Improvement of Computational Efficiency for Fast ISAR Image Simulation Through Nonuniform Fast Fourier Transform

The image-domain ray-tube integration formula (IRIF) is an efficient way to build a large database for synthetic-aperture radar automatic target recognition (SAR-ATR) because the IRIF can rapidly generate an inverse SAR (ISAR) image of a computer-aided design (CAD) model. Additionally, calculation of the IRIF can be accelerated by a nonuniformly sampled impulse train and the nonuniform fast Fourier transform (NUFFT). In a numerical implementation, upsampled ISAR images are required and the IRIF can calculate the upsampled ISAR images directly in image-domain. However, image-domain upsampling increases the computation time because the NUFFT calculates a matrix whose size is increased by the upsampling factor. In this letter, we propose a method to minimize the increase in computation time caused by image-domain upsampling. We derive a formula to approximately predict the scattered far-field under the small-angle condition and accelerate the formula by utilizing the nonuniformly sampled impulse train and the NUFFT. The NUFFT is used to calculate a far-field matrix without upsampling. Then, zero-padding in the field-domain and the FFT are utilized to generate an upsampled ISAR image. To validate the proposed method, ISAR images of a tank and a helicopter CAD models were simulated and were compared with those of the previous methods.