Wanted: Next-gen tech for weapons detection
By Charles J. Murray
EE Times
(09/14/01 14:02 p.m. EST)
PARK RIDGE, Ill. — News that terrorists used knives and box cutters as weapons in this week's hijackings has raised questions about airport metal detectors and underscored the need for a security technology capable of recognizing nonmetallic objects.
Security experts said this week that metal detectors leave a gaping hole in airport safety, mainly because determined wrongdoers know the weaknesses of such systems. The primary problem, they say, is that terrorists have access to a growing array of nonmetallic weapons, ranging from ceramic knives to plastic guns, which they can tuck underneath jackets and smuggle past metal detectors.
"You don't need to be a rocket scientist to know that metal detectors can't detect a ceramic knife," said Gabriel Rebeiz, professor of electrical engineering at the University of Michigan in Ann Arbor, and an expert in security technology. "Most people who want to bring a weapon into an airport are well aware of this."
Still, experts said that airports around the United States continue to use gateway-type metal detectors to scan individuals entering restricted areas, primarily because alternative technologies face barriers in terms of cost, reliability, safety and privacy.
For airport security personnel and the public in general, however, the inability to detect such nonmetallic objects is becoming a critical problem, especially as the list of such devices grows. "There are 9-millimeter automatic weapons that can now be easily altered to go through a metal detector," said Richard Huguenin, executive vice president and chief technology officer of Millivision LLC (Amherst, Mass.). "People buy them at gun stores and replace the metal parts with ceramics and plastics."
In addition to such easily alterable guns, consumers can also purchase knives made from ceramics or carbon fibers. Carried in a person's belt, under their pants or beneath a jacket, such weapons won't set off metal detectors, Huguenin said.
Imaging technology needed
To combat those problems, the Federal Aviation Administration (FAA) this past summer asked for proposals for technologies to sense the presence of concealed weapons. FAA officials, who were not permitted to make public statements after this week's events, are said to be considering several technologies to augment, or succeed, metal detectors.
The challenge for engineers responding to the FAA request is to develop a technology that can detect the presence of weapons under a person's clothing. Experts say that monitoring a person for weapons is a challenge that's distinctly different from that of examining baggage, because the task must be done quickly and safely, without violating anyone's privacy. At the moment, imaging systems seem to be the strongest candidates, mainly because weapons now vary so broadly in terms of size and materials.
One type of solution currently under consideration is imaging systems that use millimeter-wave technology. The millimeter wave, which lies on the frequency spectrum between microwaves and infrared waves, derives its name because it operates at wavelengths between 1 and 10 mm. Engineers have developed two types of such systems: those that use millimeter-wave receivers to measure the thermal emissions from bodies; and those that illuminate the target with millimeter waves and measure the reflected signal.
Systems that use millimeter waves to measure thermal emissions operate like the radiometers that astronomers use to measure the radio energy from distant stars. Such systems don't generate their own signal; rather, they look at the natural "black-body emissions" generated by any object above 0 Kelvin (absolute zero).
"Your body emits a signature radiation in the millimeter-wave range," said Huguenin of Millivision, which makes three different types of millimeter-wave systems. "If a person has a gun or knife tucked in their belt, that object will block the radiation emitted by the body." The contrast between the object and the person's body creates a bright outline that can easily be seen on an imaging display.
Such systems depend primarily on two parameters — temperature and emissivity — to create an image. If the contrast between those parameters is high, Huguenin said, the image is sharp. A human body and a metal gun form a strong contrast, for example, because the body's emissivity is high (about 90 percent efficiency), while that of metal objects is low (near 0 percent). The contrast between the human body and a ceramic knife is not as high, but is sufficient to create a relatively sharp image, Huguenin said.
Millivision offers a portable unit the size of a radar gun; a gateway scanner; and a video surveillance-type camera unit, all based on millimeter-wave technology. Each system uses an electronic focal-plane array to measure the amount of radio energy emitted by the source. "It's just like a camera except that it operates at very long wavelengths," Huguenin said.
Ongoing development
Such equipment contrasts with so-called active-millimeter-wave systems, like those developed by Battelle Memorial Institute's Pacific Northwest National Laboratory (PNNL).
The PNNL system uses an array of antennas that transmit and receive a millimeter wave. The sensor array illuminates the target, then captures the data from the receive array and builds an image that is transmitted to a nearby display.
To deal with the large amounts of scanning data, the system's computer employs eight Sharc digital signal processors from Analog Devices Inc. operating in parallel. The eight DSPs, each of which has a peak performance rating of 120 Mflops, yield almost 1 Gflops of performance. During operation, the system can scan a person in 1 to 2 seconds and create an image in as little as 4 seconds.
"With this technology, it's fair to say that we could augment, or even replace, a metal detector," said Doug McMakin, developer of the system and a staff engineer at PNNL.
Engineers say, however, that the primary advantage of the millimeter-wave system may, ironically, also be its weakness. The technology's ability to see through clothing and build a crisp image may create privacy issues among travelers. PNNL's system, which dates back to the late 1980s, is being redeveloped to address the privacy issue, but engineers say that solutions won't come easily.
Experts say that the road to creating a metal detector replacement could still be a long and arduous one. In addition to the privacy issue, engineers still need to deal with other inherent problems, including costs.
While metal detectors typically run about $5,000, experts believe that millimeter-wave systems will be lucky to come in under $50,000 apiece. PNNL engineers suggest that their system might cost closer to $100,000.
"They [PNNL] have good technology, but the question is whether they can even come close to $50,000 per camera," said Rebeiz of the University of Michigan. "Under the circumstances, people will now accept higher cost, but we don't know how much higher."
Makers of millimeter-wave systems argue that the initial costs aren't important. "In terms of the overall cost [of hijacking incidents], it's a drop in the bucket," Huguenin said. "The cost of personnel and potential legal action is far greater."
Experts also wonder where the expertise will come from to perfect and maintain such systems. "These systems operate at 100 GHz, and that's very, very fast," Rebeiz said. "Companies that can make transistors which operate reliably at these frequencies are few and far between."
Ultimately, aviation experts believe that real security will require far more than millimeter-wave detection systems. Some believe that airports will need to look beyond the weapon and focus on the person as well. By using such tools as biometric and physiometric profiling, officials hope to do on-the-spot background checks and look for signs of unusual, nervous behavior. Such extreme measures might one day be necessary, they believe.
Whether such techniques can be employed, however, may ultimately depend on the will of society, experts said. |
