Numerical Studies of Sea Surface Scattering With the GMRES-RP Method

In this paper, we apply the right preconditioned generalized minimal residue (GMRES-RP) method for the analysis of electromagnetic scattering from 1-D ocean surfaces. We first verify that the GMRES-RP method has the desirable features of high accuracy, robustness, and efficiency, and, hence, is well suited to such analysis. We then carry out this analysis systematically by examining the effects of wind speed, incidence angle, ocean spectrum, and the degree of inclusion of the gravity and intermediate waves. With a large surface length of 16384 λ and a noise floor pushed down below -70 dB, we have found more refined structures than suggested in the literature: At near-normal incidence angle (θ
_i
=20°), both VV and HH results remain sensitive to wind speed. Across the incidence angle range, the sensitivity is higher at small wind speed than at medium to high wind speed. Plant argued that the contribution from intermediate waves separately, not as a part of the large wave spectrum, becomes increasingly important with increasing wind speed and, accordingly, a multiscale model is desired. Our numerical analysis validates this viewpoint by showing that a normalized bistatic scattering coefficient value as large as 4 dB at certain angles for H-pol at L band can be attributed to the intermediate wave and the tilt modulation provided by large waves. The viewpoint is further validated by a comparison of the two-scale model and three-scale model against numerical results where the two-scale model shows appreciable discrepancy in the forward scattering angles whereas the three scale model shows very good agreement with MoM, hence manifesting the necessity of advancing multiscale models beyond the conventional two-scale model. This finding also implies the need for numerical studies to use adequately large surface length, at least to cover the intermediate waves, even for below intermediate incidence cases.