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

The parabolic-wave equation and its variants have provided the theoretical framework for most practical forward-propagation models. Split-step integration generates an easily obtained, robust solution for most applications. Irregular boundaries can be incorporated by using a conformal mapping technique introduced by Beilis and Tappert (1979) and refined by Donohue and Kuttler (see ibid., vol.48, p.260-77, 2000). In an earlier paper, we demonstrated an alternative method that incorporates a numerical solution to the forward-boundary-integral equation within each split-step cycle. This paper compares predictions of forward propagation obtained by these two distinctly different methods. The results confirm that the PWE-based method is very accurate for smoothly varying surfaces and that it captures the primary forward structure even in the presence of unresolved surface detail. The moderate loss of fidelity is often an acceptable trade for increased computational efficiency. There are situations, however, where the details of the surface structure are important. Furthermore, the induced surface currents are unique to the forward-boundary-integral method. We illustrate their use by calculating the bistatic scatter 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 bistatic scatter function only when the illuminating beam is tilted slightly toward the surface. We interpret this disparity as a breakdown in concept that underlies a local scattering function.