Another surface scattering model for bistatic scattering

In the development of the IEM model for surface single scattering a simplifying assumption was applied to the phase of the Green's function in the average power calculation leading to a surface scattering model in algebraic form. We remove this assumption yielding a more complex model but still in algebraic form. We show that the simplifying assumption used in the past does not cause an appreciable difference in forward and backscattering directions. However, for different incident and scatter directions there is a noticeable difference especially when the surface roughness is small and scattering is weak. For surfaces with large root mean square height the difference between the original IEM and the new IEM is generally small.