Abstract: Surface-penetrating radar (SPR) is used to obtain images of a vehicle's undercarriage and employ these to identify the vehicle (or vehicle type or class). In particular, a vehicle may have a unique undercarriage SPR “signature” that remains largely stable over time, since it is not significantly affected by buildup of dirt, moisture or light debris. Even if the signature is not sufficiently differentiated from those of similar vehicles, it can be used to identify the vehicle within a class, which may be adequate for many purposes.