Application of fully-focused SAR altimetry to the Wadden Sea

The Wadden Sea is an ecological region that is sensitive to the regional sediment balance. The sediment balance is strongly dependent on hydrodynamics, which is affected by regional sea level rise. To predict the future state of the Wadden Sea it is required to understand the hydrodynamics and to estimate future sea level rise. Based on climate models and tide-gauge records, sea level in the Wadden Sea is expected to closely follow that of the North Sea. The North Sea is densily sampled by tide gauges and since 1992 its mean sea level can be estimated using satellite radar altimetry. However, to understand the hydrodynamics, instant water level estimates are required within the Wadden Sea, which can be used to validate models or for assimilation. Unfortunately, only a handful of sparsely distributed datum-controlled tide-gauge records are available near the coasts or on the islands. Next to that, conventional satellite radar altimetry is not useful in a complex environment as the Wadden Sea, since its beam-limited footprint is typically >10 km in diameter, which causes distortions in the practically every waveform due to nearby land targets. Conventional altimetry is due to its limited resolution also not able to distinguish the narrow tidal canals from the tidal slats. We aim to enhance the spatial distribution of water level estimates using full bit rate altimetry data from the Cryosat-2 mission. By applying a recently developed Fully-Focused Synthetic Aperture Radar (FF-SAR) algorithm, the along-track resolution of this altimeter is enhanced to 0.5 meter. Firstly, this enables us to better remove static land-targets that pollute near-shore waveforms. Secondly, waveforms over tidal slats are possibly distinguishable from those over open water by waveform classification techniques. Besides the benefits for hydrodynamic modelling, the complex Wadden Sea domain is a suited location to study the novel FF-SAR technique in detail. Note that the configurations of the Sentinel-3 and the future Sentinel-6 satellites also allow for the application of FF-SAR, which could further enhance the temporal and spatial sampling of the Wadden Sea.