EFFECT OF SHADOWING ON ELECTROMAGNETIC SCATTERING FROM ROUGH OCEAN WAVE-LIKE SURFACES AT SMALL GRAZING ANGLES

A hybrid moment-method/geometrical-theory-of-diffraction technique (MM /GTD) has been implemented to numerically calculate the electromagneti c scattering from one-dimensionally rough surfaces at extreme illumina tion angles (down to 0 degrees grazing), The hybrid approach allows th e extension of the modeled scattering surface to infinity, avoiding th e artificial edge diffraction that prevents use of the standard moment method at the smallest grazing angles. Numerical calculation of the b ackscattering from slightly rough large-scale surfaces approximating o cean wave features shows that roughness in strongly shadowed regions c an contribute significantly to the total backscatter at vertical polar ization, This is observed when the shadowing obstacle is several wavel engths high, and the magnitude of the shadow-region contribution does not depend on the radius-of-curvature of the shadowing feature, Strong ly shadowed roughness does not significantly contribute to the backsca tter at horizontal polarization, although weakly shadowed roughness ne ar the incidence shadow boundary does. The calculations indicate that a shadowing-corrected-two-scale model may be able to predict the distr ibuted-surface portion of the sea-surface scattering from the ocean su rface at grazing angles down to about 15 degrees, but at lower grazing the shadowing and large-scale curvature of the surface prevent the es tablishment of a Bragg resonance and invalidate the model.