Dependency of the Sentinel-1 azimuth wavelength cut-off on significant wave height and wind speed

Sea wind and sea state estimation by synthetic aperture radar (SAR) measurements is a topic of relevance both on the scientific and user side. The new European Space Agency (ESA) Sentinel-1 constellation is meant to support marine studies and ensure highquality data. In this paper, we investigate the azimuth cut-off (lambda(C)) sea wind speed and significant wave height retrieval approach by taking benefit of two sets of multi-look SAR images with incidence angles varying from 20 degrees to 45 degrees. The images have been co-located with sea surface wind measurements acquired by the scatterometer onboard the Chinese satellite HY-2A (HSCAT) and with the European Centre for Medium Range Weather Forecast (ECMWF) operational model output. This study is meant to analyse both the empirical dependency of SAR lambda(C) on significant wave height (Hs) and wind speed (U). Several fitting geophysical model functions (lambda(C)-GMFs) are proposed and discussed. The results show that lambda(C) is strongly correlated with the significant wave height in all sea state conditions, while the correlation with the wind speed is only high for fully developed sea states. The azimuth cut-off based significant wave height retrievals are compared with independent National Data Buoy Centre (NDBC) network observations, showing a root mean square difference of about 0.5 m.