Direct And Indirect Ortho-rectification Of Space-borne SAR Image

Due to the side-looking imaging principle,there are many geometric distortions such as layover?shadow and foreshortening in the SAR image,so it is of great significance to do ortho-rectification for it.In terms of rectification methods,there are a lot of rectification method for SAR image.Currently the most widely used method is indirect ortho-rectification which is based on DEM simulation.This kind of method take the details of the terrain into consideration,thus can improve the rectification accuracy.By contrast,the application of direct ortho-rectification method for SAR image is less,because of its inaccurate positioning accuracy and the problem of existing large area of empty pixel in the corrected image,which seriously affect the application of direct geolocation method.With the attitude and orbit determination of SAR satellite is becoming more and more accurate,the accuracy of direct rectification image is greatly improved.What's more,the characteristics of its intuitive,simple,easy to understand has drew more and more attention.But how about the positioning accuracy,and how much is the difference between this method and the indirect calibration method,can rectification results directly applied to other applications is the problem that people more concerned.Because there are large empty pixel in the direct rectification image,which restricts its application to a certain extent.The traditional neighborhood interpolation method only works for the sporadic cavity pixels in flat regions,but in terms of large area of empty pixels in mountain regions,it is somewhat inadequate.In that case,it requires a new method to deal with a large area of empty pixels,make it possible to generate a rectification result similar to indirect rectification.Based on this,this paper presents a method of empty pixel processing.The principle of processing is still to fill the value with nearby pixels,but the interpolation neighborhood will self-adaption larger with the area of the cavity.by means of this method solves the problem of existing large area empty pixels in mountain regions in direct ortho-rectification image.At the same time,this paper present the simulation image based on RD model and 3 kinds of backscattering models.Also we do comparative analysis of the simulation results according to different backscattering models.At last,complete the rectification of SAR image based on the simulation image.Evaluating accuracy of the corrected image of direct and indirect rectification methods respectively based on high spatial resolution optical satellite images,evaluating the geometric accuracy of direct and indirect rectification results.Also we do visual analysis of the two methods,evaluate of the effect of processing of empty pixels in direct rectification image.The results show that the accuracy of direct rectification is slightly lower than indirect rectification,but the difference is not significant.The errors in the two different rectification image are both within one pixel.The rectification result can be improved greatly after the process of the empty pixel,and it can generate a correction result which is similar to the indirect rectification.