The recent extension of the finite difference time domain (FDTD) method to frequency-dependent media [Nickisch and Franke, 1992] allows FDTD to now be applied to ionospheric propagation. The FDTD method solves the Maxwell equations directly in the time domain by temporal integration. No approximations beyond that of finite differencing are necessary, although the direct enforcement of certain approximations is possible. By doing so the method can be used to explore the effect of many standard propagation approximations. We consider certain approximations typically applied to the problem of propagation in randomly structured ionization, such as the neglect of polarization coupling and the assumption of small-angle scattering.