CoAuthors

Adrià Gómez-Olivé (isardSAT S.L., Spain); Charlie McKeown (isardSAT Ltd., UK); Robert Molina (isardSAT S.L., Spain); Albert Garcia-Mondéjar (isardSAT S.L., Spain)

The Sentinel-6A satellite, launched in November 2020, carries the first radar altimeter operating in open burst with a PRF high enough (~9kHz) to perform the focussing of the whole target observation echoes in a fully coherent way with practically negligible impact from along-track replicas. Furthermore, such a feature allows to improve the along-track resolution down to the theoretical limit 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 previous 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 derived from such new Sentinel-6 FFSAR products. Indeed, the FFSAR Ground Prototype Processor (GPP), developed by isardSAT and based on the backprojection algorithm [1], has been used to process data over different types of inland water targets with the following objectives: (1) validate range measurements with in-situ water height data in case of nadir targets and (2) monitor water extent for off-nadir targets located within certain observation constraints. In this contribution we present results for both activities. First, we provide statistics of long-term monitoring of a series of reservoirs, which have reported global biases and standard deviations well below 10 cm. Secondly, we present a methodology to estimate water extent of small targets located on unambiguous across-track targets. We have analysed targets that present strong seasonal variability in terms of area, and validated the method by comparing water extent measurements derived from Sentinel-6 with the ones derived from optical, SAR imagery and in-situ observations. The overall work is part of the VALERIA (Validating Algorithms Levels 1A and 2 in Ebre RIver Area) project developed within the Sentinel-6 Validation Team using data from the satellite commissioning phase.

[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.