Introducing New Radar Altimetry Products from Sentinel-3 for Inland Water Monitoring

Remote sensing techniques are crucial for a continuous and comprehensive monitoring of inland waters. In particular, recent advances in satellite radar altimetry have allowed the observation of an increasing number of small and medium-sized lakes and reservoirs, even in complex topographies. The advent of nadir radar altimeters operating in Synthetic Aperture Radar (SAR) mode has significantly improved the resolution of observations in the along-track direction, from several kilometers in conventional pulse-limited altimeters to hundreds of meters in close-burst altimeters when applying unfocused SAR (UFSAR) processing, as is the case in the Sentinel-3 satellite constellation.

Inversion methods for estimating geophysical parameters, such as Lake Water Level (LWL), from the backscattered altimetry signal are commonly called retrackers. These retrackers can be empirical, such as the widely used OCOG method or physics-based, i.e.  a background waveform model is derived from the theoretical knowledge of the microwave scattering process and then fitted to the backscattered signal received on-board. Several retrackers of the second type have been developed for processing conventional radar observations, such as the Brown-type models, and also for UFSAR observations in the case, for example, of the SAMOSA model. However, one of the limitations of physics-based retrackers concerns the assumption that the radar footprint is completely covered by water, as is the case for the ocean. This assumption, which applies to large lakes, starts to degrade the accuracy of the retrieved geophysical parameters when monitoring smaller water bodies. For this reason, a retracker based on numerical simulations tailored to UFSAR observations was proposed for inland waters [1]. This latter model has the advantage of taking into account a priori knowledge of the lake contour (for example, the Prior Lake Database [2]), and, thus, only the in-water areas of the radar footprint contribute to the simulated waveform. A preliminary assessment of the performance of this retracker solution indicated a LWL accuracy better than 10 cm in most of the lakes [3].

Considerable effort has been put into making that retracker robust enough to generate demonstration altimetry products for the hydrological community. These Level-2 products, expected to be available in the Copernicus Data Space Ecosystem in early 2025, cover the entire Sentinel-3 mission time period (both A and B satellites) and provide information on more than 1200 lakes worldwide. This work will present the physical retracker basis and methodology, as well as the content and format of these new radar altimetry products, ready for use by scientific users. Finally, an extensive comparison with in-situ data will be performed to characterize the expected accuracy, with a special focus on time series for some specific lakes.

 

[1] Boy, F., et al., 2021. Improving Sentinel-3 SAR mode processing over lake using numerical simulations. IEEE Transactions on Geoscience and Remote Sensing, 60, 1-18.

[2] Wang, J., et al., 2023. The Surface Water and Ocean Topography Mission (SWOT) Prior Lake Database (PLD): Lake mask and operational auxiliaries. Authorea Preprints.

[3] Yanez, C., et al., 2023. Performance Assessment of Lake Water Level Estimation from Sentinel-3 SAR Data over 1000 Lakes and Reservoirs Worldwide. 2023 IEEE IGARSS, 2870-2873.