Abstract's details

Understanding nadir altimetry measurements over continental waters: simulations over rivers and statistical analysis of individual pulses from Sentinel altimetry missions

Sophie Le Gac (CNES, France)



Event: 2022 Ocean Surface Topography Science Team Meeting

Session: Science IV: Altimetry for Cryosphere and Hydrology

Presentation type: Type Poster

Over the last few years, satellite radar altimetry measurements have become more and more numerous over inland waters, thanks to improvements in the tracking function as well as enhanced measurement modes (from pulse-limited low resolution mode to high resolution synthetic aperture radar). This higher quality and abundance of measurements also raises the question of their processing: from historical retracking algorithms (e.g. the Offset Centre Of Gravity - OCOG retracking) to most recent innovative ones (e.g. Fully-Focused SAR). In particular, it is of common knowledge in the hydrology community that current ground segments using the OCOG algorithm provide, on frequent occasions, biased water surface height estimates and is not reliable at global scale. The need for more reliable and robust processing methods is therefore one of the biggest challenges for radar altimetry over land.
The main challenge is that contrary to ocean altimetry, the radar signal over inland waters is highly variable both in space and time, as it depends on the nature and size of the water body observed (lakes, rivers, flood plains, etc.) as well as on surface conditions.
In this study, we focus specifically on rivers where various types of waveforms can be acquired: sinc²-like peak (the most frequent ones), asymmetric peak, multiple peaks, distorted Brown-like waveform. We address the representativeness of the signal’s specularity, as it has been documented in the literature that the radar backscattered signal over rivers is often highly specular and can be modeled as a squared cardinal sine function. We use two parallel approaches: simulation of radar signals over a specular surface, and analysis of real data acquired by current Sentinel-3 and Sentinel-6 Ku-Band missions over rivers.
In the theoretical approach, we use simulations of various specular surfaces and analyze both the amplitude and phase behaviors. It is interesting to analyze the relative impact of the observing system configuration (e.g. radar bandwidth, altitude, sampling) and the nature of the surface (e.g. geometry of the observed scene, backscatter coefficient) on the simulated radar signal.
In the data analysis approach, we use a unique dataset of one hundred rivers worldwide to perform a statistical analysis of the specular nature of the signal and other characteristics in function of the scene configuration. River width is one but not the only parameter impacting the measured signal.
With this study, we aim at understanding the most significant factors controlling the measured radar signal over rivers in order to build a robust and universal processing algorithm capable of providing reliable water surface height estimates to inland waters users.
In the perspective of the Surface Water and Ocean Topography (SWOT) mission, nadir altimetry more than ever stands as an important asset for calibration and validation of this mission as well as for the design of future altimetry missions such as the Copernicus Sentinel-3 Next Generation: Topography mission, which will necessarily address the question of processing performance over inland waters.

Poster show times:

Room Start Date End Date
Mezzanine Tue, Nov 01 2022,17:15 Tue, Nov 01 2022,18:15
Mezzanine Thu, Nov 03 2022,14:00 Thu, Nov 03 2022,15:45
Sophie Le Gac