The Potential of Spaceborne GNSS Reflectometry for Detecting Ocean Surface Currents

The detectability of ocean surface currents in global navigation satellite system reflectometry (GNSS-R) observations is analyzed. We use a large dataset of spaceborne GNSS-R measurements from NASA cyclone GNSS (CYGNSS) mission. The data is collocated with ocean wind and near-surface current measurements. Our analysis reveals clear responses of the GNSS-R sigma 0 to the presence of currents. The response depends on the wind conditions and is more prominent for wind speeds below 6 m/s. A current velocity of 0.5 m/s under an opposing wind can, on average, suppress the GNSS-R sigma 0 by 0.8 decibels for low incidence angles. The interaction of the same current with a codirectional wind can enhance sigma 0 by almost the same amount. This enhancement is most visible at high incidence angles. We develop a model that improves the prediction of the GNSS-R sigma 0 in the presence of surface currents. The detected signatures of wind-current interactions highlight the potential of GNSS-R sensors onboard small satellites for observing ocean surface currents.