Investigation on Doppler Shift and Bandwidth of Backscattered Echoes From a Composite Sea Surface

In the general framework of the second-order small-slope approximation (SSA) (SSA-II), Doppler shifts of backscattered fields from time-varying sea surfaces are predicted at small and moderate incidence angles. The composite sea surface is modeled as a superposition of large-scale gravity waves and small-scale ripples. Here, the elevation of the large-scale surface component, on which each small facet travels along a closed orbit under the condition of the first-order approximation, is described by the coordinates of the small facets. The predicted Doppler shifts are compared with the results obtained by the small perturbation method (SPM), the geometrical optics method, the first-order SSA (SSA-I), and the two-scale scattering method (TSM). We can find that the predicted Doppler shifts for SPM and SSA-I in copolarized configuration are insensitive to the polarization state. However, the results obtained by SSA-II are consistent with those obtained by TSM and yield significant differences between HH and VV polarizations. Spectrum bandwidth is mainly induced by sea-surface orbit motions. In this paper, the formula of the spectrum bandwidth for the scattered echoes from the composite sea surface is also derived on the basis of TSM. The predicted bandwidths are compared with the Monte Carlo simulated results and the measured data. Furthermore, the dependences of the Doppler shift and the bandwidth on the parameters, such as the polarization, wind speed, radar frequency, etc., are also discussed.