Numerical study of shadowing in electromagnetic scattering from rough dielectric surfaces

A numerical study has been performed to examine the effects of surface self-shadowing on the electromagnetic backscattering from dielectric interfaces with two-scale roughness in one dimension. A hybrid numerical technique combining the moment method (MM) and geometrical theory of diffraction (GTD) was used in the numerical calculations. This technique was first extended to be applicable to general dielectric media as well as perfectly conducting and highly conducting, high permittivity surfaces. The numerical calculations show that, for the one-dimensional (1D) rough surfaces considered, the contribution of shadow-region roughness to vertically polarized backscatter decreases significantly as the scattering surface is changed from perfect to finite conductivity, while little change is observed at horizontal polarization. A geometrical optics (GO)-based shadowing function should be accurate down to approximately the same illumination grazing angles at both polarizations with scattering surfaces with complex dielectric constants equal to and below (in magnitude) that of sea water at microwave frequencies. At the smallest grazing angles, weakly shadowed roughness can significantly increase the backscatter from finite-conductivity surfaces at both polarizations, thereby invalidating the concept of a distinct shadow boundary. Vertical polarization is further limited by the contributions of deeply shadowed roughness that decreases with decreasing dielectric constant. As the grazing angle decreases, the shadow-corrected two-scale scattering model loses accuracy well before the contribution of the shadow-region roughness becomes significant.