We propose a polarimetric calibration procedure required to produce accurate polarimetric characteristic of the illuminating scene by GB-SAR. The correction for polarimetric distortions of the GB-SAR system employs only three types of calibration targets, i.e., trihedral, dihedral and 22.5°-rotated dihedral corner reflectors. Azimuth and frequency dependencies on polarimetric responses of a trihedral corner reflector were evaluated by numerical simulation, and we found little difference within 300 MHz bandwidth and of 2 m synthetic aperture length. The proposed polarimetric calibration method was verified by experiment. An amplitude error as low as 0.5 dB and phase error within 3° was achieved by the GB-SAR system at 17 GHz, showing excellent agreement between the theoretical and calibrated polarimetric signatures for a trihedral corner reflector. Finally, we investigate the effects of polarimetric calibration by Pauli decomposition, which indicates desirable polarimetric performance after polarimetric calibration.