Scatterometer Measurement of Differential Mueller Matrix of Distributed Targets

The recent interest in radar polarimetry has led to the development of several calibration techniques to retrieve the Mueller matrix of a distributed target from the multi-polarization backscatter measurements recorded by the radar system. Existing calibration methods rely on two major assumptions. The first is that the illuminated area of the distributed target is regarded as a single equivalent point target located along the antenna’s boresight direction, and that the statistics of the scattering from all of the measured equivalent point targets are indeed the same as the actual scattering statistics of the distributed target. The second assumption pertains to the process involving the measurement of the radar response of a known point calibration target, located along the boresight direction of the antenna, and then modifying the measured response by a constant, known as the illumination integral, when observing the distributed target. The illumination integral accounts for only magnitude variations of the illuminating fields. Thus, possible phase variations or antenna crosstalk variations across the beam are totally ignored. In this paper a new technique is proposed with which the radar polarization distortion matrix is characterized completely by measuring the polarimetric response of a sphere over the entire main lobe of the antenna. Additionally, the concept of a “differential Mueller matrix” is introduced.