Fast RFGG-FG-FFT/IKA for Scattering From Multiple Targets Above a Rough Surface

To solve scattering from multiple complex targets above the rough surface with a high root mean square, a hybrid method combining real coefficients fitting both Green's function and its gradient with the fast Fourier transform (RFGG-FG-FFT) and the iterative Kirchhoff approximation (IKA) is developed. Due to its great performance for solving complex fine structures, RFGG-FG-FFT is applied to compute scattering from complex targets, while IKA is used to solve scattering from rough surface. Interactions between the targets with rough surface are solved iteratively and these are the most time-consuming part of the entire computational process. To overcome this computational bottleneck, the combined multilevel fast multipole algorithm (MLFMA), ray-propagation fast multipole algorithm (RPFMA), and fast far-field approximation (FAFFA) are incorporated into interactions between the targets and rough surface. To effectively implement the MLFMA-RPFMA-FAFFA algorithm, the rough surface region is divided into near area, adjacent area, and far area according to distance, where the interactions between these areas with targets are solved by MLFMA, RPFMA, and FAFFA, respectively. The high efficiency and accuracy of the proposed method are verified by a comparison with the numerical method.