CALIBRATION OF POLARIMETRIC RADAR IMAGES USING ONLY IMAGE PARAMETERS AND TRIHEDRAL CORNER REFLECTOR RESPONSES

This paper describes a technique that uses the radar return from natural targets and at least one trihedral corner reflector to calibrate compressed polarimetric radar data. In this paper we specifically address calibration for relative amplitude, relative phase, absolute amplitude, and system cross talk. The cross-talk calibration method is based on the theoretical result that for natural targets with azimuthal symmetry the co- and cross-polarized components of the scattering matrix are uncorrelated, and does not require any external calibration targets to be deployed before imaging. Since compressed data are used, one is forced to model the transmitting and receiving systems as reciprocal, i.e., that the matrix describing the transmitting system is the transpose of the matrix describing the receiving system. Even though the inferred transmit and receive matrices are not each simply related to the physical transmitter and receiver, the true Stokes matrix for each pixel in an image may be accurately determined by this approach. The method is illustrated by estimating the cross-talk parameters of the NASA/JPL aircraft for different types of terrain and for two frequencies. For the C-band system, the cross talk is less than -20 dB for all ranges in the images. The cross talk of the L-band system is a function of range, however, and may be as poor as -10 dB in the near range, leading to a noticeable distortion of the polarization signatures. When the estimated cross-talk parameters at L band are compared to those expected from the antenna patterns alone excellent agreement is found. Next, it is shown how this method can be used to calibrate radar images provided that at least one trihedral corner reflector is deployed in the scene prior to imaging. Finally, it is shown that polarization signatures of trihedral corner reflectors extracted from the calibrated polarimetry data agree excellently with the theoretical polarization signature of an ideal trihedral corner reflector. © 1990 Canadian Crown Copyright