Insight into validation methods of Sentinel-3 Hydrology Thematic Products

Sentinel-3 is an Earth Observation satellite series developed by the European Space Agency (ESA) as part of the European Copernicus Programme, currently composed of the two Sentinel-3A and Sentinel-3B satellites. Both satellites carry on-board SAR Radar Altimeter (SRAL), which aims at supplying operational topography measurements of the Earth’s surface. Over inland waters, the main objective is to provide accurate Water Surface Height (WSH) measurements to support the monitoring of freshwater stocks through dedicated Level-2 Hydrology Thematic Products. As part of the ESA Sentinel-3 Altimetry Mission Performance Cluster (MPC) project, the Hydrology Expert Support Laboratories (HY-ESL) evaluates the product performance using dedicated validation methodologies and proposes potential enhancements to the Hydrology Thematic Products. 

In this study, the performance of the Hydrology Thematic Products over rivers and lakes is assessed using complementary validation methodologies to better estimate WSH uncertainties over inland waters. First, Sentinel-3 WSH timeseries are compared with in-situ WSH timeseries over rivers using nadir validation method combined with river slope estimates derived from SWOT products. These results are complemented with the innovative off-nadir validation technique that redefines the notion of virtual station, reducing WSH uncertainties induced by the river slope bias. Cross-validation is then performed between Sentinel-3 and SWOT products to leverage the large spatial coverage of the SWOT mission, resulting in a distribution of WSH differences from thousands of lakes. Statistical metrics from this distribution are analyzed with respect to lake size and specularity. Finally, mean lake surfaces inferred from SWOT products are used over large lakes to quantify WSH uncertainties due to errors in global geoid models, which is currently the main contributor to the error budget over large lakes.