SAR CALIBRATION - AN OVERVIEW

Remote sensing with synthetic aperture radars (SAR's) is a field approaching a new phase in its maturity. Over the next decade, several spaceborne SAR systems, including E-ERS-1, J-ERS-1, SIR-C/X-SAR, Radarsat, and the EOS SAR will produce imagery. The streams of data generated by these systems will pose a considerable challenge to the microwave remote sensing community in terms of relating the data to properties of the Earth's surface. There is an urgent need for the data from these sensors to be calibrated, so that the relationships between radar backscatter and geophysical parameters can be properly established. In this paper, recent progress in SAR calibration is reviewed. The difficulties of calibrating both airborne and spaceborne SAR image data are addressed. The quantities measured by a SAR, i.e., radar backscatter, are defined and mathematical formulations for the three basic types of SAR image developed. The difficulties in establishing science requirements for calibration are discussed. The measurement of SAR image quality is briefly addressed. The problem of radiometric calibration is introduced via the SAR form of the radar equation, with both internal and external calibration approaches considered. The development of algorithms for polarimetric radar calibration is reviewed and the problems involved in phase calibration of interferometric SAR are discussed. Future challenges in the field of SAR calibration are considered.