Improvement of the Accuracy of Conventional and Delay-doppler Altimetry Measurements over Continental Water Bodies with Dedicated Waveform Processing Strategies

Abstract:

The study describes recent improvements in the processing of the complex radar echoes acquired over continental water bodies for both “conventional” and delay-doppler altimetry.

The accuracy of “conventional” altimetry is at the centimeter level, but only over flat and homogeneous surfaces such as the ocean’s surface, for which this technique was originally designed and optimized. But when the altimeter footprint encompasses various terrains, the measurement accuracy is frequently degraded to several tens of centimeters. While the improvement of the altimeter corrections over continental water areas seems today rather well mastered, it is not the same for the analysis and exploitation of radar echoes. The main reason is the presence of polluting reflectors within the footprint (emerged terrain, vegetation, buildings, ...) Thus, the resulting echo is a vertical composition, sometimes very complex, of a lot of reflections from various reflective facets with variable properties (geometry, backscatter and dynamics). The retracking algorithms currently used (derived from algorithms used over the ocean or sea ice) do not fit to multi-peaks echoes and show difficulties to return the true altitude of the water body. We illustrate the variety of waveforms on various continental hydrological examples and describe the errors made by using the current retracking algorithms. But above all, we demonstrate that the useful information in the waveform is often present but poorly exploited. Secondly, we describe our two approaches used to deal with multi-peaks echoes. The first strategy is to use an a priori estimate of the water height to restrict the retracking to a subset of the echo. The second approach is based on the identification and systematic analysis of all the peaks, for which the associated water levels are analysed in terms of consistency in a second time. We show a clear improvement of the resulting water level time series, especially during low water season. In a third part, these approaches are applied to delay-doppler measurements from Cryosat-2, which higher along-track spatial resolution may enable the measurement of high-resolution water level transects across rivers and other water basins where conventional altimeters echoes are impacted by land contamination due to their larger footprint size.