The method of ordered multiple interactions for closed bodies

Boundary integral methods for calculating bistatic scatter start with a specified sample grid over the surface that defines the scatterer. Whereas standard method of moments (MOM) implementations can be applied independent of the mesh structure, the method of ordered multiple interactions (MOMI) effectively defines a surface trajectory over the sample grid. This paper introduces source-directed slice sampling whereby a variant of the MOMI method applied to three-dimensional (3D) objects becomes a strict forward-backward recursion. Applications of MOMI to 1D rough surfaces first demonstrated the rapid convergence that can be obtained when the surface sampling follows the projected direction of the incident radiation. With a 3D object source-directed sampling can be implemented with no projection, although the surface must be resampled for each new source direction. The first iteration of the MOMI recursion makes a partial backward correction. With strict forward-backward sampling, the initial forward sweep, which we call the forward-approximation, often provides a good approximation to the source currents. The results are demonstrated for the scalar problem, but the extension to the vector problem is straightforward.