In practical high-frequency RCS (radar cross section) computations, the most common method is to represent a complex target by thousands of triangular facets, and to calculate its frequency-domain scattering by the physical optics (PO) theory. The authors extend this method to the time domain, in which the incident plane wave is a pulse instead of a continued wave. The solution is based on the simple closed form formula described. A general-purpose computer code, Xpulse, using the derived formulas for field calculation and ray tracing for blockage check has been completed. A typical result from Xpulse is shown. >