On the dependence of radar signatures of surface current variations on the azimuthal radar look direction

An improved two-dimensional composite surface model for the radar imaging of underwater bottom topography in tidal waters is presented. It accounts for the hydrodynamic modulation of the entire wave spectrum by surface current variations, i.e., the effect of waves which are longer than the Bragg waves is included as well as the effect of wave components traveling normal to the radar look direction. These waves affect the radar backscatter by geometric modulation and by hydrodynamic modulation of the Bragg waves, yielding non-zero second-order contributions to the mean normalized radar backscattering cross section (NRCS) which depend on the mean square surface slopes parallel and normal to the radar look direction. The authors discuss the dependence of the resulting radar signatures on radar frequency, polarization, incidence angle, and the azimuthal radar look direction. An interesting result is that the contribution of the normal traveling wave components to the radar backscatter is negative for all incidence angles at vertical (VV) and for steep incidence angles at horizontal (HH) polarization, while the contribution of parallel traveling waves is always positive. This leads to an "inversion" of the theoretical radar signatures under certain conditions, i.e., areas of increased surface roughness can appear as dark areas in an image, where one would normally expect an increased image intensity.<
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