High Precision Altimetry Using Spaceborne Grazing Angle GNSS-Reflectometry Over Tonlé Sap Lake

This study uses spaceborne grazing-angle Global Navigation Satellite System-Reflectometry (GNSS-R) signals to measure surface height variations of the Tonlé Sap Lake in Cambodia. GNSS-R tracks collected by Spire Global from 2021 to 2024 are compared with co-located Surface Water and Ocean Topography (SWOT) and Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) measurements. There is good agreement between all, with surface height correlation coefficients >$0.7$. Over open water, the average measurement noise is 3, 2.5, and 1.6 cm for GNSS-R, SWOT, and ICESat-2, respectively. However, GNSS-R altimetry maintains a similar noise level under vegetation, while SWOT and ICESat-2 noise increase significantly to 4.6 and 6.9 cm, respectively. The ability to measure water height anomalies under vegetation in all weather and times of day highlights the strength of the GNSS-R altimetry method for inland water studies. Finally, persistent water surface anomalies as large as 12 cm and 15 cm present in all three satellite data suggest an EGM08 geoid anomaly over both the upper and lower part of the lake, and highlight the utility of these satellites for improving geoid representations over large lakes.