Water Extent Measurements with Sentinel-6 Fully - Focussed SAR data

Abstract

The Sentinel-6 mission, launched in November 2020, carries a radar altimeter operating in open burst with a PRF high enough (~9kHz) to perform the focussing of whole target observation echoes in a fully coherent way. Furthermore, such a feature allows improvement of the along-track resolution down to the theoretical limit of around 0.5 m when processing the data with a Fully-Focussed SAR (FFSAR) algorithm. This resolution increment actually represents a revolutionary step with respect to the ~300 m along-track resolution provided by current operational processors based on Unfocussed SAR algorithms, commonly used in radar altimeters with a closed burst chronogram, such as CryoSat-2 and Sentinel-3. In this contribution, we explore new applications over inland water surfaces, such as reservoirs or lakes derived from the new Sentinel-6 FFSAR products. Indeed, a FFSAR Ground Prototype Processor (GPP), developed by isardSAT and based on the backprojection algorithm [1], has been used to process altimetry data and generate FFSAR radargrams of off-nadir inland targets located within certain observation constraints. As a main outcome, we present a methodology to geo-reference and estimate the extension of water bodies located on unambiguous across-track targets and that present strong seasonal extension variability. Validation of the method has been performed by comparing the FFSAR water extent measurements derived from Sentinel-6 against optical (Sentinel-2) measurements and in-situ observations. [1] Egido, Alejandro and Walter H. F. Smith. “Fully Focused SAR Altimetry: Theory and Applications.” IEEE Transactions on Geoscience and Remote Sensing 55 (2017): 392-406.