Hybrid-Polarity SAR Architecture
R Keith Raney(1) and Anthony Freeman(2)
(1) Johns Hopkins University, 11100 Johns Hopkins Road, 20723, Laurel, Maryland, United States
(2) California Institute of Technology, MA140-404, 91109, Pasadena, California, United States
A polarimetric synthetic aperture radar may be defined as a SAR whose primary data product is a coherency matrix (or, equivalently, Stokes parameters). In the dual-polarized case, the matrix is a 2x2 array whose elements characterize the observed backscattered field. In the fully-polarized case, the resulting 4x4 matrix describes the complex scattering function. In both cases the elements of these matrices are invariant with respect to the polarization basis in which they are evaluated. It follows that the polarizations may be chosen to optimize the radar’s engineering performance. This design freedom applies to the received polarizations for a dual polarized SAR, and to both the transmitted and received polarizations for a fully-polarized SAR. Engineering criteria such as signal levels and dynamic range, minimal cross-talk between channels, ease of calibration, and hardware complexity are optimized with a radar for which the receive polarization basis bears no “like” or “cross” polarization relationship with the transmitted polarization. This logic leads to hybrid-polarity SAR architecture, in which circular polarization is transmitted, and orthogonal linear polarizations (such as H and V) are coherently received, retaining their relative phase. In the hybrid-dual-polarized case, either left-circular or right-circular polarization is transmitted, and coherent dual linears are received; the first examples of this architecture are the Mini-RF radars on Chandrayaan-1 and the Lunar Reconnaissance Orbiter. In the hybrid-quad-pol case, alternating right- and left-circular transmissions must be interleaved, and coherent dual linears received following each transmission; the first example may be JPL’s DESDynI radar proposed in response to the National Research Council’s Decadal Plan. Hybrid polarity architecture enjoys substantial objective advantages when compared to the traditional linearly-polarized approach to polarimetric SAR design, including substantial reduction of range ambiguities in the quadrature- or fully-polarized case.
Parts of the research described in this paper were carried by the Applied Physics Laboratory, Johns Hopkins University, and by the Jet Propulsion Laboratory, California Institute of Technology, under separate grants from the National Aeronautics and Space Administration.