A model to simulate the complete planetary orbital radar sounder problem is presented in this paper. The subsurface features and the ionosphere will not only bring ambiguity to the collected radar sounder data, but also critically affect instrument capabilities. These environmental uncertainties generate a compelling need for meaningful quantitative simulation of the orbital radar sounder problem. Our model combines finite difference time domain (FDTD) and analytical methods and splits the computational volume into two pieces owing to the large size of the simulation space. The near-surface and subsurface fields are computed with the FDTD method to improve the simulation flexibility of the surface and subsurface features. The two-way ionospheric propagation is treated with the simpler but accurate near-far field transformation method to maximize computational efficiency. With the capability of including all of the important radar sounder effects that can be difficult to compute analytically, the model enables accurate numerical experimentation with realistic instrumental and environmental parameters, and can handle an arbitrarily two-dimensionally inhomogeneous ground and an arbitrary ionospheric profile. Simulation results are given on the application of detecting Martian polar subsurface water, and we place bounds on the ionospheric losses and the subsurface conductivity through which water can be detected. We find that a basal lake located ~2.5 km below the surface is near the limit of detectability. The ionospheric losses should be no larger than 10 dB and the average subsurface conductivity should be no larger than 4 ¿ 10-6 S/m for the basal lake to be detectable.