Lake Water Level Estimation from Grazing GNSS-Reflectometry and Satellite Radar Altimetry over the Great Lakes

Monitoring the dynamics and surface conditions of lakes is essential for the understanding of climate change dynamics. Lake water level is one of the variables that needs a particular vigilance, requiring a global and consistent monitoring that can be achieved by spaceborne remote sensing. Satellite radar altimetry has been widely used in the monitoring of inland waters in the past decades, with recent great improvements in the spatial resolution through the new generation of Synthetic Aperture Radar (SAR) altimeters. However, current radar altimetry constellations limits the space-time sampling necessary for a systematic and regular mapping of lakes. GNSS-Reflectometry (GNSS-R) could provide complementary observations to densify the spatio-temporal coverage, allowing hydrologists to access to finer details on the lake surface evolution. In particular, grazing GNSS-R broaden the use of opportunistic GNSS signals due to the smaller elevation angles of the reflecting signals. In this letter we address this issue by comparing the altimetry profiles over Great Lakes of grazing GNSS-R data provided by Spire constellation satellites and SAR altimeter data provided by Sentinel-3 constellation satellites, as a first approach in preparation of inter-techniques hydrology lake water level products. This letter confirms the advantages of fusing observations from both types of remote sensing techniques provided that accurate regional geoid model are available. Furthermore, GNSS-R data could also provide an independent lake water level information to validate the radar altimetry measurements over sites which are not equipped with ground means.