Abstract's details

What can we learn from numerical simulations of satellite altimetry signals over rivers? Detailed study and comparison to a dataset of Sentinel-6MF individual echoes

Eliott Bourget (ISAE/SUPAERO, KTH, CNES, France)


Sophie Le Gac (CNES, France); François Boy (CNES, France)

Event: 2023 Ocean Surface Topography Science Team Meeting

Session: Science IV: Altimetry for Cryosphere and Hydrology

Presentation type: Type Forum only

Contribution: not provided


Radar altimeters are primarily designed for ocean surfaces. Over rivers, the water extent occupies a small area in the 30-km-wide antenna’s main lobe. Consequently, radar echoes over inland waters have a particular shape, very different than the one commonly observed in altimetry over oceans. Resolving the radar equation for non-linear or complex river shapes is almost impossible and one must rely on numerical simulation. This makes water level measurement challenging over rivers.
Recent developments in altimetry over inland water have allowed retrieving water levels of narrow rivers (less than 200 meters wide), but with a precision way under sea surface height measurements. The retracking process, which is the inversion of the waveform to retrieve the water level in the measured signal, also fails due to contaminations of the scene by the surrounding environment.

In this context, we investigate how a numerical simulation of the observed scene can help understanding the influence of several topographical and geophysical parameters on the shape of the measured waveforms. The goal of this study is to find out to what extent simulated signal can be used to improve retracking techniques and perform better in complex observation cases.
Using a representative dataset of rivers sampling varying widths (from 50 meters up to 1500 meters) and different geometries, we perform simulations of the altimeter signal. We use watermasks from different approaches and sources at different resolutions: from handmade masks based on satellite optical images to Pekel’s mask. This approach ensures the representativeness of our simulations and allows identifying the influence of different properties of the observed scene on the simulated signal. We compare our numerical simulations to real measurements from the Sentinel-6 Michael Freilich mission, launched in November 2020, which holds the Poseidon-4 Ku-band altimeter. This altimeter provides over certain areas and time periods a highly valuable dataset of individual echoes at 9 kHz frequency. This dataset is key to understand the fundamentals of altimetry over inland waters.

We show comparisons of the simulated vs. measured signal over rivers with variable widths, roughness, slope and surrounding environment complexity. Learning from these comparisons, we develop a heuristic method to integrate simulations in the retracking process.

This work offers great potential in the use of numerical simulations to enhance retracking techniques of altimetry over inland waters. Future work will include the use of SWOT-derived water masks to describe the observed scene and an assessment of the performance of this new retracking method with respect to other remote sensing products or in-situ data.

Eliott Bourget