Fast Scattered Far-Field Predictions for Super-Resolution ISAR Image Formation Using the Shooting and Bouncing Ray Technique

It is essential to obtain a large amount of inverse synthetic aperture radar (ISAR) image data for ISAR automatic target recognition. For a super-resolution ISAR image for a complex perfect electric conducting (PEC) computer-aided design (CAD) model, parameter estimation methods should be applied to $M \times N$ scattered far-field data over $M$ frequencies and $N$ angles, which is conventionally obtained by using the shooting and bouncing ray (SBR) technique. Therefore, $N$ -time ray tracing processes are required for $N$ angles, and $M \times N$ -time field calculations are required for $M$ frequencies. In this paper, to reduce the computation time for $M \times N$ -time field calculations, we introduce a fast scattered far-field prediction method for a super-resolution ISAR image of a complex PEC CAD model using the SBR technique. This method consists of two main ideas. First, under the small-angle approximation, we derive a DFT-based ISAR image formula in closed form, which allows the rapid acquisition of DFT-based ISAR image through just a one-time ray tracing process at the center frequency and angle. In addition, the scattered far-field data are also obtained by inverse discrete Fourier transforming the DFT-based ISAR image without $N$ ray tracing processes and $M \times N$ field calculations. Second, a field truncation method is employed to suppress errors in the scattered far-field data caused by the proposed closed-form formula. Further, by applying a parameter estimation method to the scattered far-field data, a super-resolution ISAR image is obtained quickly. Simulation results for complex PEC CAD models are in good agreement with the results for the conventional method, while the computation times are tremendously reduced.