On the question of time causality for HF ray fields traversing caustics

Summary form only given. A preliminary study has been performed which develops an exact, causal analytical solution to the physical optics (PO) approximation for the on-axis transient scattering from a 2-D parabolic cylinder whose concave face is illuminated by an axially incident impulsive plane wave. This analytical time-domain PO (or TD-PO) solution indicates, upon comparison with the corresponding TD-UTD (uniform theory of diffraction) solution, that the TD-UTD is noncausal for observation points beyond the focus (caustic), as expected; however, the causal part of the early TD-UTD solution compares very well analytically and provides numerical results which agree extremely well with the TD-PO solution, indicating that the causal part of the TD-UTD is not really affected by the noncausal portion of the response.<>