An Algorithm for Radarsat-1 Baseline Parameter Estimation Satilmis, Kucuk, Istanbul Technical University, Turkey Serdar, Demirel, Istanbul Technical University, Turkey Meftun, Unal, Istanbul Technical University, Turkey Alper, Akoguz, Istanbul Technical University, Turkey Sedef, Kent, Istanbul Technical University, Turkey Abstract InSAR stands for Interferometric Synthetic Aperture Radar and it is a technique used in remote sensing and sat- ellite geodesy. InSAR uses SAR images for generating surface maps for various applications. In these applica- tions baseline parameter is a criteria for choosing right orbital tracks. In practice baseline is the perpendicular distance between two satellite passes in repeat-pass situations. This parameter is represented in different ways such as perpendicular, parallel, horizontal and vertical baselines. This paper presents the calculation of these presentations for Radarsat-1 satellite in order to use in InSAR researches. 1 Preparation of Orbit Files for Orbit Propagation In repeat-pass researches, there are two satellite passes which are called master and slave orbits. For Radarsat-1 there must be multiples of 24 days and 343 passes between these passes. In order to calculate a correct baseline, precise spatial positions of the satel- lite for each track looking into same area (research area) have to be computed. In the first step, two-line element (TLE) data files for master and slave tracks must be prepared for the next step, orbit propagation. 1.1 Coordinate Transformation Two-line element (TLE) files for Radarsat-1, contains statistical and Kepler information for the satellite or- bit. Also in this file there are 15 state vectors meas- ured with 8 minute separation which describes the motion of the platform along the track. These vectors show the spatial positions (m) and velocities (mm/s) of the satellite in GEI (Geocentric equatorial inertial) coordinate system. Since InSAR studies try to create images of the Earth’s rotating surface, GEI coordinates must be transformed into a rotating coordinate system [1]. As a rotating coordinate system we use ECEF (Earth- Centered, Earth-Fixed) coordinate system. The ECEF is a cartesian system which the point (0, 0, 0) repre- sents the center of the earth. In this transformation, the time of interest becomes critical. Because this transformation needs UT1 time as sample time but in TLE files state vectors are given in UTC time. The values of UT1-UTC difference are obtained from NIST Time Scale Data Archive [2]. Assume that the position in GEI coordinate system is given by the vector We need a transformation matrix computed, to con- vert GEI coordinates to ECEF coordinates. Matrix A, is the transformation matrix and the position in ECEF coordinates is defined as And the transformation of the velocity is After these calculations we obtain the 15 state vectors in ECEF coordinate system for each passes. Now our data for the next step, orbit propagation is ready. 2 Orbit Propagation and Base- line Calculation 2.1 Orbit Propagation 2.1.1 Finding Closest Points Before orbit propagation there are 15 state vectors with the frequency 1/480 Hz and these vectors are spanning the whole orbital track. But it is not needed