Lake Level and Surface Topography Measured With Spaceborne GNSS‐Reflectometry From CYGNSS Mission: Example for the Lake Qinghai

This paper demonstrates inland water altimetry of spaceborne Global Navigation Satellite System‐Reflectometry (GNSS‐R) using the Cyclone GNSS (CYGNSS) mission data. From 12 tracks of raw data overpassing the Lake Qinghai, the bistatic group delay and carrier phase delay are extracted from the quasi‐specular GNSS reflections. The water levels derived from the group delay observations are consistent with the Cryosat‐2 and in situ gauge measurements. The surface topography profiles from the phase delay measurements show good self‐consistence along the coincident tracks. Decimeter‐level surface height anomalies are resolved with the phase delay measurements, which can be associated with the accuracy degradation of the geoid model in this region. Systematic errors remain in both group delay and phase delay altimetry measurements, which are attributed to the receiver orbit errors and ionospheric correction residuals. These limitations can be eliminated in further GNSS‐R missions dedicated to altimetry applications. Plain Language Summary Lakes play important roles in a variety of scientific, economic, and social applications. Satellite altimetry has been increasingly used to monitor lakes and rivers over the past decades. The use of signals transmitted by the navigation satellites after they have bounced on the Earth surface is a technique called Global Navigation Satellite System‐Reflectometry (GNSS‐R). This technique can also perform altimetry with exceptional coverage, that is, any given zone on Earth being frequently and densely observed. The National Aeronautics and Space Administration's Cyclone GNSS (CYGNSS) constellation, consisting of eight small and low‐cost satellites, is the first pathfinder mission using the GNSS‐R technique with a scientific objective. Although not designed for altimetry, the CYGNSS instruments also record the signals (called raw data) before they are processed by the receiver to explore other possible applications. With these raw data sets collected over the Lake Qinghai, we show the feasibility and performance of GNSS‐R concept for lake altimetry. The methodology and results presented in this work can provide useful inputs for the design of a potential CYGNSS add‐on or follow‐on mission to expand its capabilities toward valuable altimetric applications. Key Points This paper demonstrates inland water altimetry using CYGNSS spaceborne bistatic radar observables over the Lake Qinghai Lake level derived from CYGNSS bistatic group de