Through-the-Wall Small Weapon Detection Based on Polarimetric Radar Techniques

Wave polarization discriminates between armed and unarmed people.

Detecting concealed small weapons carried by people has received significant interest from law enforcement agencies as well as the military, most frequently for application in controlling checkpoints (in airports, border crossings, public spaces, etc.). Imaging systems for concealed weapons based on radar or other sensor technologies have been recently developed and tested. Most of the existing electromagnetic (EM) sensors suitable for this application operate at very high frequencies, usually in the millimeter or terahertz frequency bands and produce high-resolution images. Although these EM waves can penetrate through clothing (textile materials), they have very poor penetration properties through many common construction materials (such as brick or concrete). Most through-the-wall radars must operate at much lower frequencies, usually below 4 GHz, in order to “see” targets behind walls. However, at those low frequencies, the image resolution is degraded, so small weapons carried by humans may be difficult to detect directly in the image domain.

This work presented polarimetric radar approaches that could be used in detecting small weapons concealed behind walls. The techniques rely on the fact that the radar signature of targets with a large shape factor is strongly polarization-dependent. This property was exploited for weapons such as rifles or rocket-propelled grenades (RPGs). The emphasis was on weapons carried by humans. In this context, the problem could be reformulated from detecting a weapon to discriminating a human carrying a weapon from a human without a weapon. It is important to emphasize that the approaches presented do not apply to small weapons (such as handguns) that do not satisfy the large shape factor requirements. Also essential is that the weapons are passively carried by humans (as, for instance, in the port-arm position), such that the radar illuminates the target mostly at broadside. These techniques do not apply to a rifle or RPG held in the shooting position, where the radar illumination is straight along the barrel.

Four types of targets were considered that are frequently present in a sensing-through-the-wall (STTW) scenario: walls, furniture objects, humans, and rifle-like weapons. The interest in detecting these targets usually increases from walls and furniture objects (which are considered clutter) to humans, and eventually weapons carried by humans. The cross-polarization signature of these targets also increases in the same order (from walls to weapons), whereas the co-polarization signature strengths follow the exactly opposite order. This suggests the use of the cross-polarization to co-polarization ratio as a good way to enhance the response from the most interesting targets (humans and weapons), while at the same time rejecting the clutter (walls and furniture in the scene). This metric can also be used as a discrimination tool in STTW imaging radar or Doppler radar.