Convolution backprojection image reconstruction for spotlight mode synthetic aperture radar

Synthetic aperture radar (SAR) image reconstruction falls into the class of inverse (deconvolution) problems. A spotlight mode SAR system obtains line integrals (projections) of the ground reflectivity at various look angles as the radar platform progresses along the flight trajectory. The image of the ground area is then reconstructed from this set of projections. Conventionally, the SAR image has been produced by a direct Fourier reconstruction algorithm referred to here as the 2-D inverse FFT method. This method has two major problems: 1) due to the batch processing nature of the FFT, all returns must be recorded before the image processing can begin, and 2) a polar-to-cartesian interpolation, which is computationally intensive and error prone due to interpolation inaccuracies, is necessary before a 2-D inverse FFT can be performed. In this paper, a method based on a convolution backprojection (CBP) algorithm is presented. CBP is a widely used technique in computer-aided tomography (CAT). The CBP algorithm has been modified and applied to image reconstruction from SAR data. A quantitative evaluation using computer simulation of the CBP algorithm for spotlight mode SAR is presented. Its performance is then compared with the 2-D inverse FFT method with respect to the multiplicative noise ratio (MNR). Conclusions are supported by a reconstruction example on real SAR data collected by the Lincoln Laboratory's high resolution (0.3 m) radar.