A Study of Sea Surface Range-Resolved Doppler Spectra Using Numerically Simulated Low-Grazing-Angle Backscatter Data

A numerical study of sea surface range-resolved Doppler spectra using low-grazing-angle backscatter measurements is described. Backscattered fields as a function of frequency are computed using the method of moments (MOM) for a single realization of a 1-D oceanlike surface profile as the realization evolves in time. Transformation into the range-Doppler domain enables examination of properties of the resulting Doppler spectra (for both HH and VV polarizations) and their relationship to properties of the surface profile. In general, a strong correspondence between the "long-wave" orbital velocity of the surface projected along the radar line of sight and the Doppler centroid frequency is observed for visible portions of the surface, as well as some evidence of relationships between the "width" of the Doppler spectrum and variations of the projected velocity in time at a given range point. Evidence of similar relationships even in some shadowed portions of the surface is also provided. Doppler spectra from HH and VV polarizations are qualitatively similar, despite differences in total power levels, although the portion of shadowed surface points from which Doppler information is available is somewhat larger in VV polarization. A further examination is conducted using backscattered fields computed with a single-scattering method, which neglects shadowing and any multiple-scattering effects. The remarkable similarities observed between MOM and single-scattered Doppler spectra even in some shadowed portions of the surface suggest that non-line-of-sight propagation effects do not significantly influence Doppler properties in such regions.