The first objective of the Ground-Penetrating Radar (GPR) experiment on NETLANDER is to investigate the geological structures of the Martian subsurface. The aim of this paper is to present initial results obtained in the first phase of a long-term effort to build a numerical model of the GPR operation on Mars and test dedicated signal-processing algorithms on the simulated data. The simulation is based on the use of a Finite Difference Time Domain method, and we have pointed out some of its advantages that allow us to take into account complex features of the underground. This model has given reliable estimates of the power budget of the radar for a simple but still representative model of the Martian subsurface. In addition, several detailed features such as gradients and roughness at the interfaces were introduced to appraise their possible influence on the GPR performances. In the frame of a simplified geometry of both the GPR antennas and the various underground interfaces, a simple and first-order method was developed and tested on simulated data to show the ability of the GPR to retrieve a three-dimensional distribution of the underground reflectors. Based on this model, and even with some rather crude hypothesis on the subsurface electromagnetic characteristics, information on the direction and distances of the reflectors has been retrieved with a satisfactory approximation.