Advanced Processing of 3D Computational Microwave Polarimetry Using a Near-Field Frequency-Diverse Antenna

Polarimetry is typically restricted to far-field characterization of a target using beam-like waves, which results in a 2 x 2 scattering matrix representation under two orthogonal in-plane polarization bases. However, a short-range (or radiative near-field) microwave polarimetric approach can recover a 3 x 3 polarimetric matrix representing a full vector polarimetric response of the imaged object. The computational imaging method retrieves this full polarimetric response by utilizing an ensemble of randomly polarized probing fields from a cavity-backed metasurface antenna as the enabling technology. In this paper, we describe the polarization states of the non-planar vector sensing fields with three-dimensional (3D) Jones vectors and examine the polarization diversity with the polarization ellipses in 3D space. Corresponding 3D polarimetric target parameters are derived from the 3D polarimetric matrix and the diagonalization process of this matrix. The generalized 3D target parameters disclose direct details of the imaged object which are otherwise inaccessible to the conventional 2 x 2 polarimetric scattering matrix description, especially the polarimetric features along the range direction. The target parameters reconstructed in experiments validate the effectiveness of our 3D polarimetric near-field imaging framework and the parameterization. The advanced processing and parameterization of 3D polarimetry indicate great potential applications in many short-range microwave imaging scenarios.