|Paper title||The Aresys FF-SAR Service for Cryosat-2 at ESA’s Altimetry Virtual Lab|
|Form of presentation||Poster|
The Fully-Focused SAR (FF-SAR) processing, introduced in Egido and Smith (2016) allows obtaining a maximum resolution of 0.5 m in the along-track direction. It provides significant benefits for inland water altimetry investigations allowing the successful investigation of very small rivers and canals (Kleinherenbrink, 2020) that are typically harder to be analysed by using unfocused Delay-Doppler SAR (DD-SAR) data (about 300 m resolution in the along-track direction).
In its development, two major limitations were associated with the FF-SAR processing: 1) the presence of evenly spaced high sidelobes in the Point Target Response (PTR) due to the closed-loop burst mode implemented in Sentinel-3 & Cryosat-2 altimeter payloads, used for initial FF-SAR investigations, and 2) the heavy computational burden with respect to the unfocused DD-SAR processing.
The first limitation can be overcome by designing the radar system differently adopting an open-loop transmission scheme as, for instance, the one implemented in the altimeter payload of the Sentinel-6 Michael Freilich mission, launched on 21 November 2020 and operating since 21 June 2021.
The second limitation has been addressed in research works following Egido and Smith (2016) indicating that an improvement in terms of computational burden can be achieved by adopting algorithms in the frequency domain (Guccione et al., 2018).
Being the role of FF-SAR for future inland water altimetry well understood, along with the possibility to see it implemented with reduced drawbacks during the Sentinel-6 Michael Freilich mission, a collaboration has started between the ESA Altimetry Team, already hosting the successful SARvatore services portfolio for unfocused SAR & SARin altimetry, and Aresys.
Aresys has developed a generic FF-SAR prototype processor, that is able to process data acquisition from different instruments and exploiting the frequency-domain Omega-K algorithm (Guccione et al., 2018 & Scagliola et al., 2018). The Aresys's FFSAR prototype processor for CryoSat-2 allows users to process, on line and on demand, low-level CryoSat FBR products in SAR mode up to FF-SAR Level-1 products with self-customized options. Additionally a wide set of processing parameters is configurable, allowing as an example to select the along-track resolution or to obtain FFSAR multilooked waveforms at the desired posting rate.
The collaboration led to the creation of a new service for the processing of CryoSat-2 data in FF-SAR mode. Users will be able to select the following options: 1) range oversampling factor, 2) bandwidth factor (responsible for the along-track resolution value) and 3) multilook posting rate (1Hz-500Hz). Geophysical corrections and L2 estimates from both a threshold peak retracker and an ALES-like subwaveform retracker are part of the output package. In preliminary open ocean analyses, very good results on SSH noise have been obtained by the ALES-like subwaveform retracker.
In this presentation, the Aresys FF-SAR prototype processor is described and the outcome of some preliminary validation activities, performed by a group of altimetry researchers, is reported.
The service, to be soon extended to allow the processing of Sentinel-3 and Sentinel-6 data, will be made available to the altimetry community in early 2022 as part of the Altimetry Virtual Lab, a community space for simplified services access and knowledge-sharing. It will be hosted on EarthConsole (https://earthconsole.eu), a powerful EO data processing platform now also on the ESA Network of Resources (info at firstname.lastname@example.org).
Egido A., Smith W. H. F., “Fully Focused SAR Altimetry: Theory and Applications” IEEE Transactions on Geoscience and Remote Sensing, Volume: 55, Issue: 1 , Jan. 2017, doi: 10.1109/TGRS.2016.2607122.
Kleinherenbrink M., Marc Naeije, Cornelis Slobbe, Alejandro Egido, Walter Smith, The performance of CryoSat-2 fully-focussed SAR for inland water-level estimation, Remote Sensing of Environment, Volume 237, 2020, 111589, ISSN 0034-4257, https://doi.org/10.1016/j.rse.2019.111589.
Guccione P., Scagliola M., and Giudici D., “2D Frequency Domain Fully FocusedSAR Processing for High PRF Radar Altimeters” Remote Sens. 2018, 10, 1943; doi:10.3390/rs10121943.
Scagliola M., Guccione P., “A trade-off analysis of Fully Focused SAR processing algorithms for high PRF altimeters”, 2018 Ocean Surface Topography Science Team (OSTST) Meeting. https://meetings.aviso.altimetry.fr/programs/complete-program.html.