<title>Some results from geocoding and calibration of ERS-1 SAR FD images in a mountainous area</title>

Abstract

ABSTRACTAn algorithm for geometric and radiometric correction of ERS-1 Synthetic Aperture Radar images by using Digital Eleva-liOn Model data is described and applied to images over mountainous area in Norway. Several images covering the samearea are corrected and geococled into UTM map projection and the results are evaluated against map and in-situ measure-ments. The map to SAR geometric precision is given by a RMS value of 54m in the casting direction and 45m in the north-ing direction. The SAR to SAR geometric precision is given by a RMS value of 43m in the easting direction and 44m in thenorthing direction. Estimates of the radar backscatter coefficient are extracted from the geocoded SAR backscatter images.Obtained values are for wet snow -13 dB and for dry snow -7 dB which agrees favorable with earlier radar measurementsand theory. 1.0 INTRODUCTIONWith the launch of the ERS-l satellite in July 1990, a new era in spaceborne Synthetic Aperture Radar(SAR) imaging was initiated. Since then an enormous mount of SAR images of the earth surface hasbeen generated, and many new user groups have been introduced to SAR data. Unfortunately, SAR dataare not easily related to the properties of the earth surface for several reasons. First a calibration of theSAR data is needed in order to establish a relationship between the radar backscatter value and theproperties of the earth surface. Second the topography of the earth surface causes severe geometric andradiomeiric distortions in SAR images. Finally, the SAR images must be presented in a map projectionsuch that it easily can be merged with other data (time studies, multi-sensor analysis). There are twobasic concepts for geocoding of radar images. The simplest one approximates the earth surface by anellipsoid while the more advanced one takes into account the local elevation by using Digital ElevationModel (DEM) data. Geocoding of radar images using DEM data has been known for several years1'2.However, combining calibration and terrain corrections on spaceborn SAR data is still a research topicthat will become even more important in near future. In this paper a method is described for generatinga calibrated terrain corrected image in Universal Transverse Mercator (IJTM) map projection based onthe Fast-Delivery (FD) ERS- 1 SAR image product from Tromso Satellite Station (TSS)3. The resultsfrom evaluating the absolute and relative geometric precision using ground control points are alsogiven. The observed backscatter from known targets is discussed in terms of in-situ measurements ofthe surface conditions.