Multiple radio wave scattering by sea surface at low grazing angles

Abstract

The problem of very high frequency (VHF) radiowave propagation over the sea surface at low grazing angles is of a great interest for a lot of applications including radar backscattering at different polarizations. Existing theoretical approaches predict much bigger backscattering cross section for vertical polarized (TM) waves as compared to horizontal polarized (TE) waves, and are unable to give satisfactory explanation of experimental results showing that these two values occasionally can be of the same order. In this paper two main features are taken into account in a theoretical investigation of the processes of VHF radiowave backscattering by the rough sea surface at low grazing angles: a) distortion of the incident plane wave by diffraction effects at the sea wave crests prior to scattering by smallscale roughness; b) the concave form of the most part of the visible undulating sea surface. The incident wave diffraction caused by sea wave crests is considered using the simplest model of the crest shape, i.e. a perfectly conducting wedge. It is shown that the diffraction in forward direction at low grazing angles increases the strength of the horizontal polarized field as compared to the vertical polarized one just near the boundary between shadowed and illuminated parts of the surface. Back scattering of electromagnetic waves from cylindrical impedance concave statistically rough surface is studied analytically by the small perturbation method.