show a linear variation relative to wind speed. It is a signed quantity with negative at upwind and positive at downwind with greater values in HH polarization than in VV polarization for given wind speed and incidence angle. This characteristics is similar to that of Doppler Centroid Anomaly (DCA). As such, preliminary investigation is carried out, demonstrating its potential in capturing upper ocean surface current.
10:20 -
Copernicus Level-3 Wave Products Based On Sentinel-1 Spectral Wave Measurements
Husson, Romain
Husson, Romain (1); Mouche, Alexis (2); Wang, He (3); Wang, Xuan (4); Collard, Fabrice (5); Aouf, Lotfi (6); Stopa, Justin (2); Charles, Elodie (1) - 1: CLS, France; 2: IFREMER, France; 3: NOTC. Chine; 4: OUC, Chine; 5: OCEANDATALAB, France; 6: METEO-FRANCE, France
Sentinel-1A and -1B Level-2 products acquired in a specific Wave Mode are acquired routinely worldwide over open ocean basins. They contain partitioned directional swell spectra and integral swell parameters for each partition, i.e. significant wave height (SWH), peak period and peak direction. Based on these L2 SAR observations, Level-3 products are estimated and are delivered in the Copernicus Marine Environment Monitoring Services since 2018. These products, delivered on a daily basis, describe the swell integral parameters along its propagation in deep waters, from their storm source until the land away from islands and in the absence of currents. They are often referred to as “fireworks” due to their visual aspect when represented on a world map: they evidence swell generated in localized regions coinciding with strong storms, mostly corresponding to extra-tropical events, from which they propagate over thousands of kilometers across entire ocean basins.
First and using great circle theory, Level-2 observations with SWH larger than 30cm and peak wavelength larger than 200m are back-propagated in space and time. Converging swell observations, which belong to the same storm, are then gathered in swell field groups. This first step ensures the overall consistency of the selected converging observations in terms of peak direction and peak wavelength. Once this generation source is known, the significant height decay can be estimated on top of the swell propagation path. For each swell field group of propagated observations, the spatial structure of the 3 integral parameters can be analyzed along propagation to further filter out swell observations with SWH values appearing as outliers. After applying these successive filters based on the consistency of each Level-2 integral parameter with the overall swell field distribution described in the Level-3, about 18% of all the available Sentinel-1 Level-2 partitions remain.
The accuracy of the Level-3 propagated integral parameters is estimated over a 4 month dataset in 2016 co-located with numerical wave model outputs and quality controlled in situ measurements. The accuracy of the corresponding Level-2 is also estimated and compared with that of the rejected partitions, showing a great improvement of the integral parameter estimation for the selected observations with respect to the rejected ones.
Application of the Level-3 products are various, from assimilation in numerical wave models to study of swell decaying processes.
11:10 - 11:50
11:10 -
Measuring ocean waves in sea ice using SAR imagery: A quasi-deterministic approach evaluated with Sentinel-1 and in situ data
Ardhuin, Fabrice
Ardhuin, Fabrice (1); Stopa, Justin (1); Chapron, Bertrand (1); Mouche, Alexis (1); Collard, Fabrice (2) - 1: LOPS, France; 2: OceanDataLab, France
Measurements of wave heights in marginal ice zones are limited to very few in situ data. Here we revisit the linear and quasilinear theories of Synthetic Aperture Radar imaging of waves in the particular case of waves in sea ice. Instead of only working with spectra, we have developed an iterative nonlinear algorithm to estimate phase-resolved deterministic maps of wave-induced orbital velocities, from which elevation spectra can be derived. Application of this algorithm to Sentinel 1A wave mode images in the Southern Ocean shows that it produces reasonable results for swells in all directions except when they propagate at a few degrees off the range direction. The estimate of wave parameters is expected to work best when the shortest wave components, those which cause a pixel displacement of the order of the dominant wavelength in azimuth, can be neglected. Otherwise short waves produce a blurring of the image, increasing exponentially with the azimuthal wavenumber and reducing the estimated wave amplitude. Given the expected spatial attenuation of waves in ice-covered regions, our deterministic method should apply beyond a few tens of kilometers in the ice, without any correction for short wave effects. In situ data collected around the ice edge as part of the 2015 SeaState DRI cruise in the Beaufort confirm the progressive image blurring caused by such short waves, and the apparent reduction in the wave modulation. When short waves propagate from the open ocean towards the ice, this blurring can produce an unrealistic apparent increase of wave height, from the open ocean up to a few tens of kilometers inside the ice. The method is tested on two Interferometric Wide swath (IW) mode images from Sentinel 1A, and was applied to thousands of wave mode images from S1A and S1B. Automatic application requires a careful screening for ice features that could otherwise be interpreted as wave energy.
11:30 -
Sea state in the marginal ice zone Observed by Sentinel-1 and TerraSAR-X Imagery, Results from the Sikuliaq Campiagn
Lehner, Susanne
Lehner, Susanne (1); Gemmrich, Johannes (2) - 1: Germen Aerospace Center, Germany; 2: University of Victoria
In the context of a changing sea ice and wave climate, the build-up of ocean waves in ice-free parts of the Arctic seas and their impact on both the coastline and marginal ice zone gains growing attention. First indications for the possibility of wave exposure having an increasing tendency over the last few decades have emerged (Thomson et al.(2016)). Related sea state trends are governed by different factors including the timing and amount of ice retreat in summertime and the frequency and strength of wind forcing. In general, those trends are based on satellite climatology and model runs. Current efforts are undertaken to improve the representation of ice in sea state models, among others by means of observations, e.g. Zieger et al.(2015).
In recent years, several strong swell wave events in Arctic waters were recorded by in-situ measurements during ship campaigns (Collins et al.(2015)). Consistency checks with models were performed to interpret observations in Ardhuin et al.(2016). Moreover, case studies on swell wave events in the MIZ were conducted using SAR imagery from satellites (Schulz-Stellenfleth and Lehner(2002), Ardhuin et al.(2015), Gebhardt et al.(2016)).
During the ONR Sea State DRI Project a campaign was performed by the research vessel Sikuliaq and the interaction of sea state and sea ice in the MIZ was investigated. Several wave events including swell penetrating into the sea ice as well as wave build up in off ice wind fetch conditions were measured and at the same time imaged by Sentinel-1 SAR imagery .For the cruise report see Thomson(2015). Further information on the cruise support by TS-X satellite images can be found in Lehner and Gemmrich(2016).
Different sources of wavelength variability, both ice and non-ice related, were identified and the results were cross-checked with a model hindcast from ECMWF (European Centre for Medium-Range Weather Forecasts).
Thomson, J., Fan, Y., Stammerjohn, S., Stopa, J., Rogers, W. E., Girard-Ardhuin, F., Ardhuin, F., Shen, H., Perrie, W., Shen, H., Ackley, S., Babanin, A., Liu, Q., Guest, P., Maksym, T., Wadhams, P., Fairall, C., Persson, O., Doble, M., Graber, H., Lund, B., Squire, V., Gemmrich, J., Lehner, S., Holt, B., Meylan, M., Brozena, J., and Bidlot, J.-R.: Emerging trends in the sea state of the Beaufort and Chukchi seas, Ocean Modelling, 105, 1-12, http://dx.doi.org/10.1016/j.ocemod.2016.02.009, 2016.
Zieger, S., Babanin, A. V., Rogers, W. E., and Young, I. R.: Observation-based source terms in the third-generation wave model WAVEWATCH, Ocean Modelling, 96, 2-25, http://dx.doi.org/10.1016/j.ocemod.2015.07.014, 2015.
Pleskachevsky, A., Rosenthal, W., and Lehner, S.: Meteo-marine parameters for highly variable environment in coastal regions from satellite radar images, ISPRS Journal of Photogrammetry and Remote Sensing, http://dx.doi.org/10.1016/j.isprsjprs.2016.02.001, 2016
Gebhardt, C., Bidlot, J.-R., Gemmrich, J., Pleskachevsky. A., Lehner., S., and Rosenthal., W.: Wave observation in the marginal ice zone with the TerraSAR-X satellite, Ocean Dynamics, 66, 839-852, doi:10.1007/s10236-016-0957-8, 2016
Gemmrich, J., Pleskachevsky, A., Lehner, S., and Rogers, E.: Surface waves in arctic seas, observed from TerraSAR-X, 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), Milan, 2015, pp. 3615-3617, doi: 10.1109/IGARSS.2015.7326604, IEEE, 2015
11:50 - 12:20
13:20 - 14:20
13:20 -
Capabilities of the Chinese GaoFen-3 Synthetic Aperture Radar for Ocean Observation
Li, Xiao-Ming
Li, Xiao-Ming - Institute of Remote Sensing and Digital Earth, CAS, China, People's Republic of
The GaoFen-3 (GF-3) is the first Chinese spaceborne SAR in C-band for civil applications, particularly for ocean, coast and polar observations. In the paper, we provide an overview on capabilities of the GF-3 SAR for ocean and coastal observations, particularly focusing on some representative applications, such as polarimetric feature of typical tidal flat in Subei shoal, observation of offshore wind turbine wake in the North Sea and East China Sea, observation of internal waves generating in the Luzon Strait and their propagation to DongSha Atoll. Derivation of marine-meteo parameters in high spatial resolution is the one of the most attractive applications of spaceborne SAR for ocean observation, such as sea surface wind, wave and current. This is a primary purpose of launching the GF-3 SAR, we also presented the current problem of using GF-3 for retrieval of oceanic dynamic parameters and some possible solutions.
13:40 -
Ocean Winds and Waves from Chinese SAR Mode Sensors
Yang, Jingsong
Yang, Jingsong; Ren, Lin; Wang, Juan; Chen, Peng; Zheng, Gang - Second Institute of Oceanography, SOA, China
GF-3 (GF stands for GaoFen, which means High Resolution in Chinese) is the China's first C band multi-polarization high resolution microwave remote sensing satellite. It was successfully launched on Aug. 10, 2016 in Taiyuan satellite launch center. The synthetic aperture radar (SAR) on board GF-3 works at incidence angles ranging from 20 to 50 degree with several polarization modes including single-polarization, dual-polarization and quad-polarization. GF-3 SAR is also the world’s most imaging modes SAR satellite, with 12 imaging modes consisting of some traditional ones like stripmap and scanSAR modes and some new ones like spotlight, wave and global modes. GF-3 SAR is thus a multi-functional satellite for both land and ocean observation by switching the different imaging modes.
TG-2 (TG stands for TianGong, which means Heavenly Palace in Chinese) is a Chinese space laboratory which was launched on 15 Sep. 2016 from Jiuquan Satellite Launch Centre aboard a Long March 2F rocket. The onboard Interferometric Imaging Radar Altimeter (InIRA) is a new generation radar altimeter developed by China and also the first on orbit wide swath imaging radar altimeter, which integrates interferometry, synthetic aperture, and height tracking techniques at small incidence angles and a swath of 30 km. The InIRA was switch on to acquire data during this mission on 22 September.
This paper gives some preliminary results for the quantitative remote sensing of ocean winds and waves from the GF-3 SAR and the TG-2 InIRA. Comparisons to the buoy and reanalysis data show good agreements but more valuable details. The quantitative analysis and ocean wave spectra retrieval have been given from the SAR imagery. The image spectra which contain ocean wave information are first estimated from image’s modulation using fast Fourier transform. Then, the wave spectra are retrieved from image spectra based on Hasselmann’s classical quasi-linear SAR-ocean wave mapping model and the estimation of three modulation transfer functions (MTFs) including tilt, hydrodynamic and velocity bunching modulation. The wind speed is retrieved from InIRA data using a Ku-band low incidence backscatter model (KuLMOD), which relates the backscattering coefficients to the wind speeds and incidence angles. The KuLMOD model was developed based on Ku-band low incidence angle Tropical Rainfall Mapping Mission (TRMM) precipitation radar (PR) data. The ocean wave spectra are retrieved linearly from image spectra which extracted first from InIRA data, using a similar procedure for GF-3 SAR data.
14:00 -
The Sea surface Kinematics Multiscale monitoring mission (SKIM): a pathfinder radar mission for mapping waves and currents
Ardhuin, Fabrice
Ardhuin, Fabrice (1); Chapron, Bertrand (1); Collard, Fabrice (2) - 1: IFREMER, France; 2: Ocean Datalab, France
The Sea surface Kinematics Multiscale monitoring mission (SKIM) is a low incidence Ka-band Doppler radar to measure surface currents,
ice drift and ocean waves at spatial resolution of about 40km, with snapshots at least every day
for latitudes 75 to 82, and every few days otherwise. The use of
incidence angles at 6 and 12 degrees allows a measurement of the
directional wave spectrum which yields accurate corrections of
the wave-induced bias in the current measurements. The instrument principle, algorithm for current
velocity and mission performance are presented
here. The proposed unfocused rotating SAR instrument can reveal features on tropical ocean and marginal ice zone dynamics that are inaccessible to existing
remote sensing measurements, as well as a global monitoring of the ocean
mesoscale that surpasses the capability of today's nadir
altimeters. Measuring ocean wave properties facilitates many applications, from wave-current interactions and air-sea fluxes to the transport and convergence of marine plastic debris and assessment of marine and coastal hazards. This mission can ideally complement measurements of sea surface
height (nadir altimeters and SWOT) and surface density (combining
temperature and salinity) for a better understanding of upper ocean
dynamics.
14:20 - 15:30
14:20 -
Summaries and Roadmap
Engdahl, Marcus - ESA-ESRIN, Italy
10:50 - 11:10
12:20 - 13:20