Autonomous off-road navigation and obstacle avoidance are central to applications of unmanned ground vehicles (UGV). This requires the sensors to have the ability to sense obstacles outdoors or in unstructured environments. Radar sensors, which have the ability to operate in all weather, all lighting, and foliage-covered conditions, have been widely used. We focus on the perception ability of forward-imaging interferometric synthetic aperture radar (InSAR), which can provide three-dimensional scene information of unstructured environments for UGV. An InSAR signal processing flow, similar to the techniques of stereo pair airborne InSAR, is tailored to acquiring the height information of the scene. The InSAR can output three different kinds of images: the scattering image, the coherence image, and the elevation image. Simulation and real data experiments have been carried out on a terrain scene where there are positive and negative obstacles to validate the proposed method and the perception performance of the InSAR, and the special feature differences of obstacles are prominent in the InSAR images.