ASAR WSS product verification using derived image mosaics

A program has been written to create image mosaics from ASA_WSS_1P ASAR-WSS level-1 products, providing an overview of the imaged area. The ability to generate WSS mosaics facilitates the study of several WSS product features. This study focuses on their radiometric and geometric characteristics. The incidence angle variation of 16 to 43 degrees across beams SS1 through SS5 creates large differences in the nominal near- and far-range backscatter intensities. Radiometric calibration is applied to the mosaics, taking the range-spreading loss as well as the incidence angle effect into account, normalising for systematic radiometric trends. The five WSS beams acquire data with a substantial overlap (typically several hundred range samples). These overlap regions are of interest because the same ground targets are imaged by two different beams. We calculate the mean radiometric differences between the two beams for each overlap region, and draw conclusions based on the statistics. WSS geolocation accuracy is assessed for the image mosaics as well, using two methods. First, predicted positions of transponders in the Netherlands and Canada are compared to measured positions. Second, terrain geocoding of selected WSS scenes is performed, using DORIS precise state vectors and a digital terrain model (DTM). The geolocation accuracy is estimated using survey points or ground control points (GCPs) derived from topographic maps. The radiometric and geometric investigations confirm a high quality of the level-1 WSS products