NUMERICAL STUDY OF SHADOWING IN ELECTROMAGNETIC SCATTERING FROM ROUGHDIELECTRIC 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 numeric al technique combining the moment method (MM) and geometrical theory o f diffraction (GTD) was used in the numerical calculations. This techn ique was first extended to be applicable to general dielectric media a s well as perfectly conducting and highly conducting, high permittivit y surfaces. The numerical calculations show that, for the one-dimensio nal (1-D) 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 conductiv ity, while little change is observed at horizontal polarization. A geo metrical optics (GO)-based shadowing function should be accurate down to approximately the same illumination grazing angles at both polariza tions with scattering surfaces with complex dielectric constants equal to and below (in magnitude) that of sea water at microwave frequencie s. At the smallest grazing angles, weakly shadowed roughness can signi ficantly increase the backscatter from finite-conductivity surfaces at both polarizations, thereby invalidating the concept of a distinct sh adow boundary. Vertical polarization is further limited by the contrib utions of deeply shadowed roughness that decreases with decreasing die lectric constant. As the grazing angle decreases, the shadow-corrected two-scale scattering model loses accuracy well before the contributio n of the shadow-region roughness becomes significant.