Abstract: External calibration is an effective way to calibrate absolutely the synthetic aperture radar(SAR) system,but the external calibration method used for conventional SAR system cannot be implemented for low frequency ultra wideband(UWB) SAR system.In this paper,three characteristics of UWBSAR are fully taken into account: low frequency,ultra wideband and large processing angle,and using the math model of the external calibration,an external calibration scheme is suggested.The selection of reference scenes,location of calibrators and calibration errors are discussed.