The Fast Calculation of RCS of Electrically Large Open-Ended Cavities Combine IPO, FMM, RPFMM, FaFFA
IPO结合FMM，RPFMM，FaFFA方法快速计算电大腔体的RCS

IPO+FMM method can rapidly solve the scattering of three-dimensional cavity. In the conventional Fast Multipole Method (FMM), at all directions of translation between two groups should be computed, it is expensive. The Ray-Propagation Fast Multipole Method (RPFMM) is applied to simplify the translation when two groups are well separated. In fact, the amount of translators to be calculated should be decrease with the increase of distance between two groups, to take full advantage of the characteristics; a self-adaptive ray-propagation fast multipole algorithm which can modulate the bound of cone zone of RPFMM is adopted. When the distance between two groups is enough large, Fast Far-Field Approximation (FaFFA) is used to further simplify the translation. The numerical results show this method is accurate enough and can improve efficient computing speed than IPO+FMM.