I. Introduction
The spectral projection model (SPM) was developed with the goal of relating the electromagnetic scattering to the physical properties of a target or surface [1], [2]. In SPM, the addition theorem for Bessel and Hankel functions [3] is employed to describe the relationship between source currents and the corresponding induced fields as a projection of spectral signatures [1], [2]. The field at an observation point is written as the projection of the spectral signature representing the source point on to the spectral signature representing the observations point where the field is to be estimated. For two-dimensional objects, these spectral signatures are described as vectors in terms of Bessel or Hankel functions and the appropriate trigonometric functions [1].