Geometric distortion correction in the subaperture processing for high squint airborne SAR imaging

Basic subaperture processing for synthetic aperture radar (SAR) imaging consists of generating a set of low-resolution images and adding them coherently to form the final high resolution image. For an airborne high squint imaging mode SAR, the coherent addition processing suffers from the geometric distortion in the subaperture images caused by range-Doppler interaction and the irregularities in aircraft motion. This paper presents a method to compensate these effects based on the geographical coordinate transform, which is used as a middle processing prior to the coherent summation of the subaperture images. It mainly involves coordinates calculations between the focus target plane and the image display plane under the flat-Earth model, and then a 2D interpolation is carried out in the image domain. A quantitative evaluation using point-target simulations of the coherent subaperture imaging algorithm for a squint single of 60deg is also provided. Its performance successfully demonstrates the validity of the proposed method. The further advantage of implementing this approach is that the crucial step of compensating the spatially-variant residual phase error can be done at the same time, and the resultant images with constant sample spacing are ready to be mosaiced to produce the full-strip image