Accuracy assessment of elevation data obtained from Radarsat stereo images

Abstract

Radar stereo mapping or radargrammetry using high resolution spaceborne SAR data may be used to obtain terrain elevation maps. In the mapping context, SAR systems have an added advantage of being ranging devices. The geometric relationship of a terrain point with respect to the SAR is intrinsically encoded in the image by its position on the image. Indeed, the pixel coordinates specifies the distance r of the satellite from the imaged point p and the azimuth position of the satellite at that instant. The authors present some preliminary assessment on the quality of elevation data generated from a stereo-pair of Radarsat images. They first compare the range values of control points, whose ground coordinates are accurately known, with the calculated values. This gives a measure of the inherent inaccuracies in the satellite's position vector which are obtained from the orbit ephemeris information provided with the SAR data. They show that the the values obtained warrants some adjustments in the satellite's orbit. To this end, they model the two-body Keplerian orbit with a cubic polynomial. The parameters characterising the orbit are initially estimated from the ephemeris information. Using suitable ground control points these parameters are further refined using a least-squares adjustment technique. The coordinates of arbitrary points are then evaluated using the standard radargrammetric equations and compared with values obtained from a DEM of the test site.