Absolute radiometric correction in rugged terrain: a plea for integrated radar brightness

Rigorous intercomparison of multimode SAR imagery requires not only geometric correction (terrain geocoding), but also radiometric normalisation for terrain-induced variations in the local illuminated area caused by differing imaging geometries. Particularly in steep Alpine terrain, foreshortening and layover significantly increase the local illuminated area. Some SAR applications (e.g. biomass estimation) include a science requirement for an absolute radiometric calibration accurate to within 1 dB. Fulfilment of this requirement in areas that are not flat requires compensation for terrain-induced effects. Simple "homomorphic" local incidence angle based radiometric models do not satisfactorily account for the variations; instead an integrative faceted "heteromorphic" SAR image simulation approach is used, whereby simulated SAR images are used to normalise for variations in the area of illuminated terrain. Although radar shadow must be considered in the model as a special case, accounting for the increased area within layover areas requires no extraordinary bookkeeping. A test site providing a wide variety of terrain slopes was selected in Switzerland. RADARSAT imagery are used to illustrate the need for normalisation in preparation for multimode image comparison. The effect of varying the resolution of the reference digital elevation model (DEM) on the normalisation factor is discussed. Pre- and post-correction images are shown, as well as comparisons between simulated SAR images calculated using DEMs of varying resolutions.