EXTERNAL CALIBRATION OF POLARIMETRIC RADARS USING POINT AND DISTRIBUTED TARGETS

Polarimetric radar calibration algorithms using a combination of point targets and reciprocal distributed targets are developed. From distributed target reciprocity, a matrix equation is derived which can be converted into an equivalent point target response, and can also be utilized to make a polarimetric radar reciprocal. The equivalent point target corresponds physically to a 90-degree polarization rotator which is non-reciprocal and rotation-invariant. Due to this equivalent-point-target interpretation, the cases of polarimetric calibration using two point targets and one distributed target then reduce to those using three point targets, which have been solved in previous research. Regarding the calibration using one point target and one reciprocal distributed target, two cases are analyzed with the point target being a sphere (trihedral reflector) or a polarimetric active radar calibrator (PARC). For both cases, the general solutions of system distortion matrices axe written as a product of a particular solution and a matrix with one free parameter, and then an additional assumption about the distributed target is made to determine the free parameter. For the trihedral-reflector case, when the particular solution is applied for calibration, the measured polarimetric data can be calibrated to the level that only rotation errors remain, and once azimuthal symmetry is assumed for the distributed target, an iterative scheme is devised to solve the rotation parameter. For the PARC case, the residual error is like the channel imbalance after the data are calibrated by the particular solution, and the free parameter can be determined by knowing one ratio of two covariance matrix elements of the distributed target. Numerical results were simulated to demonstrate the validity of the algorithms developed in this paper.