Geometric and Radiometric Calibration of RADARSAT Images

Interpretation of SAR images in areas with significant relief requires rigorous calibration considering local illuminated area and incidence angle effects if one is to be able to perform meaningful multitemporal analysis using images acquired with different geometries and/or sensors. Given the large swaths made available by RADARSAT, even comparison of the near vs. far range sections of the same image requires such calibration. We present here results from geometric and radiometric calibration of single beam mode RADARSAT SAR images, as applied to data provided under the Application Development and Research Opportunity (ADRO) programme. A high resolution elevation model is used first to terrain-geocode the data from ground range radar coordinates into the reference map coordinate system. Tiepoints are used to coregister both the slow and fasttime axes to a global Earth-centred rotating coordinate system. Radiometric calibration is performed by compensating for effects of local illuminated area and incidence angle on the local backscatter. In particular, the effects of multiple DEM slopes (“facets”) within a single radar geometry pixel (which can grow to over a kilometre in steep terrain!) are considered. The utility of the SAR data for thematic interpretation is presented, both before and after calibration.