A comparison of forward-boundary-integral and parabolic-wave-equation propagation models

The parabolic wave equation and its variants hale provided the theoretical framework for most practical forward-propagation models. Split-step integra tion generates an easily obtained, robust solution for most applications, I rregular boundaries can be incorporated by using a conformal mapping techni que introduced by Beilis and Tappert [1] and refined by Donohue and Kuttler [2], In an earlier paper, we demonstrated an alternative method that incor porates a numerical solution to the forward boundary-integral equation with in each split-step cycle [3]. This paper compares predictions of forward pr opagation obtained by these two distinctly different methods. The results c onfirm that the PWE-based method is very accurate for smoothly varying surf aces and that it captures tl;e primary forward structure even in the presen ce of unresolved surface detail, The moderate loss of fidelity is often an acceptable trade for for increased computational efficiency. There are situ ations, howe icr, where the details of the surface structure are important. Furthermore, the induced surface currents are unique to the forward bounda ry-integral method. We illustrate their use by calculating the bistatic sca tter that would he measured from an isolated surface segment. We show that the scattered field measured in this way can be normalized to form a bistat ic scatter function only when the illuminating beam is tilted slightly towa rd the surface. We interpret this disparity as a breakdown in concept that underlies a local scattering function.