Thomas Moreau (Collecte Localisation Satellites, France); Nicolas Taburet (Collecte Localisation Satellites, France); Franck Borde (European Space Agency, The Netherlands); François Boy (Centre National d’Etudes Spatiales, France); Sophie Le Gac (Centre National d’Etudes Spatiales, France); Nicolas Picot (Centre National d’Etudes Spatiales, France)

The access to fresh water becomes increasingly difficult for many local populations. This natural and incompressible need for living beings leads to economic and geopolitical stakes. The knowledge of inland water resources represents a major challenge to anticipate floods hazards, assess inland waterways sailing conditions and estimate freshwater stocks or rivers discharge. At the same time, public in situ data are decreasing and heavy modifications of the water cycle are expected due to the current global warming and intensive deforestation. It is against this background that the use of altimetry data to monitor surface water levels over continental areas has been considered by altimetry experts since the beginning of the spatial altimetry in the 1990s.

Inland water observation has never been the main objective of altimetry mission. However, consistent water surface height measurements have been provided from these satellites especially using SAR-mode altimeters. SAR-mode altimetry is currently used on-board operational missions such as Cryosat-2, Sentinel-3A and Sentinel-3B. It brings significative improvements when comparing with conventional altimetry over ocean but also inland waters areas. Based on the scientific community feedbacks, a greater use of SAR-mode altimeter is foreseen in the future (as for the upcoming Sentinel-3C/D and the Sentinel-6 missions of the Copernicus programme).

In the operational ground segments, the current SAR-mode processing is based on the so-called unfocused SAR altimeter (UFSAR) processing. It performs the coherent summation of pulses over a limited number of successive pulses (64-pulses bursts of a few milliseconds in length). The possibility to realize coherent pulses summations has been recently [Egido and Smith, 2017] extended to the whole illumination time of the surface (from few milliseconds to more than 2 seconds). It allows the increase of the along-track resolution from 300 m (UFSAR) to the theoretical limit of approximately 0.5 m (FFSAR). Improving the effective number of looks, the capability for obtaining consistent measurements over reflective surfaces of small size is thus enhanced. As part of ESA and CNES project a so-called SMAP open-source software (FFSAR Standalone Multi-mission Altimetry Processor) has been developed.

The purpose of this talk is to assess the FFSAR processing performances over more than 700 water bodies (French rivers, the Amazon, the Congo, the Niger …) within a 1-year period (April 2019 – April 2020) using Sentinel-3A measurements acquired in Open-Loop mode. The benefits from a user perspective will be addressed in terms of measurements precision, timeseries stability and ability to track water bodies of small sizes. The advantage related to the improved along-track resolution to select consistent measurements will be shown. Metrics will be presented to understand the main differences when comparing with the UFSAR. “Replicas” issues of Sentinel-3A and their implications for the FFSAR processing will be also discussed.