Multilevel Fast-Multipole Algorithm for Scattering from Conducting Targets Above or Embedded in a

An extension of the multilevel fast multipole algo- rithm (MLFMA), originally developed for targets in free space, is presented for the electromagnetic scattering from arbitrarily shaped three-dimensional (3-D), electrically large, perfectly conducting targets above or embedded within a lossy half space. We have developed and implemented electric-field, magnetic-field, and combined-field integral equations for this purpose. The nearby terms in the MLFMA framework are evaluated by using the rigorous half-space dyadic Green's function, computed via the method of complex images. Non-nearby (far) MLFMA interac- tions, handled efficiently within the multilevel clustering construct, employ an approximate dyadic Green's function. This is expressed in terms of a direct-radiation term plus a single real image (representing the asymptotic far-field Green's function), with the image amplitude characterized by the polarization-dependent Fresnel reflection coefficient. Examples are presented to validate the code through comparison with a rigorous method-of-moments (MoM) solution. Finally, results are presented for scattering from a model unexploded ordnance (UXO) embedded in soil and for a realistic 3-D vehicle over soil.