A convenient polarimetric radar calibration technique is presented and used to calibrate an X band polarimetric radar. The technique uses a distortion matrix model for the errors introduced by the transmitter and receiver, and it is applicable to any polarimetric radar system where the concept of an invariant distortion matrix is valid. A sphere and two arbitrary passive targets are used to calibrate the radar with respect to the unknown polarization transmitted when the v-polarized channel is energized. The unknown transmitted polarization is then recovered by measuring any nondepolarizing target. Because knowledge of the scattering matrices for the two arbitrary targets is not required, the technique is insensitive to errors in the orientation of calibration targets. Experimental results are presented, and they indicate magnitude and relative phase measurement errors of less than 0.2 dB and 2°, respectively.