The Ground-Based Synthetic Aperture Radar (GB-SAR) facility in the UK provides high-resolution, fully polarimetrically calibrated L- through X-band SAR imagery, principally of targets of remote sensing interest such as soils and vegetation. The facility consists of an indoor laboratory and a portable outdoor imaging system. Details of the polarimetric calibrations of both systems are discussed, with consideration given to the special requirements of field operation. Because of the need to mechanically scan the real antenna to build up a synthetic aperture, the SAR imaging process is significantly longer than its airborne and satellite counterparts. Some of the extended imaging schemes, such as those used in three-dimensional tomographic imaging and diurnal monitoring campaigns, can last from hours to days. However, calibration is normally only possible just prior to, and just after, imaging, leaving the data susceptible to nonlinear system sensitivity fluctuations during the imaging process itself. To address this problem, a novel scheme is discussed that utilizes the signal that arises from the imperfection in the rf isolation of the antenna head as a diagnostic to account for sensitivity fluctuations. Variations of several decibels were seen on a time scale of hours over an extended 2 day measurement. Excellent agreement was found with radar cross section (RCS) fluctuations retrieved from contemporaneous SAR imagery of reference trihedrals placed in the scene
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