There is much interest in vehicle mounted array GPR systems for landmine detec- tion and work is being carried out on various national, because high speed array GPR systems can sweep a large area in a relatively short time to improve performance and efficiency. We devel- oped the vehicle mounted stepped-frequency (SF) CMP antenna array GPR system. The system has three pairs of Vivaldi antennas that configure a symmetrical CMP antenna array. Based on this configuration, multi-offset CMP data gather can be acquired at every measurement posi- tion. Migration is an important signal processing method that can improve signal-clutter ratio and reconstruct subsurface image. Diffraction stacking migration and Kirchhoff migration sum amplitudes along the migration trajectory that generally is hyperbolic. But when the ground surface varies acutely, we have to modify the migration trajectory by the technique of ray tracing. We compute the travel time between transmitter, receiver and each subsurface scattering point, and search the propagation ray depending on the Fermat's principle. The method is tested by an experiment data acquired by the SF CMP antenna array GPR system. The target is a metal ball that is buried under a sand mound. A nice result of ray tracing is shown in the case.