Preventing Aircraft Bombings

Advanced explosion-detection technologies are beginning to make a positive change worldwide in the quality of aviation security.

Commercial aviation is an integral part of the American economy. More than 546 million passengers flew within the United States in 1997 (Air Transport Association, 1998), an average of more than 1.5 million people a day. An additional 52 million people boarded international flights that same year. Of the world's 10 busiest airports, 8 are in the United States. With a continually growing population of Americans on the move, the Federal Aviation Administration's (FAA's) mission to promote and ensure the safety and security of air travel in the United States proves more difficult every day. In spite of the challenge, the FAA has fostered an air travel system that is safer than virtually any other means of transportation (White House Commission on Aviation Safety and Security, 1996, 1997).

Civil aviation security in the United States is the shared responsibility of the FAA, the air carriers, and the over 450 airports that have scheduled commercial flights. The FAA is responsible for developing the regulations that ensure security, assuring compliance with these regulations, interacting with international security partners, conducting research to develop technology to improve security, and most recently, purchasing and deploying this advanced security technology. Air carriers are in charge of the in-flight safety of passengers; they accomplish this by screening passengers, baggage, and cargo to prevent explosives, weapons, or other dangerous objects from getting on an airplane. Airports are responsible for providing a safe and secure location for air travel and air travelers. The need to ensure passenger safety while also providing opportunity for air carriers and airports to make a profit results in a dynamic tension among the three parties.

One area of FAA focus has been the development of technology to detect weapons and explosives contained in passenger baggage or carried on passengers themselves. An increase in hijackings in the late 1960s led to establishment, through the Air Transportation Security Act of 1974 (P.L. 93-366), of the FAA anti-hijacking program. The program spurred the introduction in U.S. airports of the now-familiar metal-detection screening portals for passengers and the X-ray inspection systems for carry-on baggage. In the early 1970s, hijackings were occurring at a rate of more than 1 every 2 weeks. More recently, events such as the 1988 destruction of Pan American Airlines Flight 103 over Lockerbie, Scotland, by a terrorist bomb and the 1996 catastrophic loss of TWA Flight 800 by still-undetermined causes, have added urgency to efforts to develop new technologies that could find a single bag containing explosives among the millions of bags loaded onto airplanes each day.

There are many obstacles to deploying such technologies. For example, the introduction of walk-through metal detectors and carry-on baggage screening X-ray systems in the early 1970s created negative public perceptions, raised questions about the legality and safety of the searches, and prompted concerns about the cost of security-system deployment and use.

Ultimately, the public accepted that the FAA, air carriers, and airports had to take steps to reduce the number of dangerous objects allowed onto airplanes. Questions about the constitutionality of the searches were addressed by subjecting each passenger to the same search criteria and by limiting the scope of the search to only those objects that could be a danger to the aircraft, crew, or passengers.

The initial deployment of screening devices was paid for by the federal government; subsequent purchases have been the responsibility of the air carriers. The expenses associated with operating the devices also falls to the carriers. Standards were established for the X-ray baggage-screening equipment and the walk-through metal detectors to ensure there was no health hazard to either the public or the security work force. The United States currently has a total of about 1,400 walk-through detectors and a similar number of carry-on baggage screening systems. Minimizing operational costs by matching the number of open checkpoints to the peaks and valleys of passenger flow through the day is a challenge for air carriers.

Funding for Advanced Security Technologies
Issues such as initial cost, level of personnel required, and matching personnel to demand must be addressed every time a threat-driven decision is made to deploy an additional piece of technology. To raise the level of aviation security throughout the country and to introduce state-of-the-art equipment, the FAA is again funding the procurement of advanced security technologies. This recent deployment includes automated explosive-detection systems, X-ray systems that alert the screener to threat items, and highly mobile trace explosive-detection devices.

Identifying explosives within a passenger bag has some parallels to classic fault inspection. Such tasks as finding cracks in the walls of nuclear containment vessels, identifying cold-welds in automobile body pieces, or verifying proper shape and depth of contact holes in integrated circuits have driven large and successful efforts to develop inspection techniques and related computer analysis. However, explosives detection is different from these inspection tasks in two important ways: There is no way to know in advance what a passenger's bag should look like, and there is no way to know in advance what a terrorist bomb will look like.

Passengers' bags can contain anything that can be purchased, and a great deal more. And there is no standard method for packing a bag. Convolving the number of items you might expect to find in a passenger's bag with the variety of ways that these items can be arranged within that bag, not to mention the diversity of bags available, leads quickly to the realization that it is not possible to construct a "typical bag" that could be used as a model system. For the "fault" being detected, the FAA has identified a wide range of materials that could damage an aircraft, each of which could take on many different shapes. Many of the techniques developed for fault inspection, especially advancements in multidimensional analysis, are useful in explosives detection. But the most challenging task by far is identifying a property of explosives that can be distinguished quickly and easily from the properties of the benign contents of a passenger's bag.

Bulk and Trace Detection
The goal of passenger and passenger-baggage inspection is to get people and their bags on airplanes quickly, without passing a person or bag that is carrying a threat to the aircraft. As anyone who travels by air today knows, the current system of passenger and baggage inspection is prone to identify belt buckles, steel-shanked shoes, and liquids in suitcases as potential threat objects that warrant further inspection. The result is delay to that passenger, and likely to all who are lined up behind. An ideal weapon- or explosive-detection technology would only identify threat objects, never sounding an alarm for benign materials, and would never miss a detection. The FAA has supported the development of a wide variety of technologies with the aim of identifying methods that promise to move the aviation security system toward that ideal.

Explosives-detection technologies can be divided into two categories depending on what material properties the technology exploits. Bulk detection technologies remotely sense a physical or chemical property of the object under investigation; trace detection technologies physically sample particles or vapor from the object under investigation. Because of their individual strengths and weaknesses, bulk- and trace-detection technologies are developing niche applications, which can be combined to offer more complete detection capabilities.

The remainder of this paper lays out some of the primary technologies for detecting explosives. We will first discuss technologies used to screen checked and carry-on baggage. These systems use enhanced X-ray, thermal neutron analysis, quadrupole resonance, computed tomography, and trace explosive-detection techniques. We will then address technologies for passenger screening. These approaches utilize low-intensity microwave energy, quadrupole resonance, millimeter-wavelength imaging, X-ray backscatter, and trace detection.

The first baggage-screening systems employed simple X-ray attenuation to produce a shadowgraph of the object being screened. This approach works well for high-contrast targets such as handguns, but is not as effective for more subtle targets like explosives. In the early 1990s, devices were developed that could probe using two different X-ray energies. These machines could differentiate materials of relatively high atomic number, such as the iron of a gun, and materials with low atomic numbers, such as explosives, from benign materials. These dual-energy systems are in use today. Typically, the bag being screened is imaged in discrete segments and those segments are automatically evaluated against explosive-threat criteria. Images obtained from bags containing segments that are not clearly distinguishable from the threat criteria can be inspected by a human operator. An important attribute of the dual-energy automated X-ray systems is their high throughput.

Throughout the 1980s, thermal neutron analysis (TNA) was explored for the detection of explosives concealed in checked baggage and cargo. Neutrons from radioactive decay or an electronic neutron generator were used. The neutrons react with the nitrogen in commercial and military explosives to produce a high-energy gamma ray. This 10.8 MeV gamma ray is rare and stands out from the background, allowing an estimation of the amount of nitrogen present. Following the downing of Pan American Flight 103, TNA systems were deployed in six different airports. The performance and operational availability of the systems were good, but they were not accepted by the air carriers because of their size, cost, relatively low throughput, and limited ability to detect small amounts of explosives.

So-called fast neutrons, which have been employed in the detection of contraband materials such as drugs, can also be used to detect certain explosives. The type of gamma rays given off as a result of fast-neutron scattering are characteristic of the elements encountered by the beam. Timing the arrival of a gamma ray following the interaction of the fast neutron with a nucleus allows one to determine the element's location in space. Combining the elemental and locational information allows for identification of explosives. Typical commercial and military explosives can be recognized by their characteristic ratios of oxygen, carbon, and nitrogen. Elements present in improvised explosives, for example chlorine and high levels of oxygen, may assist in the detection of explosives manufactured by an individual.

Transmission shadowgraphs can also be done using broad-energy-range fast neutrons. Specific elements in the beam will scatter selected neutron energies. Determining which energies are absent allows one to discern which elements are in the beam line, potentially indicating the presence of explosives. All of these neutron-based detection approaches are in the experimental stage. The pulsed fast-neutron technology is the most mature with an operational prototype under construction.
Quadrupole resonance uses radio-frequency radiation to excite the nuclei of selected atoms. Commercial equipment using this technology has been produced and tested on checked baggage and mail. The primary advantage of quadrupole resonance is also its major disadvantage: It is very specific, with discrete frequencies and pulse sequences for each explosive. There are virtually no false alarms, but the optimum pulse sequencing and frequencies must be discovered for each explosive. This can be challenging given the variety of explosives available and the compositional variation in even commercially produced explosives.

In 1994, FAA certified InVision's CTX-5000, an automated explosive detection system. The CTX-5000 takes selected tomographic slices through the object being screened and uses the information to make a decision on the presence of an explosive threat. The system is the only one demonstrated to detect threat quantities across the broad range of commercial and military explosives required for FAA certification. CTX-5000 systems are now deployed in airports in the United States and abroad. The FAA is in the process of purchasing over 50 units for air carriers to use in screening checked baggage. InVision and other vendors are developing systems with higher speed and lower costs.

R&D on an "Electronic Dog"
Experience and research have shown that individuals who are transporting explosives often have traces of the material on their hands and clothing. Dogs, which have very sensitive olfactory capabilities, are capable of detecting the characteristic scent of explosives or other ingredients in explosive formulations. For routine baggage screening, however, the animals present a variety of operational problems, including a short attention span and reduced detection capability when ill.

Scientists have been working to develop an electronic equivalent to the dog since the early 1970s. Several detection technologies similar to those used to screen baggage are able to detect traces of explosives. The current problem with these approaches relates to how the sample is collected. Previous research focused on collecting vapors given off by the explosives themselves. This approach will work for volatile explosives like the nitroglycerin in smokeless powder or dynamite, but it will not work for plastic military or commercial explosives. In the latter instances, intimate sampling may be necessary. Such sample collection could be done using contact paddles or an air shower. Indirect, nonintrusive sampling could be achieved by inspecting a handled object such as a passport or boarding card. Systems under development use either chemiluminescence or ion mobility to identify substances of interest.

Current commercial technologies employ ion-mobility spectroscopy or chemiluminescence detectors. Sample collection depends on vacuuming or wiping exposed surfaces with a glove or other collection medium. The shortfall of current technology is the need for intimate sampling (i.e., direct contact with surfaces that contain the residues of low-vapor-pressure military explosives).

Some of the systems employ very fast (typically 5-10 seconds) gas chromatography to separate the explosive molecules collected from all the other materials that may interfere with detection. Trace explosive-detection systems have been operationally evaluated in airports and are most commonly used to examine electronic items for concealed explosives. The nuisance alarm rate is less than 0.25 percent with a majority of false alarms attributable to the legitimate presence of explosive residues. The FAA purchased over 400 trace detectors and is deploying them in U.S. airports. These systems have been used in airports in Germany and other locations and to protect selected federal installations.

In addition to looking for traces of explosives on a person's body or clothing, several technologies are being developed that can detect threat quantities of explosives concealed under clothing. This type of detection presents challenges different from those of detecting these substances concealed in baggage. People have the expectation of being quickly and efficiently moved through security checkpoints with no perceived or actual insult to their personal privacy, health, or safety.

Low-Intensity Microwave
Low-intensity microwave energy can be used to measure the dielectric constant of objects present on the body. In one approach, an array of sensor elements is rotated around the person being screened, and changes in the dielectric characteristics of the field are measured. If explosives are present, their reflection or absorption of microwaves is recognized, an alarm sounds, and the position of the anomalous object appears on a wire-frame representation of the person. No human interpretation is required, and no actual images of the person being screened are presented.

The detection of explosives by quadrupole resonance is commercialized in a baggage-screening configuration. Quadrupole resonance uses a radio-frequency field to cause characteristic absorption and emission, giving a specific signature for the molecules of interest. Work is under way to adapt the technology to screen people using either a walk-through portal or a hand-wand sensor. The approach uses acceptable levels of electromagnetic fields and does not produce an image of the person being screened.

Millimeter wavelength electromagnetic radiation is given off by any warm body, penetrates clothing, and can be used to form an image. Active systems, where the person being screened is illuminated and objects on the body are imaged, are reaching maturity. One of the problems with this approach is that the image provides considerable detail, raising concerns about personal privacy. Scanning takes a few seconds with a low-intensity source. Passive systems, in which the human being is the radiation source, are in the research phase.

Very low intensity X-rays can be used to image objects on the body. Using the flying spot approach, a backscatter image of the body is taken. Fat and water, which make up most of the underlying surface of the body, are good low-energy X-ray scatterers. Objects on the body show up as areas of increased scatter or, if metallic, as X-ray absorbers. The approach currently requires that an operator interpret an image of the person being screened. The X-ray dose used is equal to a few minutes background at sea level. Nevertheless, the use of X-ray along with the resulting high-resolution image may pose a public acceptance problem.

Advanced explosive detection technologies are beginning to make a positive change worldwide in the quality of aviation security. Automation of baggage screening has the potential to further improve passenger safety and the convenience of air travel. The automated detection of explosives concealed on people is already becoming a reality. There will be challenges during the transition to automated systems. The size, speed, and high capital cost of the automated explosive-detection equipment are significant obstacles. However, the long-term survival of the aviation industry will depend on public confidence, which will be enhanced with the widespread use of highly reliable high- technology security systems.


References
Air Transport Association. 1998. [Online]. [October 12, 1998].

White House Commission on Aviation Safety and Security. 1996. Initial Report to the President, September 9. [Online]. [October 12, 1998].

White House Commission on Aviation Safety and Security. 1997. Final Report to the President, February 12. [Online]. [October 12, 1998].