Validating SWOT water elevations in a dynamic estuary environment

Monitoring water levels in dynamic estuary environments is exceptionally challenging: ground stations are sparse and traditional nadir altimeters have wide (~100km) track spacing and cannot capture spatial dynamics.  The Surface Water and Ocean Topography (SWOT) mission uses an imaging Ka-band radar interferometer to address these issues by providing WSE measurements over estuarine areas at high spatial resolution with unprecedented accuracy and precision. However, the vertical accuracies of these advanced systems remain largely unverified, underscoring the necessity for standardized and repeatable field procedures to validate remotely sensed water elevations. To that end, the Bristol Channel and the Severn River-Estuary has been selected for an extreme edge case for validation studies due to its ~14m tidal range, the second largest in the world. Between April and June 2023, two airborne LiDAR systems collected five independent sets of WSE measurements concurrent with SWOT overpasses. These measurements encompassed a wide range of tidal scenarios, from low tides at -3.6 m relative to the EGM2008 geoid to high tides reaching 5.5 m. LiDAR surveys were conducted along and perpendicular to the SWOT trajectory, covering approximately 35 km and 55 km, respectively, each with a swath width of about 1km. Initial raster-by-raster comparison between SWOT Level-2 HR Raster-100m datasets and LiDAR points (within a distance of 50 m and a UTC time difference of less than 20 s, for Lidar WSE values between 2.1 and 5.2 m) demonstrated a good performance, with a correlation coefficient of up to 0.96 and an RMSE of 0.27 m.  Subsequent correction of LiDAR water levels to the time of the SWOT overpass using the spatial field of water height change from a 500m resolution coastal ocean model allowed a much larger sample for comparison and yielded an RMSEs of 0.16m and 0.40 for the Raster 100m data and SWOT pixel cloud (PIXC) data respectively. Comparison of SWOT data to ground tide gauge elevations resulted in an RMSE of 0.13m.  These results underscore the significant potential for enhanced accuracy in measuring water surface elevations in dynamic coastal regions through the application of Ka-band radar interferometry.