Radiometric correction of RISAT-1 data

Today's spaceborne synthetic aperture radar (SAR) systems require innovative technologies to calibrate and process the SAR images. To perform accurate antenna pattern correction during image formation, an antenna model is needed for different antenna beams generated by active antenna steering. The application of such antenna model is successfully demonstrated in RISAT-1 mission, the first microwave remote sensing satellite built by ISRO, India. The basic configuration of RISAT-1 is an active antenna based multimode SAR system with imaging capability ranging from spot, strip to scanSAR To ensure radiometric performance, proper antenna pattern correction is required in all modes of imaging. It becomes especially important to have an accurate knowledge of antenna pattern for ScanSAR imaging mode, where images of multiple beams are joined to have larger swath coverage, for seamless geometry and radiometry. Statistical antenna pattern correction does not correct the radiometry in heterogeneous scenes. Whereas, calibrated antenna patterns works well for all scenes in all condition, if extracted properly. Conventional method of extraction of antenna patterns from calibrated patterns does not work effectively in all cases, due to any change in imaging geometry at the time of acquisition. For extraction at precise location, an innovative methodology is adopted, which extracts the pattern using the information of the actual imaging scene. Two biases, one corresponding to change in actual imaging height and imaging height in calibrated pattern formation, and the other, corresponding to change in ideal imaging off nadir distance and actual off nadir distance for a particular beam, are used for extraction of proper antenna pattern. This extracted antenna pattern is fitted to the required swath used for imaging and applied for antenna pattern correction. Considerable improvement in radiometric correction with calibrated patterns was noted for all modes. Results are illustrated with RISAT-1 data for different modes of operation.