For quick detection of “open field” targets and landmines in tussock grass over wide area AMUSAR system must operate in low frequency below 500MHz and high frequency above 2.5GHz. The previous system calibration method used in AMUSAR adopts a compromise in single calibration object such as 0.1m trihedral. But for the frequency band between 500MHz and 1GHz, the 0.1m trihedral locates in Relay region with low signal-to-noise ratio (SNR). In order to improve performance of penetration, we make some modifications as follows. The data between 1GHz and 2.5GHz is calibrated by 0.1m trihedral. Since the 0.3m trihedral exhibits small fluctuation and large RCS between 500MHz and 1GHz, it ensures the SNR of calibration data. So the data is calibrated by 0.3m trihedral in this frequency band. The results show that the proposed method calibrates the system errors more efficiently and thus enhance the performance of imaging.