High Precision Altimetry using Spaceborne Grazing Angle GNSS-Reflectometry over Tonle Sap Lake

This study uses spaceborne grazing angle Global Navigation Satellite System-Reflectometry (GNSS-R) signals to measure surface height variations of the Tonl´e Sap Lake (TSL) in Cambodia. The GNSS-R tracks are collected by Spire Global from 2021 to 2024, and GNSS-R altimetry retrievals are compared to 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 correlations > 0.7. Over open water, the average measurement noise level in terms of standard deviation 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 when measuring water height under vegetation, while SWOT and ICESat-2 noise increases significantly to 4.6 and 6.9 cm, respectively. The ability to measure water height under vegetation in all weather and times of day highlights the strength of the GNSS-R altimetry method for inland water studies. Finally, signatures suggestive of the mean lake surface (MLS) of the TSL along tracks in the upper and lower TSL are identified. The TSL was chosen due to its local importance and unique hydrology, but the techniques of this research can be applied to many large lakes.