Measurement and calibration of differential Mueller matrix of distributed targets

Abstract

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 multipolarization backscatter measurements recorded by a radar system. Because a distributed target is regarded as a statistically uniform random medium, the measurements usually are conducted for a large number of independent samples (usually spatially independent locations), from which the appropriate statistics characterizing the elements of the Mueller matrix can be derived. 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 (representing the spatially independent samples observed by the radar) are indeed the same as the actual scattering statistics of the distributed target. The second assumption pertains to the process by which the actual measurements made by the radar for a given illuminated area are transformed into the scattering matrix of that area. The process involves measuring the radar response of a point calibration target of known scattering matrix, 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 (between orthogonal polarization channels) across the beam are totally ignored, which may compromise the calibration accuracy. To rectify this deficiency of existing calibration techniques, 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, rather than along only the boresight direction. Additionally, the concept of a "differential Mueller matrix" is introduced, and by defining and using a correlation-calibration matrix derived from the measured radar distortion matrices, the differential Mueller matrix is accurately calibrated. Comparison of data based on the previous and the new techniques shows significant improvement in the measurement accuracy of the copolarized and cross-polarized phase difference statistics.

Investigation of polarimetric SAR data acquired at multiple incidence angles

“…It is also noted, that for the bare field an increasing phase difference is observed for C-band and a decreasing for L-band as a function of incidence angle. Similar results have been repormi for measurement of backscatter from bare soils using a truck mounted scatterometer [4].…”

Role of Polarimetric SAR data for discrimination/biophysical parameters of crops based on canopy architecture

“…This result finds explanation due to presence of large leaves in crops with low plant density as in case of mustard and cotton. Large leaves generate, particularly at high frequencies, a "facet" scattering, whose properties are similar to those attributed to surface scattering in the literature (Bouman, 1991, Sarabandi et al, 1992b. Depolarization effects lower than those produced by inclined stems, high correlation between VV and HH channels for planophiles (Macelloni et al, 2001).…”

Analysis of Temporal Polarization Phase Difference for Major Crops in India

“…For a linear polarization mono-static SAR that coherently transmits and receives in both horizontal (h) and vertical (v) polarizations, when scattering reciprocity is assumed (i.e., S hv = S vh ), the scattering matrix S can be written in the form [4,5,7] S = e iφ vv…”