Measuring and Modeling the NRCS of the Sea for Backscatter

Recent ship-based measurements using a calibrated, coherent, dual-polarized, X-band radar illuminated the behavior of the normalized radar cross section of the sea, NRCS or s₀, at incidence angles of 88deg to 89deg. For wind seas s₀(VV) at these incidence angles behaves much like those at lower incidence angles while s₀(HH) behaves very differently, being largest looking upwind and smallest looking downwind. Fits of a multiscale scattering model to these data and data at lower incidence angles show that over the range of incidence angles from 0deg to 89deg, s₀(VV) is adequately explained by the model while s₀(HH) is generally higher than the model predicts at incidence angles above about 45deg. Thus scattering phenomena exist on the ocean that affect HH backscatter very strongly at high incidence angles while impacting VV backscatter only slightly. We show that when the ocean surface is disturbed by currents s₀(HH) can exceed s₀(VV) by as much as 15 dB for long periods of time and over large spatial areas. We examine phenomena that might account for this behavior and how they might affect the wind sea case.