The effective detection of concealed handguns and knives in open spaces is a major challenge for police and security services round the world. Here an automated technique for the detection of concealed handguns that relies on active swept illumination of the target to induce both scattered fields and aspect independent responses from the concealed object is presented. The broad frequency sweep permits information about the object's size to be deduced from transformations into the time/distance domain. In our experiments we collect multiple sweeps across the frequency range at very high speed, which produces a time evolved response from the target, from both normal and cross polarized detectors. From this we extract characteristic signatures from the responses that allow those from innocent objects (e.g. mobile phones, keys etc) to be distinguished from handguns. Information about the optical depth separation of the scattering corners and the degree and shape of cross polarization allows a neural network to successfully concealed handguns. Finally this system utilizes a range of signal processing techniques ranging from correlation between cross and normally polarized scattering through to a neural network classifier to deduce whether a concealed weapon is present. INTRODUCTIONThe detection of concealed weapons, especially handguns, is a long standing problem for many police authorities round the world, especially when a portable non-imaging system is required, for example for use in random checks in an urban environment.Much recent work has concentrated on the use of passive imaging technology in the mm-wave or sub-THz wavebands where the thermal emission of the body or illumination by the cold sky is used to form images 1 . These techniques are becoming adopted and are useful for indoor environments such as airports or other public venues but their size 2 and the difficulties of interpreting images when operated outdoors have restricted their use. Another potential technology and methodology (SEM, Singularity Expansion Method) that could form the basis for such a deployable gun detection system has been in place for decades and could be based, for example, on techniques used for the identification of aircraft by extracting aspect independent information from the late time response (LTR) of an aircraft or from buried landmines illuminated by a pulse or chirped pulse. This technique is well described in the literature 3,4,5 . However the approach of collecting and analysing the transient response of the target presents problems when that target has a small conducting surface from which surface currents can radiate and the illuminating pulse power is limited, as shown by the work of Novak and Gashinova 6,7 , who use Vector Network Analysers in Time Domain Reflectometry mode in an attempt to determine the pulsed response of handguns and other concealed weapons and explosives strapped to the body. The effectiveness of any target identification system is based on the aspect invariance of the information gathered and t...
An active technique for the standoff detection and identification of concealed conducting items such as handguns and knives is presented. This technique entails illuminating an object with wide range stepped millimetre wave radiation and inducing a local electromagnetic field comprised of a superposition of modes. The coupling to these modes from the illuminating and scattered fields is, in general, frequency dependent and this forms the basis for the detection and identification of conducting items. The object needs to be fully illuminated if a full spectrum of modes and therefore a full frequency response are to be excited and collected. The scattered EM power is measured at "stand off" distance of several metres as the illuminating field is frequency swept and patterns in frequency response characteristic to the target item being sought are looked for. This system relies on contributions from the aspect independent late time responses employed by Baum 1 together with aspect independent information derived specifically from gun barrels and polarisation from scattering effects. This technique is suitable for a deployable gun and concealed weapons detection system and does not rely on imaging techniques for determining the presence of a gun. Experimental sets of responses from typical metal or partially conducting objects such as keys, mobile phones and concealed handguns are presented at a range of frequencies.
A method of detecting concealed handguns and knives, both on and off body, has been developed. The method utilizes aspect-independent natural, complex resonances (poles) excited by illuminating the target with frequency swept, ultrawide band microwaves in the range 0.5 -18 GHz. These natural resonances manifest as a Late Time Response (LTR) that extends significantly (~ 5 ns) beyond the direct reflections from the human body (the Early Time Response) and are of the form of a superposition of exponentially decaying sinusoidal waveforms. Two handguns are examined, both on the human body and in isolation, by the established methodology of applying the Generalised-Pencil-Of-Function to the late time response data of the target. These poles allow the weapon to be effectively classified. Out of plane polarized (cross-polarized) scattered response is used here as this gives improved discrimination between the early and late time responses. Determination of the presence or absence of particular weapons concealed under clothing, on the human body, is demonstrated. A novel bow-tie slot antenna is described which has good pulse and frequency response over the range 0.3-1 GHz and which is suitable for excitation of the fundamental natural resonances.
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