Abstract: Synthetic aperture radar (SAR) altimetry has been widely used in inland water monitoring, but the echoes from inland waters are complex and need to be retracked to retrieve effective water levels. The SAR waveforms from inland waters usually contain multiple sub-waveforms, The main idea of retracking is to find the optimal sub-waveform. A reasonable water level may be obtained under specific conditions, but the wrong sub-waveform is likely to be picked in complex waters or small water bodies, which leads to abnormal water levels. To solve this problem, we proposed a spatial and temporal constrained multi-subwaveform (SaTCoM) retarcker considering time continuity and spatial stability of inland water level changes,which is different from traditional methods that finding the optimal sub-waveform by along-track or external data. Firstly, to find the sub-waveform reflected from water bodies effectively, SaTCoM interpolates and smooths the waveform. Secondly, the reference water level is obtained by filtering the sub-waveform level point clouds from all cycles in the time direction. Then, the along-track sub-waveform water levels are divided into several intervals and all along-track water levels are put into the nearest interval. Under the constraints of the maximum fluctuation from the reference water level, the indicator function characterizing the along-track spatial stability of the water level is used, which can guide the selection of the optimal water level intervals by maximum.
In this paper, we selected 68 hydrological stations with situ level measurements of large, medium, and small water bodies in China as validation, the nearby Sentinel-3A/3B/6A satellite SAR waveforms data were used for five retrackers:SaTCoM, OCOG, ICE1, threshold, and MWaPP+. The results show that the mean RMSE of the inland water level obtained by using SaTCoM is 0.34 m, the relative RMSE is 7.8%, and the correlation with the situ level is 0.95, SaTCoM also performs excellently in most inland water surface and has an absolute advantage over the other retrackers, especially in small and medium-sized water bodies. At the same time, the analysis showed that the SaTCoM can still effectively capture the rapid water level changing signals under spatiotemporal constraints, so it indicates that the proposed retracker SaTCoM can effectively solve the difficulty of retrieving effective water levels in small water bodies, SaTCoM greatly expands the application of inland altimetry data.

Key words: waveform retracking, inland water level, SAR, hydrological station, satellite altimetry