Looking beyond nadir: Measuring densely sampled river elevation profiles with the Sentinel-6 altimeter

For almost 30 years radar altimeters provide water elevations of rivers and lakes only where a target intersects the satellite’s ground track, called Virtual Stations (VS). This way, the observations have both limited temporal and spatial resolution, because on one hand such intersections occur by chance and because on the other hand the repeat cycle of the orbit ranges from 10 to 35 days, depending on the mission. Boy et al. [1] illustrated recently that river signals may also be captured when the river is located at cross-track distances of several kilometers, when utilizing high resolution SAR altimeter products (particularly fully-focused SAR [2], FFSAR). Therefore, the concept of the altimeter river measurements can be revisited completely. Based on the idea presented in [1], we developed a novel algorithm to calculate water surface elevations (WSE) of rivers within a ground swath of approximately 14 km width, and with along-track resolutions as fine as 10 m from the Sentinel-6 altimeter signal. All that is needed additionally to the FFSAR-processed signal is an a-priori river polygon or centerline to correct for non-zero cross-track distances. Our algorithm can provide WSE along most parts of the river that fall within the swath, thus delivering densely sampled WSE profiles instead of a few point measurements over only the nadir crossings (VS). This marks a drastic improvement in the number of available WSE observations and opens completely new research possibilities, as water surface slopes and level changes due to rapids and dams can be studied directly. Essentially, these new Sentinel-6 WSE measurements resemble the river WSE product obtained with the recently launched SWOT mission (albeit with more limited coverage). As such, they can be exploited in similar manners to provide much additional information for hydrological research, e.g. for assimilation in hydrological models and more reliable estimation of river discharge. We demonstrate and validate the new measurement approach and our algorithm over two rivers in France, the Creuse river and the Garonne river, showing biases that are typically on the order of +-4 cm and random errors on the order of 5 cm, both on 30 m along-track resolution. In our presentation, we will concentrate our attention on the new challenges of the method, including a sophisticated signal detection algorithm, the altered error budget of off-nadir WSE measurements and the limitations due to signal folding, clutter, lacking contrast and the complexity of the scene. [1] Francois Boy et al. “Measuring longitudinal river profiles from Sentinel-6 Fully-Focused SAR mode”. In: Ocean Surface Topography Science Team (OSTST) meeting. Nov. 2023. doi: 10.24400/527896/a03-2023.3781. [2] Alejandro Egido and Walter H. F. Smith. “Fully Focused SAR Altimetry: Theory and Applications”. In: IEEE Transactions on Geoscience and Remote Sensing 55.1 (Jan. 2017), pp. 392–406. doi: 10.1109/TGRS.2016.2607122.