Rough-surface polarimetry in companion SAR missions

Bistatic scattering from rough surfaces is typically approached through the analysis of the scattered field in the conventional H and V polarization basis, which coincides with the zenith and azimuth unit vectors in a spherical reference frame. This study delves into the impacts of different choices of the transmit and receive linear basis on the performance and design of a SAR mission receive-only companion. The paper formalizes the rotation of the scattered wave orientation at the antenna axes of the companion with respect to the transmitted one and introduces a novel set of linear polarizations, named principal polarizations, in transmit and in receive, deemed more suited to represent the scattering mechanisms of rough surfaces. Such set is defined by the polarization bases that maximize the radar cross section. It is shown that the theoretical estimates from the proposed geometrical framework provide a good agreement with analytical and numerical simulations, performed considering state-of-art numerical solutions. In addition, the paper promotes the hypothesis that a bistatic radar configuration, defined through the conventional H and V linear basis, presents a strong similarity, from a target information retrieval standpoint, to a monostatic compact φ-pol mode, i.e., with the transmission of a linear polarization rotated by an angle φ. The rotation φ varies over the swath and as a function of the satellite separation. For baselines of 250-300 km, such as those envisioned by the ESA Harmony Earth explorer candidate, and for steep incidence angles, an equivalent π/8-pol can be achieved for rough surfaces.