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A Review of Nonimaging Stand-Off Concealed Threat Detection with Millimeter-Wave Radar [Application Notes]

There is now, more than ever before, a need for technologies that enable the screening of people from a distance. A wide variety of weapons can be easily concealed under clothing and carried into crowded public sites to target national infrastructure, spread fear, and inflict mass murder and casualties. The most feared and devastating terrorist weapon is the suicide bomb or person borne improvised explosive device (PBIED). Such devices are relatively simple to conceal on the body, and successful detection is required at considerable distance or stand-off range before the bomber reaches the target area.

ON BODY CONCEALED WEAPON DETECTION USING A PHASED ANTENNA ARRAY

The detection and identiflcation of metal items and, in particular weapons, of linear size ‚ 10cm, concealed upon the human body, is demonstrated as being entirely feasible by using a phased array of suitably ultra wide band transceivers. The complex natural resonances and especially the fundamental resonance, are excited by ultra wide band, stepped frequency continuous wave illumination of the target, using a phased array of antennae to focus the radiation. Broadband illumination of the target with microwave radiation of suitable frequency range (Typically 0.3{3GHz for handgun sized objects) excites low order complex natural resonances and the late time response of the concealed item can be spatially located using phased array imaging techniques. Further processing of the late time response enables classiflcation of the concealed object, based on the complex natural resonant frequencies of the object, so that threat items such as handguns and knives can be difierentiated from benign items such as mobile phone handsets and cameras.

Detection of concealed explosives at stand-off distances using wide band swept millimetre waves

Millimetre waves in the range 20 to 110 GHz have been used to detect the presence and thickness of dielectric materials, such as explosives, by measuring the frequency response of the return signal. Interference between the reflected signals from the front and back surfaces of the dielectric provides a characteristic frequency variation in the return signal, which may be processed to yield its optical depth [Bowring et al, Meas. Sci. Technol. 19, 024004 (2008)]. The depth resolution depends on the sweep bandwidth, which is typically 10 to 30 GHz. By using super-heterodyne detection the range of the object can also be determined, which enables a signal from a target, such as a suicide bomber to be extracted from background clutter. Using millimetre wave optics only a small area of the target is illuminated at a time, thus reducing interference from different parts of a human target. Results are presented for simulated explosive materials with water or human backing at stand-off distances. A method of data analysis that involves pattern recognition enables effective differentiation of target types.

Active millimeter wave detection of concealed layers of dielectric material

Extensive work has been published on millimetre wave active and passive detection and imaging of metallic objects concealed under clothing. We propose and demonstrate a technique for revealing the depth as well as the outline of partially transparent objects, which is especially suited to imaging layer materials such as explosives and drugs. The technique uses a focussed and scanned FMCW source, swept through many GHz to reveal this structure. The principle involved is that a parallel sided dielectric slab produces reflections at both its upper and lower surfaces, acting as a Fabry-Perot interferometer. This produces a pattern of alternating reflected peaks and troughs in frequency space. Fourier or Burg transforming this pattern into z-space generates a peak at the thickness of the irradiated sample. It could be argued that though such a technique may work for single uniform slabs of dielectric material, it will give results of little or no significance when the sample both scatters the incident radiation and gives erratic reflectivities due to its non-uniform thickness and permittivity . We show results for a variety of materials such as explosive simulants, powder and drugs, both alone and concealed under clothing or in a rucksack, which display strongly directional reflectivities at millimeter wavelengths, and whose location is well displayed by a varying thickness parameter as the millimetre beam is scanned across the target. With this system we find that samples can easily be detected at standoff distances of at least 4.6m.

A swept millimeter-wave technique for the detection of concealed weapons and thin layers of dielectric material with or without fragmentation

Active millimetre wave systems, operating at frequencies up to 110 GHz have been used to detect the presence of both concealed dielectric and metallic objects at standoff distances. Co- and cross-polarized superheterodyne or direct detectors are used to differentiate between metallic and purely dielectric objects. The technique determines the thickness of a dielectric target and detects the presence of concealed handguns or fragmentation by utilising the pattern of the responses from both the co- and cross-polarized detectors. The returned signals are processed and analysed by an artificial neural network, which classifies the responses according to their correspondence to previous training data.

A MICROWAVE MEASUREMENT SYSTEM FOR METALLIC OBJECT DETECTION USING SWEPT-FREQUENCY RADAR

Guns and knives have become a significant threat to public safety. Recently, a variety of techniques based on Electromagnetics (EM) have been used for their detection. For example, walk-through metal detection has been used in airports; X-ray and THz detection systems have been used for luggage screening. Different EM frequencies for metallic object detection have demonstrated different merits. This paper reports on a 1-14 GHz swept-frequency radar system for metallic object detection using reflection configuration. The swept frequency response and resonant frequency behaviour of a number of metallic objects, in terms of position, object shape, rotation and multiple objects have been tested and analysed. The system working from 1 to 14 GHz has been set up to implement sensing of metal items at a standoff distance of more than 1 meter. Through a series of experimental investigations, it can be found that the optical depths derived from the Fourier Transform of the power spectrum profile is in close relation with the relative location of the metallic object. The cross correlation between coherence-polarisation and cross-polarisation RF returns can be used to distinguish different objects. Therefore the optical depth and the cross correlation can be used as useful features for metallic object detection and characterisation in this portion of the microwave frequency spectrum.

Quantitative millimetre wave spectrometry. Part I: Design and implementation of a tracked millimetre wave confocal Fabry-Perot cavity spectrometer for gas analysis

This paper describes the design criteria and construction of a compact Fabry-Perot cavity spectrometer with frequency modulation (FM) signal recovery, for use in the 70–220 GHz region. A solid state Gunn oscillator was frequency locked to a radiofrequency synthesiser and used as the primary source. The confocal cavity comprised two brass spherical mirrors, one made part of the sample chamber and the other attached to a precision translation stage with piezoelectric displacement. The sample chamber used a PTFE cup with a central thin window 30 mm diam. supported by the rest of the cup structure which also acted as a spatial filter to suppress non-axial modes. The high quality factor cavity (Q = 1 × 104) was maintained resonant with the source frequency by locking the detected FM signal in phase with the incident FM impressed upon the source, in a control loop which adjusted the cavity dimensions using the piezoelectric transducer. The first harmonic (2F) of the FM (F) was measured to recover the second derivative of the spectral line profile, as the source was scanned across it. Examples of spectra arising from 9 ppm HDO, 2.7 ppm OCS (in an excited vibrational state), HCH18O, (2000 ppm natural abundance), HCH17O (373 ppm natural abundance) and acrylonitrile at approximately 300 ppm, demonstrate the high sensitivity and versatility of the technique.

Ultra wide band detection of on body concealed weapons using the out of plane polarized late time response

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.

Quantitative millimetre wave spectroscopy. part II: Determination of working conditions in an open Fabry-Perot cavity

Abstract A confocal Fabry-Perot frequency modulated cavity spectrometer of quality factor 1.25 × 10 5 operating inside a chamber maintained at ambient temperature and pressure of 1 Pa to 1 KPa was employed for spectrometric measurements in the region of 72 GHz and 140 to 160 GHz. The spectrometer used a spatial filter to suppress unwanted, non-axial modes. The solid state microwave source frequency was derived from a phase-locked frequency synthesizer and detection was by a liquid helium cooled bolometer. Transitions in acrylonitrile, formaldehyde, and sulphur dioxide were studied demonstrating parts per million sensitivity for these species in atmospheric samples, whilst carbonyl sulphide samples were detected at sub-parts per million concentration. The effect of pressure on line intensities was studied in order to determine the optimum operating regime. It was found that the technique was not restricted to the 5–50 Pa region characteristic of centimetric wave spectroscopy, but was able also to function in the 0.1 to 1 KPa regime. Furthermore the intensities in this latter region were found to be not critically dependent on sample pressure. A treatment of the effect of pressure and depth of frequency modulation on absorption signals was carried out and the resulting theory applied to the observed intensity-pressure relationships. There was good quantitative agreement between the frequency modulation depth and cavity response characteristics and qualitative agreement between the pressure, frequency modulation and spectral line intensity characteristics. It became clear that the possibility of power saturation, coupled with the non-uniform power distribution within the cavity, was affecting the fits of theoretical curves to the observed data, and that taking this into account produced marked improvement in the fits. Nonetheless the treatment permitted some practically useful conclusions: at low modulation depths and pressures, the sharp spectral absorption peak makes identification of the target species easy, but extraction of quantitative information more difficult, as the intensity will depend critically on the power level in the cavity. At pressures in the 100 Pa region however, the signal obtained is maximal, power broadening minimal and comparative intensity measurements possible over a range of sample species and concentrations.

A multifaceted active swept millimetre-wave approach to the detection of concealed weapons

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 objects 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.