Frithjof Ehlers (Geoscience and Remote Sensing, Delft University of Technology, Netherlands, Netherlands); Marcel Kleinherenbrink (Geoscience and Remote Sensing, Delft University of Technology, Netherlands, Netherlands); Marcello Passaro (Deutsches Geodätisches Forschungsinstitut, Technical University of Munich (DGFI-TUM), Arcisstraße 21, Munich, 80333, Germany, Germany); Florian Seitz (Deutsches Geodätisches Forschungsinstitut, Technical University of Munich (DGFI-TUM), Arcisstraße 21, Munich, 80333, Germany, Germany); Cornelis Slobbe (Geoscience and Remote Sensing, Delft University of Technology, Netherlands, Netherlands)

Estimating significant wave height (SWH) from satellite altimetry in the coastal zone has been a great challenge so far. The received radar echoes exhibit large interference from strongly reflective targets such as mud banks, sheltered bays, ships etc. Fully-focused SAR (FF-SAR) processing exhibits a theoretical along-track resolution of less than half a metre. This unfolds great potential for a manifold of coastal applications.

We have developed a Sentinel-6 Michael Freilich (S6-MF) FF-SAR processor and fit the zero-Doppler beam of the SAMOSA2 waveform model. In order to account for coastal interference, we apply the CORALv2 coastal retracker to estimate the SWH.

For validation of our processing and retracking strategy, we have selected five coastal S6-MF crossings along the coast of the Dutch Wadden See as well as the Eastern and Western Scheldt to analyse the characteristics of the processed signals. The analysis is performed with regard to different crossing angles of the actual satellite track with the coastal shoreline and local features that induce spurious signals within the processed signals. We present an assessment of different processing approaches to exploit FF-SAR processing for improving the SWH estimates in the close vicinity of the shoreline and put them into comparison with UF-SAR-processed signals.

The assessment is accompanied by a statistical analysis that was adopted from the European Space Agency Sea State CCI project. Several metrics are assessed such as outliers, intrinsic noise, a correlation analysis with a high-resolution wave model, and number of valid points. The latter is based on the quality flag, which is an output of the retracker and represents a crucial indicator on how many useful measurements are estimated. The results are compared against the official delay-Doppler-processed products and serve as a valuable reference of validation.