Point Target Correction Coefficients For Absolute SAR Calibration

Abstract

A synthetic aperture radar (SAR) system measures the radar reflectivity of the earth's surface for a multitude of scientific and commercial applications. In many cases, accurate absolutely calibrated images are required by end-users. The backscattering from man-made point targets like passive corner reflectors and active transponders is often used as a radiometric calibration standard for synthetic aperture radar (SAR) calibration. As new systems emerge and the demand for more accurate systems increases, it becomes necessary to better understand the effects of real or imperfect targets on the calibration results. Therefore, a point target SAR simulator is presented which models the complete external radiometric calibration process. It incorporates a number of target properties like frequency response, transponder internal calibration strategies, noise, and interference signals, and it takes the SAR mode settings into consideration. Thereby, the relevant target backscatter variation as observed in the processed SAR image with respect to an ideal or any other target can be determined. The simulation results are relevant both during the design process of a new target as well as during the actual calibration. Based on these point target simulations, correction coefficients can be stated for each target and SAR mode, therefore decreasing the remaining radiometric point target uncertainties. Also, the cross-calibration between two different SAR systems becomes more accurate.