Estimated surface-wave contributions to radar Doppler velocity measurements of the ocean surface

For simulated ocean conditions, we estimate the magnitude of the Doppler velocity contributions produced by unresolved surface waves that typical spaceborne synthetic aperture radars (SAR) would measure. The mechanism for generating Doppler velocities is the correlation between wave phase and radar cross section. The contributions analyzed include those of linear gravity waves, second-order wave–wave interactions, Bragg-wave scatterers and breaking waves. For gravity waves, we consider both wave tilt and hydrodynamic modulation transfer functions (MTFs). We find that for nominal sea conditions, the Doppler velocity is significant, on the order of 1 m/s, and exhibits large variation as a function of incidence angle and look with respect to the sea direction. The most important contributors are gravity waves and the Bragg scatterers, followed by sea spikes. Effects produced by second-order wave solutions are argued to be inconsequential.