Analysis of scattering from rough surfaces at small grazing angles using a periodic-surface moment method

The moment method is used to calculate electromagnetic backscattering from one-dimensionally rough surfaces at near-grazing incidence (angles of incidence up to 89°). A periodic representation of the scattering surface is used to prevent edge effects in the calculated scattering without the use of an artificial illumination weighting function. A set of universal series common to all elements of the moment interaction matrix are derived that allow the inclusion of an extremely large number of periods in the surface description with only a slight increase in computation time. The scattering from surfaces with Gaussian roughness spectra and various roughness scales is calculated at both horizontal and vertical polarizations and compared with the theoretical predictions of the small-perturbation model (SPM) and Kirchhoff approximation (KA). SPM yields extremely accurate predictions of the horizontal-polarized backscattering from small-scale surfaces at incident angles up to 89°. At vertical polarization micro-shadowing causes a rapid drop in the scattering at large incident angles that is not predicted by SPM. KA provides accurate estimates of the scattering from large-scale surfaces at incident angles up to 83° at horizontal polarization and up to 70° incidence at vertical polarization. Surface shadowing becomes important at higher incidence angles, rapidly decreasing the validity of the Kirchhoff approximation. Compensation for surface shadowing decreased the accuracy of the Kirchhoff approximation