CROSS-CALIBRATION EXPERIMENT OF JPL AIRSAR AND TRUCK-MOUNTED POLARIMETRIC SCATTEROMETER

When point calibration targets are used to calibrate a SAR image, the calibration accuracy is governed by two major factors. The first facto r stems from the stringent requirement on the radar cross section (RCS ) of the point calibration target. To reduce the effect of radar retur n from the background, the RCS of a point calibration target must be m uch larger than that of the background. Calibration targets with large RCS require large physical dimensions for passive targets or high amp lifier gain for active targets, which in practice leads to uncertainty in the nominal RCS of the targets. The second factor is related to th e fact that point calibration targets are used to develop a calibratio n algorithm which is applied to distributed targets. To this end, accu rate knowledge of the impulse response (ambiguity function) of the SAR system is required. To evaluate the accuracy of such a calibration pr ocess, a cross-calibration experiment was conducted at a test site nea r Pellston, MI, using the JPL aircraft SAR and the University of Michi gan truck-mounted polarimetric scatterometer. Five different types of distributed surfaces, all in the same area, were chosen: three of thes e were bare surfaces with varying roughnesses, and the other two were covered with vegetation (one with short grass and the other with tall grass). Trihedral corner reflectors were used for calibrating the airc raft SAR, and the UM scatterometer was calibrated using a metallic sph ere. The scatterometer data were collected at L and C bands immediatel y after the aircraft flew over the test site. This paper presents resu lts of the cross calibration between the polarimetric SAR and ground-b ased polarimetric scatterometer measurements at L and C bands. Compari son of the data measured by the two radar systems shows that SAR calib ration with trihedrals may lead to unreliable results. It is shown tha t coherent and incoherent interaction of the ground with a trihedral r eflector can significantly alter the expected RCS of an isolated trihe dral. A distributed-target calibration technique is introduced and app lied to the data with good results.