Incidence Angle Correction Of SAT Sea Ice Image Based On Locally Linear Mapping

To obtain more land information in a wider range in one flight, many SAR (Synthetic Aperture Radar) systems have the ability to work on the ScanSAR (Scanning Synthetic Aperture Radar) mode. In this mode, the scanning width in the range direction is more than 100 km, the variation range of the side-looking angle (incidence angle) is between 20°and 40°. It is necessary to perform a correction of the incidence angle effect before the interpretation of ScanSAR data.Based on the correction method of incidence angle effect studied in the previous literatures, a new approach is proposed in this thesis. Two sea ice SAR data sets tested in the article were acquired over the Gulf of Saint Lawrence by RADARSAT-2 and the Bohai Sea by ENVISAT ASAR, respectively. To achieve a correction with little ground information, we firstly dividing the original image into several blocks on the range direction and performing a primary unsupervised classification which is block-based. The mask of each ground type is then extracted from the classified image, and the proposed class-based LLM (locally linear mapping) technique is performed on the individual class types. As the distribution of backscatter values in azimuth lines is discrete and non-linear, the LLM technique has to numerically sort the values of each class in the azimuth lines, and approximately divide the sorted values into several subsets averagely to make the distribution locally linear. The locally linear mapping is therefore can be completed between the reference line and the azimuth lines. All the parameters in the algorithm can be self-adapting produced in the procedure of image analysis and used for automatic correction. This technique is also transplantable because all the priori knowledge we need is the amount of ground types. The results demonstrate that the performance of LLM is better than HIST-alike method, it makes the corrected ScanSAR image appears more detailed texture. Furthermore, this thesis has discussed the correlation between the azimuth line interval and the change rate of mean of backscatter values.