Bomb and Weapon Detection by Millimeter Wave Imaging
By John McNicol, Director of Business Development, MMIC Solutions UK

Terrorism has become a constant threat in recent years. Gun and knife crime is also on the increase in many countries. So, the detection of weapons and explosives concealed under clothing has emerged as one of the most urgent security issues of our time. However, the technical challenges in successful detection are substantial. In some scenarios, large numbers of people must move rapidly through airport concourses, railway stations, and other buildings. In others, subjects cooperate by divesting themselves of phones, coins, etc. In still others, people must be screened at a “stand-off” distance, sometimes covertly without their knowledge.

The traditional approach is detection of the electromagnetic signature of metal objects. Developed over many years, metal detector systems can now detect very small quantities of both ferrous and non-ferrous metals in, for example, foodstuffs. Metal detector archways are used to screen people in airports worldwide, balancing sensitivity and detection against the rate of false alarms which severely constrict passenger throughput. However, metal detectors are ineffective against new, non-metallic, threats such as plastic and ceramic weapons and explosives.

New Threats – A New Technology
In the face of new challenges, new approaches are being developed. Imaging using low levels of back-scattered X-rays has been proposed for cooperating passenger checkpoints. Excellent resolution is countered by slow throughput and public resistance to being irradiated with X-rays. A very promising new approach uses millimeter waves (MMW) which pass through most fabrics to form images of concealed metal and non-metallic objects.

Ka and W-bands are both used in imaging. The higher frequency band enables higher resolution, but also requires more expensive components. Between 20 and 30GHz an “active” radar-like approach is needed for resolution good enough to detect objects of a few centimeters (one inch). Low levels of MMW radiation are transmitted, and the reflected signals used to build an image of the contours of the body. Remote screeners examine the image for anomalies and inform checkpoint operators, as the level of detail and artefacts in the image make automated detection with low false alarm rates much more difficult.

Systems at W-band are usually entirely passive, using naturally occurring MMW, and the inherent “temperature” of the body, to build an image similar to those from infra-red thermal cameras. Working on the contrast between objects and the background body, passive MMW imagers have far fewer problems with concerns over personal privacy. In addition, threat detection can often be performed automatically by software analysis of images, reducing dependence on the human factors.

Ka band imagers for passenger checkpoints can mechanically rotate a detector array around a standing person to provide a 270° image. In this case, the benefits of close-range, almost 3D, images are balanced against the time taken to perform the scan on queuing passengers. Recent Ka band imagers are now using phased array techniques to scan subjects with large numbers of beam steering nodes. At W-band, “pseudo optics” are common, using mirrors or lenses to focus the MMW onto an array of receivers (a Focal Plane Array). To avoid sudden changes in direction at the end of each line of an X-Y scan, off-axis mirrors rotating at constant speed are a common scanning solution.

Resolution and Sensitivity
From experience with camera phones and digital cameras, everyone knows that an image made from more pixels will contain more detail and be more useful (e.g. for detection purposes). Resolution is the key parameter both in still images and in video clips. Optical effects, such as diffraction, affect the resolution of MMW imagers. The smallest object an imager can resolve is driven by the aperture of the system, the target range, and the wavelength. Most imagers for detecting weapons and explosives at a range of two or three meters achieve a “spot size” of a few millimeters, using apertures less than one meter.

Resolution is also very dependent on the number of receivers, which happen to be the most expensive component of MMW imagers. Unlike optical cameras whose chip sensors comprise millions of pixels, MM-wave imagers scan 100-200 discrete detectors across the field of view between four and ten times per second to provide near real-time video.

In passive MMW imaging, the time the receivers have looking at each ‘pixel position’ (the integration time) affects their sensitivity, the minimum “temperature” difference they can detect. This obviously determines the type of threats such an imaging system can detect. Today’s best W-band receivers can distinguish a Noise Equivalent Temperature Difference (NETD) of 0.4 Kelvin in a room temperature scene around 300K. Enhancing threat detection by improving sensitivity requires both better receivers and extended integration times. Constrained by the need for high speed image acquisition for near real-time video, higher numbers of receivers address both the sensitivity and the resolution requirements of future MMW imagers.

MMW Receivers
The phase and polarisation can also be useful at a system level to distinguish objects in the MMW images. Ka-band imagers often use heterodyne receivers, which can provide some additional information (e.g. phase). However, at W-band the greater complexity and cost of a coherent down conversion is prohibitive, and W-band imagers almost exclusively use lower cost “direct detection” receivers, measuring the total power received across the operating frequency band.

The amount of energy emitted by the human body is 100 times less at, for example, W-band than the level of infra-red radiation. For this reason, Low Noise Amplifiers (LNAs) made from advanced compound semiconductors such as Gallium Arsenide (GaAs) and Indium Phosphide (InP) are required to give high gain (some W-band receivers require more than 50dB gain). Highly responsive detector diodes are also required to give high output voltage from the received MMW energy. Low noise figure is also vital as the receiver’s own noise directly affects its sensitivity, the minimum “temperature” difference it can detect.

In addition to the active components, MMW receiver modules are expensive due to precision machining of metal, and manual assembly, tuning, and test. The receiver array is almost always the largest cost component of a MMW imaging system, which limits the number of receivers and the system resolution and sensitivity.

MMIC Solutions in the UK is specialising in MMW imaging. Its innovative technology enables the use of lower cost materials and module construction techniques, as well as allowing automated manufacturing of the receiver modules. By managing the feedback paths, oscillations, and resonances that occur inherently at MMW (when the wavelength is the same size as the features of the semiconductor packaging), its receiver modules are circuit boards rather than machined metal boxes, screwed together. This enables a substantial reduction in cost, and also a smaller form factor to allow receivers to be more tightly packed in an array, supporting higher resolution imager systems. The new MSi200 series of modules are thought to be the smallest W-band receivers available anywhere. For the future, the technology also enables higher levels of chip integration than has so far been possible at MMW frequencies.

Stand-Off versus Portals
Threat detection by MMW imaging is now being deployed at checkpoints worldwide where cooperating passengers walk-through “Portals.” With a similar Concept of Operation (ConOp) to metal detector archways, the subject is less than two meters from the imager, and turns 360° in a well-controlled indoor environment to maximize the probability of detection. Two imagers can also be set up in a walk-through area to provide a simultaneous front and back view without the subject having to stop and turn.

The new challenge facing scientists is to extend the range to support “Stand-Off” imaging of people in queues or in waiting areas, or approaching a checkpoint, both indoors and outdoors. This is obviously invaluable for detecting large person-borne explosives, such as suicide vests, which can cause injury and damage over a wide area. W-band systems with an aperture under one meter and a range greater than seven meters are now being used. To achieve good resolution and longer range, without a considerable increase in size of the aperture and equipment, even higher frequencies are now being used.

For medium-range Stand-Off screening of crowds, queues, etc., smaller handheld MMW systems are also being developed. This kind of covert threat detection will be crucial for intelligence-led operations by law enforcement and security services.

Conclusions
Due to the ability to penetrate clothing, MMW screening is fast becoming the best approach for detecting the new threats, non-metallic weapons and explosives, concealed under clothing in a wide range of deployment scenarios. With new, low-cost, receiver technologies now becoming available, future systems will undoubtedly have much larger numbers of receivers providing substantial increases in resolution and sensitivity to enhance detection of these threats.

MMIC Solutions UK
www.mmicsolutions.com
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