GPOD SENTINEL-3 SARvatore Software Prototype User Manual
Introduction
The G-POD Service, SARvatore (SAR Versatile Altimetric Toolkit for Ocean Research & Exploitation) for Sentinel-3, is a web platform that provides the capability to process on line and on demand Sentinel-3 SRAL L1A data products in SAR mode until Level-2 geophysical data products.
The service is based on Sentinel-3 SARvatore Processor Prototype, heritage of the CryoSat-2 SARvatore Processor Prototype.
The data products in input to the service are the Sentinel-3 SRAL L1A data products. The manifest of these products are regurarly ingested in the G-POD storage facility so that they are made available in the G-POD data catalogue in order to be processed on line.
The Sentinel-3 SAR Processor has two stages: L1b & L2.
The Processors will make use of the G-POD (Grid-Processing On Demand) distributed computing platform to deliver timely the output data products.
These output data products are generated in standard NetCDF format (using CF Convention), and they are compatible with BRAT (Basic Radar Altimetry Toolbox) and other NetCDF tools (for instrunctions on how to open them with BRAT, see the dedicated section within this wiki).
Using the G-POD graphic interface, it is possible to easily select the geographical area of interest along with the time of interest. After the task submission, it is possible to follow, in real time, the status of the processing task.
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You find more information about how to use the generic G-POD graphic interface at the G-POD wiki page available here.
The on line SAR service is available here.
List of option entries in the G-POD graphic interface
The processor prototype is versatile in the sense that the users can customize and adapt the processing, according their specific requirements, setting a list of configurable options.
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L1b Processing Options:
Option Name | Option Value | Option Description |
Data Posting Rate 20Hz/80Hz |
- 20 Hz - 80 Hz |
User can decide here the posting rate (20 Hz or 80 Hz) of the L2 data published in output |
Hamming weighting Window |
- Apply only in coastal zone
- Yes, apply it
- No. do not apply it
|
User can decide here whether to apply a Hamming weighting window on the SAR burst data, do not apply it, or to apply it only for surface location in coastal area (more info at REF1) |
Exact Beam-Forming |
- Approximated
- Exact
|
User can decide here whether to operate an exact Beam-Forming or an approximated Beam-Forming (more info at REF1) |
FFT Zero-Padding |
- Yes, apply Zero-Padding - No, don't apply Zero- Padding |
User can decide here whether to operate the Zero-Padding prior to the range FFT (section 4.8 in REF1). Zero-Padding is indicated for coastal zone and sea-ice analysis |
Radar Receiving Window Size |
- 128 range bins (No Extension) - 128X2 range bins (Extension by factor of 2) - 128X3 range bins (Extension by factor of 3) - 128X4 range bins (Extension by factor of 4) |
User can select here the size of the radar receiving window: 128 range bins (standard) or 128XN range bins (extended N times). Extended window with N=2 is indicated for coastal zone and sea ice analysis. N>2 may be indicated only for inland water over very steep topographic regions. |
Antenna Pattern Compensation |
- NO -YES |
User can decide here whether to apply the Antenna Pattern Compensation on Stack Data prior to make the multilooking |
Dump SAR Stack Data in output |
-NO -YES, with only power -YES, with power and phase |
User can command here to post in output SARStack Data Products for each selected pass. Users can select whether to dump just the power of the Stack data or the power and phase. Beware: SAR Stack Data Products are quite bulky (usually 2 GB for 10 minute of data); we recommend to process SAR Stack Data singuarly or max 5 passes at the time: contact G-POD team for massive production of SAR Stack Data |
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L2 Processing Options:
Option Name | Option Value | Option Description |
Restrict the re-tracking on specific surfaces |
- Process all
- Process only open sea points
- Process only water
points
|
User can decide here whether to process the whole pass, only points in open sea or only water points (points in open sea, costal zone and inland water) |
PTR width alphap parameter |
- LUT
- Constant
|
User can decide here whether to use a LUT (Look-Up Table) or a constant for PTR (Point Target Response) alphap parameter |
SAMOSA Model Generation |
- Use SAMOSA2 - Use SAMOSA3 - Use SAMOSA+ |
User can decide here which SAMOSA generation model to use in the processing (SAMOSA2, SAMOSA3, SAMOSA+).
The SAMOSA 3 is a truncated version of SAMOSA 2 (only zero-order term); more info at REF2. SAMOSA+ is a enhancement of the SAMOSA2 solution providing better performances over inland water, coastal zones and sea ice; more info at REF3.
|
Dump RIP in output |
-NO -YES |
Flag to append Range Integrated Power (RIP) in the output netCDF data product |
Dump SAR Echo Waveforms in Output |
- NO -YES |
User can command here to attach in the output product the SAR Echo Waveforms |
Single-Look or Multi-Look Model |
- Multi-look - Single-look |
User can decide here to retrack the waveform with the Multi-look waveform model or the single-look waveform model. Single-look waveform model is an acceptable approximation over inland water or sea-ice scenario. The Single-Look option is quicker than Multi-Look option. |
Choose the Default Tide Model |
-FES2014b -TPXO8 -TPXO9 |
User can decide here which tide model to use as default in the product between FES2014b, TPXO8-ATLAS, TPXO9-ATLAS |
Choose the Default Mean Sea Surface Model |
-DTU18 -DTU15 -CLS-CNES15 |
User can decide here which mean sea surface model to use as default in the product between DTU18, DYU15, CLS-CNES15 |
Adopted Conventions
- The Reference Time for the TAI Datation is 01/01/2000 00:00:00
- The Vertical Datum for altitude reference is the WGS84 Ellipsoid
- The Curve Best-Fitting Scheme, used in the re-tracking stage, is a
Bounded Levenberg-Marquardt Least-Squares Estimation Algorithm (LEVMAR-LSE).
- The SAR Power Return Waveform Model, used in the re-tracking stage, is the SAMOSA Model (REF2).
- Static bias has been applied to the range, sigma nought and antenna
mispointing measurements. The values of the static biases are reported
in the output NetCDF data products.
- The product orbit latency/source is the one coming from the SAR L1A data file used in input.
- The orbital altitude has been corrected for a Time Tag Bias. The value of the applied time tag bias is reported in the output NetCDF data products.
- The measurements are posted both at rate of 20 Hz and of 1 Hz.
- The sea state bias has not been applied to the sea level anomaly measurements.
- No a priori data editing has been applied to the 20 Hz measurements.
- The misfit between SAR Waveform Model and SAR Waveform Data has been computed as:
Output Format
The L2 data products generated in output are in standard NetCDF format (CF-convention), fully compatible with BRAT.
The file naming convention for those files is :
RES_INPUT_FILENAME.nc
where:
INPUT_FILENAME is the filename of the SAR L1A data file given in input
(for more details on the input filename, please check the product specification)
The netCDF format is self-explanatory with all the data field significance described in the attributes.
Along with the NetCDF file, it is provided in output a .png picture of the radar data image (Radar Echogram) and a kmz file of the satellite pass ground-track.
Selecting the corresponding option, user can decide to post in output also the SAR Stack Data Products, still in netcdf format.
The naming convention for those stack data products is :
STK_INPUT_FILENAME.nc
Opening SARvatore products in BRAT
GPOD SARvatore products can also be opened in the Broadview Radar Altimetry Toolbox (BRAT). However, when products are loaded, the track is not correctly displayed on the BRAT map and an error is reported:
- all altimetry data from official data centres (from ERS-1 and 2, Topex/Poseidon, Geosat Follow-on, Jason-1, Envisat, Jason-2, Cryosat and Sentinel-3, from Sensor Geophysical Data Record to gridded merged data);
- any netCDF file.
In the latter case, a small edit is needed to correctly visualize the track associated with the input products.
Considering GPOD SARvatore products, the netCDF file is associated with the NETCDF_CF / Generic NetCdf Variant 1 format:
Therefore, to correctly display the track on the BRAT map, users shall close the program, open the bratXX/bin/data folder:
- change the latitude to latitude_1Hz ;
- change the longitude to longitude_1Hz;
- change the time to time_counter_1Hz (or those at 20Hz).
Once the track is loaded again in BRAT, it will be correctly displayed and no error will be reported:
More information can be found in Chapter 5 of the BRAT user manual available here.
Afterwards, GPOD SARvatore products can be processed in BRAT using the Advanced option in the Operations menu.
To learn how to use BRAT, please consider the material available at the following links:
or, equivalently, the material associated with the lesson: "SARvatore & BRAT Demos".
Contact Us
For any question, bugs and support, please contact us at:
altimetry.info@esa.int
For G-POD specific questions please contact:
eo-gpod@esa.int
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Join & Share Forum
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In order to express feedbaks, place requests and share information, you can
consider to consult the SARvatore Join & Share Forum here
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References
REF1: Guidelines for the SAR (Delay-Doppler) L1b Processing, ESA, 2013, available here
REF2 : (SAMOSA Model Paper) Ray, Chris & Martin-Puig, Cristina & Clarizia, M.P. & Ruffini, Giulio & Dinardo, Salvatore & Gommenginger, Christine & Benveniste, Jerome. (2015). SAR Altimeter Backscattered Waveform Model. Geoscience and Remote Sensing, IEEE Transactions on. 53. 911-919. 10.1109/TGRS.2014.2330423.
REF3 : Dinardo, Salvatore & Fenoglio, Luciana & Buchhaupt, Christopher & Becker, Matthias & Scharroo, Remko & Joana Fernandes, M & Benveniste, Jérôme. (2017). Coastal SAR and PLRM altimetry in German Bight and West Baltic Sea. Advances in Space Research. 62. 10.1016/j.asr.2017.12.018.