The recent tragic events at the Boston Marathon illustrate the growing and critical need for the detection of person-borne improvised explosive devices (PBIEDS). Such systems, in fact, are currently under development under the guidance and supervision of the Department of Homeland Security. These sophisticated next-generation radar systems are being designed to detect bombers at long distances, in crowded areas, and even at non-fixed locations.
Unfortunately, PBIEDS are often shaped from a variety of metals and concealed under clothing or in backpacks, making them extremely difficult to detect. With a typical blast ratio of 50 meters or more, close-up detection methods such as airport-style scanning booths and pat-downs are of limited value, given that detonation would still claim many innocent victims.
Instead, improving the ability to detect explosives at a distance demands sophisticated standoff detection systems capable of scanning individuals in a crowd at a distance in mere milliseconds. The systems already in development are occurring under the guidance and support of the Department of Homeland Security (DHS), including by the ALERT (Awareness and Localization of Ex plosives Related Threats) Center, a partnership of academic, industrial, and government entities dedicated to improving the detection, mitigation, and response to explosives-related threats.
The concept currently being developed by ALERT involves multiple radar units that can be pointed in the direction of crowds of people that are approaching a venue, checkpoint, or other area of entry. The system would scan each individual at a distance of 50 meters or more to identify suicide bombers who appear to be dressed normally, but are concealing IEDs strapped to their bodies.
Fulfilling the need for detection in large gathering areas — such as concerts, parades, political rallies, protests, and sporting events — the portability of the equipment is another key component. For this, the ALERT radar could be mounted to a van or truck for wide-ranging field use. Permanently mounted solutions would also be available for high-security buildings, checkpoints, or border crossings.
In 2008, the DHS selected Northeastern University as one of 11 universities for a DHS Center of Excellence. The $12 million grant established the ALERT Center at Northeastern. “For the suicide bomber problem, we need a high-performance radar system that can send out very specific types of signals half a football field away and identify specific features under clothing,” said Dr. Carey Rappaport, Distinguished Professor of Electrical and Computer Engineering at Northeastern.
According to Dr. Rappaport, various methods of wave detection systems, including X-ray and black-body radiation, were initially researched and investigated before he ultimately selected a millimeter-wave-based system.
Although X-ray-based systems are widely used for passenger screening at airports, an X-ray-based system for standoff detection presents many challenges and concerns. Traditional transmission X-ray images like those used by the medical industry result when X-rays pass through an object to a detector located on the other side. Objects with greater X-ray density block or absorb more Xrays than objects with lesser density, creating an image. And while X-rays can be beamed effectively at a distance of 50 meters, they produce ionizing radiation, which is a known carcinogen, and the dangerous dose level is hard to quantify.
The other option was X-ray backscatter scanners like those used for passenger screening. This type of scanning uses high-energy X-rays, which instead of passing through, reflect (scatter back) from objects. The amount of backscatter is picked up by detectors from the same general direction as the source to yield a high-resolution image designed to reveal concealed objects. Xray backscatter units have come under intense scrutiny over concerns of lack of privacy as they reveal much of the human anatomy.
The ALERT Center’s research eventually led it to select a millimeter-wavebased system design. Millimeter waves are a subset of the microwave band, which in turn is part of the larger radio wave band. These waves operate within a frequency range of 30-300 GHz. Unlike X-rays, millimeter waves are non-ionizing and universally considered non-carcinogenic. Until recently, millimeter wave technology has largely been used by the military until plummeting hardware costs have opened up this band to more commercial applications.
Collaboration with Industry
In early research, the ALERT Center has successfully demonstrated that standoff detection of person-borne IEDs concealed under clothing can be accomplished with millimeter-wave radar and advanced synthetic aperature radar processing techniques (the technique used by NASA spacecraft to take photos of a planet’s surface as it flies by). However, the next phase in the testing required development of a complete radar sensor, one that involved a number of components including multiple millimeter wave transmitters, receivers, and antennas.
HXI, a subsidiary of Renaissance Electronics Corp. and a supplier of millimeter- wave products, components, and subsystems, learned of the ALERT Center’s mission and proposed to develop and provide all the necessary “proof-of-principle” radar modules for the project.
To increase the field of view and pick out fine depth features, the millimeter-wave radar sensor needed to be multi-static — containing multiple radar components located in separate locations with a shared area of coverage. The spatial diversity allows for different aspects of a target to be viewed simultaneously, and the data fused together to generate an image with high resolution. However, the concept of multiple radars in different locations far apart creates challenging triangulation issues when attempting to focus on a single individual within a crowd.
Instead, the multi-static system would involve multiple radars mounted with as much distance as possible, but still designed to fit on a single vehicle roughly the size of a delivery truck. With even several meters of separation between antennas, the system could effectively distinguish features on people at a range of 50 meters.
The strength of the millimeter band is its unique set of properties, many of which lend themselves to improved resolution for multi-static radar systems using advanced synthetic aperature radar processing techniques such as this one. Image resolution is based on the cross-range and range measurement resolution that can be achieved by the system, both of which are tied to the bandwidth of the signal — the greater the bandwidth, the higher the resolution.
Fortunately, there is a tremendous amount of available bandwidth in the millimeter-wave spectrum — much more than at lower frequencies. Transmission of large volumes of data is possible at lower frequencies, but they lack the continuous bandwidth allocations. At lower frequencies, for example, typical allocations are 2-5 MHz only. In the millimeter-wave spectrum, there are 5, 7, 10, 15, or even 20 GHz of allocation with a total potential up to 250 GHz. This bandwidth availability means that a very short pulse can be used to interrogate a target. Measuring the time it takes for this pulse to echo back from the target lets one accurately determine how far away it is. For this project, HXI is delivering the multi-static radar system that operates in the 70-77 GHz range.
“The larger bandwidth means you can get narrower slices of distance, and this is irrespective of how far the target is from you,” said Dr. Rappaport. “So, for example, at a higher bandwidth, you can distinguish something that is 2 centimeters in front or behind another object. If you are looking for a one-inch-diameter pipe bomb strapped to a human body, that would be sufficient.”
The cross-range resolution is also improved based on the high frequency of the millimeter wave, because the width of the aperture is measured in wavelengths. However, through sophisticated signal processing, the system would be able to collect enough information in the radar signal to delineate an object that doesn’t meet the smooth contours and characteristics of skin; for example, is metallic, or otherwise meets other characteristics of person-borne IEDs. If such inconsistencies were flagged, the individual could be detained and searched before reaching more populated areas.
With its phased array, the system will be able to focus in on a specific small scene area of one person or a crowd of people. “By adjusting how much signal goes to each antennae, you can electronically ‘steer’ a beam back and forth,” explained Dr. Rappaport. “In doing so, you can bounce between an individual, the person next to him, and maybe three or four people off to the side.”
Agencies that could benefit from a standoff millimeter-wave detection system include the Transportation Security Administration (TSA), the military, security companies, and other agencies involved in homeland security.
This article was contributed by Renaissance Electronics, Harvard, MA. For more information, Click Here