Investigations on the Polarimetric Behavior of a Target near the Soil

Abstract

The diffraction of a plane wave by an object situated close to an interface of a two-layer medium has been investigated by many authors and an extensive literature exists on this topic. However, only a few publications have been devoted to the analysis of the polarimetric behaviour of the diffracted field. In general, very simple ray models based on the Geometrical Optic (GO) are considered involving single and double bounced waves for explaining some experimental results. Here, this problem is again investigated by using a more refined ray model based on the Geometrical Theory of Diffraction (GTD). As a result a ray system composed of 13 different rays was implemented for the monostatic case by applying the principle of Fermat. The different spatial and creeping waves give a physical insight in the mechanisms involved in the entire scattering process. Depending on the geometrical properties of the target, lit and shadow regions arise in the backscattered GTD field for a variation from perpendicular to grazing incidence for a monostatic alignment of the transmitting and receiving antenna. The separating line of such a lit and shadow region is called a Surface Shadow Boundary (SSB). A special emphasis is attributed to the transition regions near the shadow boundaries where the reflected spatial waves disappear and transform into creeping waves leading to a strong attenuation. The diffracted field for look angles related to the transition zones has a characteristic polarimetric behaviour which is represented on the Poincaré sphere. The typical locations on the sphere can be exploited in order to get information about the geometrical parameters of the target and its height above the ground.