Inland Water Body Surface Height Retrievals Using CYGNSS Delay Doppler Maps

CYGNSS satellites record the power of GNSS reflection signals in delay and Doppler shift bins. Myriad delay Doppler maps (DDMs) have been collected since 2016. Research has shown that reflection range delays can be derived from CYGNSS DDMs and their metadata for satellite altimetry. However, the performance of these DDMs in estimating inland water levels has not been thoroughly evaluated. This study leverages the coherent reflection-dominated DDMs from 2020 to 2022 to estimate water levels at five lakes. Compared to radar altimetric (RA) observations, the CYGNSS results have an overall bias of 2.0 m, a root mean square difference (RMSD) of 3.1 m, and an unbiased RMSD (ubRMSD) of 2.4 m, when combining measurements from all study sites over the three-year period. During 2020-2021, the bias is lower at 1.1 m, the RMSD is 1.9 m, and the ubRMSD is reduced to 1.6 m. Smaller bias, RMSD, and ubRMSD values are also observed in 2022 nighttime data. However, the daytime data from 2022 show higher water level estimations compared to RA observations with a bias of 4.7 m, an RMSD of 6.1 m, and an ubRMSD of 3.9 m. The CYGNSS overestimations are linked to an increase in ionospheric total electron content (TEC). This study demonstrates the feasibility of spaceborne GNSS-R for measuring inland water levels and highlights the usefulness of the current version of CYGNSS DDMs in monitoring reservoirs and inundation events characterized by substantial water level changes.