Water Surface Height Measurements with Sentinel-6 Fully-Focussed SAR Over Inland Targets

The Sentinel-6 mission, launched in November 2020 is a satellite mission carrying an altimeter operating in open-burst, the Poseidon-4 altimeter. This altimeter has a PRF of approximately 9 kHz, able to perform focussing of whole target observation echoes in a totally coherent way. This means that not only we are obtaining measurements with very reduced contaminant contributions coming from along-track replicas, but also the along-track resolution can therefore be narrowed down to its theoretical limit (∼0.5 m) when processing the data with a Fully-Focussed SAR (FF-SAR) algorithm. The latter is what is of interest, especially when compared to the ∼300 m along-track resolution provided by other operational processors based on Unfocussed SAR algorithms, widely used in the bast majority of satellite missions with radar altimeters that operate with closed-burst (e.g. Sentinel-3).

In this study, we apply this algorithm to perform measurements of the water surface height (WSH) over a series of inland targets including relatively small reservoirs and lakes, with typical sizes between 0.1 and 10 km. To do so, a FF-SAR Ground Prototype Processor (GPP) developed by isardSAT and based on the back projection algorithm [1] has been used to process the Sentinel-6’s altimetry data and generate FF-SAR L1B records of the nadir targets being evaluated. This study will present the methodology defined to obtain the WSH measurements using the FF-SAR products alongside the validation process, based on comparison of results with in situ water height measurements.

 

[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.