1
Introduction

BACKGROUND

The security of the U.S. commercial aviation system has been a concern since the 1970s when hijacking became a serious problem. A number of aviation security programs have been implemented at public airports throughout the United States. However, weaknesses continue to exist. These weaknesses were observed and exploited by terrorists on September 11, 2001, enabling them to hijack four commercial aircraft, with tragic results. Terrorists and others with similar intent can be expected to continue to



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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons 1 Introduction BACKGROUND The security of the U.S. commercial aviation system has been a concern since the 1970s when hijacking became a serious problem. A number of aviation security programs have been implemented at public airports throughout the United States. However, weaknesses continue to exist. These weaknesses were observed and exploited by terrorists on September 11, 2001, enabling them to hijack four commercial aircraft, with tragic results. Terrorists and others with similar intent can be expected to continue to

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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons examine transportation security operations, both overtly and covertly, to find weaknesses that can be exploited.1 With hundreds of commercial airports, thousands of commercial aircraft, tens of thousands of daily flights, and millions of passengers using the system daily, providing security to the nation’s commercial aviation system is clearly a daunting challenge. Figure 1-1 illustrates some of the threat vectors that may exist in the nation’s largest airports. At the security checkpoint,2 different kinds of screening are used for passengers and for carry-on luggage. FIGURE 1-1 Generic airport diagram showing various airport spaces and some likely sites for attacks. Certain locations are particularly vulnerable to a terrorist attack by explosion or release of chemical and biological agents; these are shown by the explosion symbol and the type of threat in boldface type near that symbol. Prior to the terrorist bomb that brought down Pan Am Flight 103 over Lockerbie, Scotland, in December 1988, the focus of airline passenger security checkpoints was the detection and interdiction of metallic weapons, either carried on the person or concealed 1 Al Qaeda Training Manual. Available at http://www.fas.org/irp/world/para/manualpart1.html. Accessed June 25, 2006. 2 “Security checkpoint” is the term that the Transportation Security Administration uses to describe the point at which passengers and their carry-on baggage are checked.

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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons within carry-on items. Security equipment included transmission x-ray systems for carry-on bags and walk-through metal detectors for passengers. However, with this tragic event, the focus immediately turned to technologies capable of also interdicting explosives devices in carry-on and checked bags. Since the introduction in December 1994 of the first certified explosive detection systems (EDSs) for checked luggage, a technology based on computed x-ray tomography, explosive threat detection has significantly improved. Furthermore, the development and insertion of threat image projection capability into security systems such as those used for scanning passenger carry-on bags have also served to improve screener performance and awareness. On November 19, 2001, President George W. Bush signed into law the Aviation and Transportation Security Act (ATSA) (Public Law No. 107-071), which mandated the federalization of passenger and baggage screening at more than 440 commercial airports in the United States by November 19, 2002, and the EDS screening of all checked baggage. On March 1, 2003, the Transportation Security Administration (TSA) was transferred from the Department of Transportation to the newly created Department of Homeland Security, as required by the Homeland Security Act of 2002 (Public Law No. 107-296). Virtually all aviation security responsibilities are assigned to the TSA. These responsibilities include conducting passenger and baggage screening and overseeing security measures for airports, commercial aircraft, air cargo, and general aviation. The TSA programs dealing with these matters are intended to form a layered system that maximizes the security of passengers, aircraft, and other elements of the aviation infrastructure. The TSA has undertaken several programs to measure and improve the performance of passenger screeners in the detection of threat objects. In March 2004, the General Accounting Office (now the Government Accountability Office) completed a study of the performance of the passenger-screening system that identified numerous performance deficiencies, such as inadequate staffing and poor supervision of screeners. These deficiencies in performance were the result of a lack of skills and knowledge, low motivation, an ineffective work environment, and wrong or missing incentives. The TSA is taking steps to remedy these deficiencies, and although it is making progress in its checked-baggage screening operations, it continues to face technical, operational, and funding challenges in accomplishing the EDS screening of all checked and carry-on baggage as mandated by ATSA.3 The 9/11 Commission Report,4 issued in 2004, recommended that the Transportation Security Administration and the Congress improve the way screeners look for explosives at airports. “As a start, each individual selected for special screening should be screened for explosives.” Former Navy Secretary John Lehman, a member of that commission, told the House Aviation Subcommittee that the prospect of suicide bombers boarding U.S. aircraft is “a very real threat.” He said that it is more likely now 3 U.S. Government Accountability Office. 2004. Aviation Security: Improvement Still Needed in Federal Aviation Security Efforts. GAO-04-592T. Washington, D.C. Available at http://www.gao.gov/cgi-bin/getrpt?GAO-04-592T. Accessed June 25, 2006. 4 Available at http://www.9-11commission.gov/report/index.htm. Accessed June 25, 2006.

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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons that terrorists will try to smuggle explosives aboard U.S. airplanes because commercial airplanes have been made more secure against other threats.5 Current requirements for the security screening of passengers were developed in response to an increase in hijackings prior to 1972. Inspection systems at airport security checkpoints include metal detectors for passengers and x-ray systems for hand-carried items. While effective, these systems have shortcomings, including the inability to address evolving threats. The current x-ray inspection and trace-explosive screening, or swabbing, of carry-on baggage provide some limited capability against explosives in luggage and carry-on items, although in effect this capability may be more a matter of deterrence than of detection. Conventional metal detectors are usually limited in capability to detecting metal targets, such as ordinary handguns and knives. The effectiveness of these detectors can vary depending on the size, shape, orientation, and type of material of the object in question. Furthermore, because no discrimination is possible between simple, innocuous metallic items (e.g., eyeglasses, belt buckles, keys, coins, and prostheses) and actual threats, a high number of nuisance alarms occurs. Possible threats have evolved to include plastic or ceramic handguns and knives as well as explosives, none of which are detectable with metal detectors. STATEMENT OF TASK The Transportation Security Administration provided the National Research Council (NRC) with the following statement of task for this study: This study will explore opportunities for technology to address national needs for transportation security. While the primary role of the committee is to respond to the government’s request for assessments in particular applications, the committee may offer advice on specific matters as required. The committee will: (1) identify potential applications for technology in transportation security with a focus on likely threats; (2) evaluate technology approaches to threat detection, effect mitigation, and consequence management; and (3) assess the need for research, development, and deployment to enable implementation of new security technologies. These tasks will be done in the context of current, near-term, and long-term requirements. The committee will perform the following specific tasks: Identify potential applications for technology in transportation security with a focus on likely threats derived from threat analyses that drive security system requirements. Review security system developments structured to meet the changing threat environment. Assess government and commercial industry plans designed to address these threats. Evaluate technology approaches to threat detection, effect mitigation, and consequence management. Delineate the benefits of the insertion of new technologies into existing security systems. Evaluate the trade-offs between 5 Leslie Miller. August 28, 2004. Most air passengers not screened for bombs. AP News. Available at http://209.157.64.200/focus/f-news/1201875/posts. Accessed November 17, 2006.

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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons effectiveness and cost, including the cost of changing the security system architectures. Assess the need for research, development, and deployment to enable implementation of new security technologies. Review and assess the potential benefit of existing and advanced detection technologies, including scanning technologies, sensing technologies, and the use of computer modeling and databases. Review and assess emerging approaches to effect mitigation and consequence management. APPROACH OF THE COMMITTEE An overarching goal of this committee has been to provide timely reports that meet the technology-evaluation priorities of the Transportation Security Administration for defeating terrorist threats. The committee judged that this could best be done by issuing a series of short reports. In consultation with the TSA, the committee selected the following four topics for review, of which this report is the third: Opportunities to improve airport passenger screening with mass spectrometry, Defending the U.S. air transportation system against chemical and biological threats, Millimeter-wavelength and terahertz technology for the detection and identification of concealed explosives and weapons, and Fusion of sensor data to improve airport security. Taken together, these reports will satisfy the first part of the statement of task, which calls for an identification of applications for technology in transportation security. Individually, each report assesses its particular technology focus and identifies additional research needs. By mutual agreement between the committee and the sponsor, the broad focus on “transportation security” in the statement of task was narrowed to the threat of attacks on the air transportation system. While potential attacks on all modes of transportation are of concern, the Committee on Assessment of Security Technologies for Transportation believes that the U.S. air transportation system continues to have a high priority for counterterrorism resources, both because of its economic importance and because of the intensified public perception of risk following the September 11, 2001, attacks. The air transportation system can also serve as a testbed for the development of defensive technologies and strategies that can subsequently be applied to other transportation modes. SCOPE OF THE REPORT The increasing level and variety of threats to transportation systems have resulted in an urgent search for effective security screening. The electromagnetic spectrum from radio frequencies up to gamma rays has been studied with varying degrees of success.

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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons The region with wavelengths from approximately 3 mm to 12 µm has been investigated with somewhat limited success because of the lack of electromagnetic sources and detectors. The shortage of high-quality instrumentation has also resulted in limited reliable information about the properties of materials from which system performance can be estimated. As requested by the TSA, this report serves as an evaluation of the potential of millimeter-wavelength and terahertz technology for threat detection and as an assessment of the research needed to bring this promising technology into wide-scale use. There has been widespread misuse of the term “terahertz” owing to the significant interest in what terahertz technology might potentially achieve. The committee reviewed papers that used the term for frequencies as low as 10 GHz (X band) and as high as 150 THz, which is in the infrared region. Historically the millimeter band extended from 30 GHz to 300 GHz and the submillimeter band extended from 300 GHz to 3 THz. In the current literature the terahertz region has subsumed the submillimeter band and extended it to 10 THz. In this report the term “submillimeter” is used only to be consistent with historical citations, and the committee has chosen the following nomenclature for use elsewhere (see Figure 1-2): Millimeter: the region from 30 GHz to 300 GHz; Submillimeter: the region from 300 GHz to 1,000 GHz; and Terahertz: the region from 1,000 GHz to 10,000 GHz. FIGURE 1-2 The three regions—millimeter wave, submillimeter wave, and terahertz—of the electromagnetic spectrum considered in this report. SOURCE: Courtesy of ESA (European Space Agency). Additionally, the committee uses the following definitions with regard to threat interdiction: Detection: the process for discriminating objects of possible interest from their surroundings, Identification: determination of the threat, and Classification: determination of the threat’s characteristics.

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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons The millimeter-wave through the terahertz region is now the subject of aggressive university and national laboratory research driven in part by the availability of short-pulse generators, which produce a wide spectrum of frequencies through this region. The experience of the Transportation Security Laboratory (TSL) with these techniques is very limited apart from experience with the Manhattan II projects,6 which were initiated by the TSL. Hence the need for this NRC report. Based on the state of the art of this technology and on input from the sponsor on how the committee can best address its needs, this report addresses the following four areas: The physical phenomena limiting or enhancing imaging and spectroscopy performance of millimeter-wavelength and terahertz instrumentation and an assessment of what unique capabilities may result to complement other sensors. These phenomena include the interaction of electromagnetic radiation with the atmosphere, clothing, and explosives. The state of the art of electronic components in the millimeter-wavelength and terahertz region of the electromagnetic spectrum and what advances will result in the enhanced performance of these components, or may even be the minimum required to provide a useful capability. Descriptions of the operation of developmental systems, including short-pulse systems and active millimeter-wave passenger scanners. An implementation strategy for the development of millimeter-wavelength/terahertz technologies for application to aviation security. While numerous references are cited throughout the report they are not all-inclusive, as the literature on millimeter-wavelength and terahertz technology is voluminous. A recent Internet search on the word “terahertz” alone generated over a million hits, and a formal bibliographic reference search will result in several hundred citations. When reviewing the literature describing the ability of these technologies to find and identify explosives and concealed weapons, the reader must carefully examine the data that are presented and the conclusions that are extracted from the data. In many cases, the information, although correct, may be incomplete and/or misleading. The combination of a sense of urgency about addressing the emerging terrorist threats and the perceived availability of new funds to bring potential technologies to address these threats has yielded a plethora of technological proposals. Unfortunately, some of these proposals are neither well founded on the principles of physics nor have they been successfully demonstrated by sound experimentation and testing. Some of these proposals also appear to have been driven by exaggerated claims made by others who may lack a sound understanding of the technology and of its strengths and limitations. This situation has resulted in some unsubstantiated and unfulfilled performance expectations regarding the application of some of this antiterrorism 6 The ongoing long-term research and development program focusing on developing highly efficient and fast next-generation EDSs.

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Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons technology. One example is the claim of being able to image through building walls.7 Another is the claim of being able to identify shipwrecks and their contents on the ocean floor.8 While millimeter-waves can “see through” some wall material such as drywall or dry plywood, they cannot see through structural materials (see Chapter 2, Figure 2-5). In light of such misconceptions, the committee decided that this report should briefly and systematically address expectations, both real and fictional, and help bring into focus cases in which this technology has promise and instances in which it offers no potential benefit as an antiterrorism technology. 7 Robert Roy Britt. 2003. First Image from Revolutionary T-Ray Camera: Sees Through Fog, Clothing and into Deep Space. Available at http://www.space.com/businesstechnology/technology/t-ray_camera_020613.html. Accessed June 25, 2006. 8 Gaiacomm International Corporation. 2004. 20 Business Applications That Will Benefit from Gaiacomm’s TeraHertz-Based Technology: A Technical Discussion of TeraHertz Technology. Available at http://www.gaiacomminternational.com/GIC_20Apps.pdf. Accessed September 13, 2006.