Evaluation of the RFM as a replacement for the Rigorous Geometric Model of ASLAT-2A imagery

The exploitation of the full geometric capabilities of the High-Resolution Satellite Imagery (HRSI), require the development of an appropriate sensor orientation model. The aim of geometric modelling is to describe the relationship be-tween image and ground coordinates for a given sensor. A rigorous sensor model (RSM) for Alsat-2A has been developed; this model is based on the time dependent collinearity which integrates attitude, orbital data and other information extracted from the metadata provided with the images. To handle this geometric model a program has been developed and tested in previous works and provides an acceptable accuracy. But for the practical use of the developed model to orthorectify the imagery or the extraction of DSM from stereo images; this model must be integrated into dedicated software. Since the majority of these softwares support the rational function model (RFM), this paper deals with the use of the RFM as a replacement of the RSM. After calculating the RSM parameters, we can trace the ray from any image pixel (x,y) to the ground by using the interior and exterior orientation parameters; this ray is intersected at a defined ground elevation (Z) to get the horizontal ground position (X,Y). At least ten well distributed image points are chosen for ray tracing, these rays are then intersected with four different Z level, so at least 40 points are generated and used to calculate the RFM parameters with an accuracy of less than 0.1 pixel. the calculated parameters are written in an RPC file that can be read directly by the softwares to generate orthos and DSMs. This approach allow the replacement of the RSM by an equivalent RFM without affecting its accuracy, which permit the use of the resulted RFM to generate accurate orthoimages and DSMs using a commercial software.