Electromagnetic scattering of a target with strong or weak coupled underlying oceanic surface: a FEM approach with FFT accelerated iterative robin boundary condition

To study electromagnetic scattering of a target at high altitude above rough oceanic surface, the iterative robin boundary condition (IRBC) is employed to separate and enclose two isolate regions of the target and underlying surface. The fields in these two enclosed regions are strongly or weakly coupled by iteratively solving the scattering field integral equation in the finite element method (FEM) with updating the right-hand side residual of the robin BC. Most time consuming of the algorithm is spent on evaluating the robin BC on the fictitious planar boundary over the large-scale rough surface. The scattering field integral equation is written as the 1D (one-dimensional) convolution form and is solved efficiently by using the fast Fourier transform (FFT). This study presents a numerical description for the scattering mechanism associated with strong or weak coupled interactions of a volumetric target at various altitudes above randomly rough surface.