A model for aspect-sensitive clutter from field-aligned ionospheric irregularity structure has been incorporated into the over-the-horizon radar performance prediction code clutter effects model [Lauer et al., this issue]. These coherent scatter contributions are modeled using scattering cross sections computed in the Born approximation. It has not been known, however, whether the Born formula for the scattering cross section adequately predicts the correct level of backscatter return at HF. The model required validation, and this has now been accomplished by computing scattering amplitudes using the finite difference time domain (FDTD) method. No approximations are made in the FDTD computations other than that implicit in the use of discrete temporal and spatial grids, so these computations serve to test the validity of the approximations used to develop the Born result. We find that under certain conditions it is necessary to use a generalization of the Born formula which incorporates the profile function of the irregularity region.