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Millimeter-wave imaging may be used to assist in\ud the problem of wastepaper recognition during the sorting and\ud recycling process. Currently, there are a variety of techniques,\ud which are used within materials recovery facilities to sort waste\ud paper and card; however, improvements in sorting capability can\ud be realized using a multiple sensors to provide complementary\ud (orthogonal) data. This paper discusses the issues related to\ud the processes, which are currently used for sorting of recycled\ud wastepaper and the phenomenology for millimeter-wave imaging\ud of a wastepaper material. Experimental measurements and\ud images in the millimeter wave band are presented, which support\ud the proposed application.

We present a millimeter radar for threat level evaluation developed for the detection concealed threats, such as guns and person borne improvised explosive devices (PBIED). The system uses a Gaussian optic lens antenna to achieve stand-off ranges up to 25 m. Ultra-wideband swept frequency radar, using direct detection receivers, is implemented to achieve short radar range resolution . The system is capable of detecting a wide range of objects positioned in front of the body by interpretation of the scattered waveform. Threat detection is rendered autonomously by a neural network that processes the scattered polarimetric, depth domain radar waveforms. The system may be configured to alarm or reject certain classes of objects, allowing for the detection of specific or broad spectrum threats. The radar system is portable and manually steered by the operator to enable standoff monitoring of walking human targets in real time. A video feed provides the operator with a wide field of view that allows tracking of persons and greatly facilitates aiming of the sensor. Rapid (1 ms) radar sweep times and fast signal acquisition and processing are implemented to provide threat detection at video frame rates (30 fps). Performance parameters for the detection of hand-guns and simulated PBIED are presented for ranges up to 25 m.

This paper describes how information about the electro- magnetic structure of targets can be obtained from direct detection radar techniques, where the relative phase of the transmitted and re- ceived signals is not measured. A comparison is made between the re- solved structure of a simple test target from an ultra wide band, pulse synthesis direct detection radar system at 14{40GHz and an equiv- alent heterodyne radar receiver where phase information is recorded. The test targets employed are wax sheet of thickness 20mm and 80mm which are illuminated alone and in contact with the human body. A vector network analyser is used as the radar system. The simplicity of constructing ultra wide band direct detection radar systems combined with their cost makes the use of such radar systems appealing for appli- cations such as concealed threat detection and non-destructive testing, where absolute range to the target, if required, can be determined by other methods.

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.

The feasibility of screening hand luggage for concealed threat items such as Person-Borne Improvised Explosive Devices (PBIED's) both metallic and non-metallic, together with handguns and at millimetre wavelengths is investigated. Previous studies by the authors and others indicate that hand baggage material and fabric is much more transmissive and has less scattering at lower millimetre wave frequencies and the ability to use K-band active imaging with high spatial resolution presents an opportunity to image and hence recognise concealed threats. For this feasibility study, a 1.6 m aperture, 35 GHz security screening imaging system with a spatial resolution of 2.5 cm and a depth of field of around 5 cm is employed, using spatially incoherent illuminating panels to enhance image contrast. In this study, realistic scenarios using backpacks containing a realistic range of threat and non-threat items are scanned, both carried and standalone. This range of items contains large vessels suitable for containing simulated home-made PBIED’s and handguns. The comprehensive list of non-threat items includes laptops, bottles, clothing and power supplies. For this study, the range at which imaging data at standoff distances can be acquired is confined to that of the particular system in use, although parameters such as illumination and integration time are optimised. However, techniques for extrapolating towards effective standoff distances using aperture synthesis imagers are discussed. The transmission loss through fabrics and clothing that may form, or be contained in baggage, are reported over range of frequencies ranging from 26 to 110 GHz.

At airports, security screening can cause long delays. In order to speed up screening a solution to avoid passengers removing their shoes to have them X-ray scanned is required. To detect threats or contraband items hidden within the shoe, a method of screening using frequency swept signals between 15 to 40 GHz has been developed, where the scan is carried out whilst the shoes are being worn. Most footwear is transparent to microwaves to some extent in this band. The scans, data processing and interpretation of the 2D image of the cross section of the shoe are completed in a few seconds. Using safe low power UWB radar, scattered signals from the shoe can be observed which are caused by changes in material properties such as cavities, dielectric or metal objects concealed within the shoe. By moving the transmission horn along the length of the shoe a 2D image corresponding to a cross section through the footwear is built up, which can be interpreted by the user, or automatically, to reveal the presence of concealed threat within the shoe. A prototype system with a resolution of 6 mm or less has been developed and results obtained for a wide range of commonly worn footwear, some modified by the inclusion of concealed material. Clear differences between the measured images of modified and unmodified shoes are seen. Procedures for enhancing the image through electronic image synthesis techniques and image processing methods are discussed and preliminary performance data presented.

This paper investigates the use of Late Time Response (LTR) analysis for detecting multiple objects in concealed object detection. When a conductive object is illuminated by an ultra-wide band (UWB) frequency radar signal, the surface currents induced upon the object give rise to LTR signals. The LTR results from a number of different targets are presented. The distance between the targets within the same radar beam has been adjusted in increments of 5cm to determine the point at which the individual objects can be distinguished from each other. The experiment was performed using double ridged horn antennas in a pseudo-monostatic arrangement. Vector network analysers (VNA) are used to provide the UWB stepped frequency continuous wave radar signal. The distance between the transmitting antenna and the target object is kept at 50cm for all the experiments performed and the power level at the VNA was set to 2dBm. The targets in the experimental setup are suspended in isolation in a non-anechoic environment. To allow for the de-convolution of the signal and the removal of background clutter Matlab was used in post processing. The Fast Fourier Transform (FFT) and Continuous Wavelet Transform (CWT) are used to process the return signals and extract the LTR features from the noise clutter. A Generalized Pencil-of-Function (GPOF) method was then used to extract the complex poles of the signal. In the case of a single needle these poles can be found around 1.9GHz.

The authors discuss a phased array antenna design consisting of multiple, modular, flat panels the number of which is determined by specific application requirements. Each panel has 512 antenna elements which are driven by their own Application Specific Integrated Circuits, in order to control the signal phase needed for electronic beam steering. Furthermore, the antenna elements can transmit (or receive) in orthogonal polarisation states. In the first embodiment, Ku-band microwave signals (receive over 10.95-12.75 GHz and transmit over 14.0-14.5 GHz) are phase-aligned, down converted to baseband and summed, permitting the receiver and the digital control circuitry to be fabricated on one low-cost SiGe ASIC. It is envisaged that this approach will be a cost-effective alternative to mechanically steered dish antennas for radar, satellite communication and point to point communications, while bearing the advantages of flat or conformal structure and no moving parts.

The use of active millimeter wave radar has proven successful in the field of Concealed Weapon Detection. Time resolved signals acquired from the radar scans are pre-processed and classified using an Artificial Neural Network. A problem with this method occurs in the training of the ANN, where the network must be trained on each weapon type that it is to reliably classify. Any deviation from the training weapon sub-class type leads to a decrease in the classifier's performance. This is illustrated in the experimental results. A possible improvement on this method is outlined in the form of including signals reconstructed using Principal Component Analysis into the training set.

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.

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 ob ject 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 s cattered 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. Keywords: Concealed object detection, microwaves, standoff detection, aspect independent.

The decomposition of the late time, transient response of metallic objects illuminated with broadband microwave energy into damped sinusoidal signals which are characterised by aspect-independent poles, is used extensively as a method of radar identification of aircraft and missiles. This method is utilised here for the detection and discrimination of handguns and non-threatening items, such as mobile phone handsets, cameras, keys and pens. It is of critical importance to explore the limits of this method, if it is to be utilised for the detection of such objects concealed on the human body or in bags carried on or close to the body that the limits of this technique is explored. Here, investigation is made by computer modelling of the complex natural resonances of two different handguns and these are compared with those for two non-threat items: a mobile phone handset and a fountain pen. Dependence of pole positions on small details present on a handgun is also investigated as is the effect of combining threat and non-threat items in close proximity. A way of enhancing the natural resonances by the use of a cross polarised receiver is also investigated.