The paper is devoted to the development of a mathematical model of the terrain radar image created by the onboard radar. The model is based on the ray tracing method and takes into account the reflective properties of the surface using a special radar texture function The radar texture function is complex-valued function of the three 3D-vector arguments: a point of illuminated surface, a position of transmitter, a position of radiation receiver. This function describes amplitude and phase of reflected plane wave. The radar texture function is needed to simulate radar images formed in motion and to account for the reflective properties of various types of surfaces. The results of comparing the real radar image with the constructed model are also described.