On Removal of Sea State Contribution to Sentinel‐1 Doppler Shift for Retrieving Reliable Ocean Surface Current

Abstract

AbstractThe Doppler frequency shift acquired by Synthetic Aperture Radar (SAR) contains information about ocean surface motion induced by waves and the underlying ocean currents in the radar range direction. An accurate estimate of the wave‐induced contribution is therefore required to derive a reliable estimation of the ocean surface current. In this study, we developed an empirical model for estimating the wave‐induced Doppler shift based on Sentinel‐1B Wave Mode (WV) Level 2 Ocean products acquired from December 2017 to January 2018 collocated with wind field from ECMWF and wavefield from WAVEWATCH III. We found that the relationship between the wind field at 10 m height and the Doppler shift from Sentinel‐1 is in agreement with previous findings based on ASAR observations. Retraining of the conventional CDOP model for the Sentinel‐1 observations (CDOP‐S) yields distinct improvements. We speculate that the improvement is due to different sensor properties and hence biases in the data. Moreover, combing wave and wind information into the model yield considerable improvements especially for the Southern Ocean and the North Pacific. Given accurate wave bias correction, the ocean surface radial velocity maps based on 2 months of Sentinel‐1 acquisitions agree with ocean surface current climatology derived from multiyear drifter observations. This suggests that Sentinel‐1 Doppler shift observations can be used to study ocean surface currents with 20 km spatial resolutions at a monthly time scale.