Detection of Explosives for Commercial Aviation Security (1993)

The technology for detecting explosive material in checked baggage is continuing to advance. Several instrumental methods have demonstrated, or are close to demonstrating, at least some operational capability. These explosive detection devices (EDDs) will become essential building blocks for an explosive detection system (EDS) that could reasonably be installed in airport terminals and operated day in and day out at predictable performance levels.

Countermeasures can degrade system performance, perhaps fatally. Considerations of countermeasures are not to be addressed in this report, but must be taken into account during the design and implementation of an EDS.

This chapter discusses two important considerations for an explosive detection system.

Explosive Detection Systems cannot be the sole means used to counter the threat posed to commercial aviation by small, concealed, highly energetic explosive devices. Within the larger frame of reference, this threat can be responded to by a wide range of activities involving deterrence, aircraft hardening, and detecting and removing concealed explosive devices. Within the airport environment, the definition of the role of explosive detection in the overall security program provides important inputs for defining the internal EDS architecture. The most important of these inputs include: (a) amount and morphology of each type of explosive material required to be detected (i.e. the threat); (b) specified degree of confidence in finding the explosive material; (c) maximum acceptable false alarm rate; (d) processing speed capability as reflected by the required through-put parameters of passenger and baggage volume, rate and type; (e) method for resolving alarms; (f) likely countermeasures; and (g) provision for future change as advancements occur and requirements evolve.

The EDS systems engineering challenge is to combine sensors and procedures in such a way as to meet the detection probability and throughput

rate requirements with acceptable false alarm rate and cost without being vulnerable to defeat by likely countermeasures. These requirements will strongly influence the architecture of an explosive detection system.¹ To date, no single instrumental method has been shown to be a "silver bullet" which can satisfy all of the requirements. Therefore, it is highly likely that two or more complementary instrumental methods (i.e. EDDs) will be required. The following discussion summarizes the key considerations relating to the internal logic of integrating the responses from multiple detectors which sense different physical characteristics so that the responses are independent of one another.

The trade-off between the acceptable system level of detection probability (when there is an explosive) and the false-alarm rate (when there is no explosive) is crucial to the system architecture; specific trade-off decisions cannot be made without data on individual instruments. Thus, statistically sound, detailed parametric performance data for individual EDDs must be available for the analyses. Chapter 2 discusses key considerations for obtaining such data.

It is generally assumed that all baggage made available for EDS testing will indeed be examined by the EDS. However, in the analogous industrial context of monitoring the quality of manufactured products and seeking to find defective items, one can easily imagine inspection sampling plans in which some portion of products bypass examination. Such a plan conceivably could be cost effective when the financial costs of sampling and failing to detect a flawed item are well understood and when the latter penalty is relatively low (e.g., routine warranty costs and no possibility of excessive litigation). But, in the screening for an explosive in baggage the actual and perceived costs of failing to detect an explosive are no doubt extremely high, and exact values cannot be definitively quantified. One can imagine the public outcry that would arise if an airplane is destroyed by an explosive that could have been readily detected by the EDS, but was instead randomly designated to bypass testing.

Moreover, any potential gain in efficiency attained via subsampling is necessarily limited by the prescribed high value for the EDS probability of detection, P_D. For example, if the required P_D threshold is 0.95 then clearly no more than five percent of the baggage can be permitted to bypass inspection (even when the EDS is infallible).

All alarms will have to be cleared. At least three alternatives are available: re-run the bag through the unit; operator interpretation of the EDS-generated image of the bag; or hand searching the suspect bag. The impact of each of these alternatives could be examined with the simulation tools. Additional complexity in the search strategy can be envisioned. For instance, the results from passenger profiling as part of a pre-screening operation could be used to cue the detector threshold levels in the instruments (i.e. lower the detection level for the higher "risk" passengers). In some

instances, the viewing angle of luggage could be changed to facilitate a particular aspect of the inspection process based on a "clue."

In order to protect deployed EDS equipment against countermeasure attack, the FAA should work with the airline industry and the EDS equipment suppliers to secure an agreement that the configuration of particular EDS equipment at a particular location will not be made available to the general public. If this cannot be done voluntarily, then appropriate enabling legislation should be sought by the FAA.

Computer simulation can provide very powerful analysis tools in developing design alternatives, trade-off strategies, and expected behavior of complex systems. The application of simulation also imposes a solution framework on the problem, which itself can be a very useful way to define and structure the problem in an iterative fashion. Simulations which can aid the assessment of incorporating various explosive detection strategies into current airport terminal operations will be of value to the FAA, carriers, and airport operators.

Therefore, the concept of using simulation for designing an airport explosive detection system is appealing. Once the tools are developed, and the supporting databases are fully populated, the consequences of changing assumptions, system input parameters, system details, etc., can be quickly explored. The result can be a set of well-analyzed alternative EDS configurations with the appropriate advantages and disadvantages described.

Simulation, however, is not a panacea. The development of good simulation models is an expensive, time consuming effort which requires the dedication of high caliber experts. Simulation can give very good, or very bad, results, depending on how it is used and how faithfully the underlying simulation models represent the "real world." The results of simulation must, by their nature, be imprecise, but there may be a tendency to attribute greater precision to the numerical results than is warranted. In the following discussion, no attempt has been made to characterize the difficulty and potential pitfalls of developing and applying different simulation models. The discussion focuses on describing WHAT is required. But the assumption should not be made that the process of developing and validating these models is either easy or quick. The committee recommends that the FAA initiate work on developing these simulation tools as soon as possible.

The overall requirements for a robust set of simulation tools fall into two categories:

• analysis of the internal EDS architecture; i.e. providing support to accomplish the analysis mentioned in the previous section; and

• selection and integration of the best EDS choice for a specific airport environment; i.e. providing insight into the most cost-effective combination

of specific FAA-certified EDS configurations for screening all passengers and their baggage.

The previous section described the general input requirements for the simulation of an EDS architecture. As mentioned, no single explosive detection device is expected to meet all the performance requirements. For instance, a combination of devices involving x-ray analysis to delineate a suspicious object and nuclear methods to identify high nitrogen concentrations has been suggested as potentially offering a satisfactory technological approach at present.²

The FAA will collect performance data for the various explosive detection devices obtained from independent testing organizations to the extent it is available. The data should include the sensitivity of the detection methods to different types of explosives, different quantities and different shapes. Since some of these data may be classified, the simulation model should be able to access classified information without retaining it so that its products could remain unclassified. But, depending on FAA's security classification guidelines, the output of the model may sometimes have to be classified.

Simulation runs should be made over a range of worst case to best case parameters (e.g. varying the amount of explosive material in a bag at different through-put rates) for each device in order to determine the realistic performance from various device combinations and operating sequences.

The following airport-specific input requirements to the simulation model would include: (a) airport operations scenario; (b) available location and amount of space for the EDS units; (c) unique constraints of the air terminal; and (d) cost.

With the appropriate simulation model, various EDS configurations for checked and carry-on baggage, and for passengers, could be simulated in the context of a specific airport, and the resulting performance assessed under varying conditions. Outputs from the computer model would include: number of EDS units required for the passenger load, overall system cost, detection effectiveness, false alarm rate, peak and average throughput, and specific impacts on airport and airline operations including environmental effects. This information would assist in developing alternatives and in decision-making for deploying these systems into existing terminals. This simulation capability would be extremely useful for future airport terminal design where the number of constraints would be small.

The simulation tools should be available to the FAA, policy makers, air carriers, airport operators, airport designers, and manufacturers of explosive detection equipment to provide guidance for making rational decisions for deployment of inspection hardware, personnel, etc., into an effective overall plan within acceptable air carrier and airport operational requirements.

The simulation model should be constructed in modules so that the various levels of analysis can be performed in a loosely coupled way. One of the modules should be capable of representing a wide variety of existing terminal configurations, baggage handling and operating systems. The module should be flexible enough to handle unusual baggage check-in configurations. This module should provide an analysis of passenger and baggage screening measures.

In order to gather the necessary airport-specific data, a number of terminal and airport configurations should be studied and used to develop a common architecture, or generalized framework, for airport operations that would affect the explosive detection system. This framework could then be specialized for particular airports of interest.

A detailed functional assessment and database development for airport environments should be accomplished. The committee suggests that as many as 12 existing U.S. airport terminals be analyzed for the purpose of developing a framework which contains the key functional aspects of an airport that an EDS would affect, and designing a database which would contain the relevant details of the airport. As an example of the level of detail required, the framework should be comprehensive enough to model the space required to operate checked baggage inspection systems within various ticketing and check-in counter baggage handling configurations and carry-on baggage inspection systems. This information would form the basis for developing realistic simulations of EDS operations in widely diverse airport and air carrier operating environments so that the most efficient use of space can be determined, as well as the best way to integrate an EDS with the existing baggage handling systems.

A primary goal of the modeling activity is to incorporate a sufficient range of space and operational circumstances to provide analytical support for airport architects who can either modify existing space or design space to accommodate the new baggage screening equipment. The architects want to minimize system costs and the number of screening systems and to avoid unnecessarily compromising the efficiency of the terminal baggage check-in operations, passenger services, and/or overall terminal service operations.

Care should be taken to select an appropriate cross-section of terminal buildings that vary in size, number of air carriers, number of common checked baggage systems, passengers per peak hour to be served, operating power, other operational environments, structural support systems, space dimensional needs, distances to baggage check-in areas, and bags per hour (peak and average).

The criteria recommended for selection of the 12 different airport terminals are summarized in Table 1.

• In the worst case categories, two of the airports should be among the five busiest airports in the United States with domestic and international operations, and with substantial complexity of operational and space environments.

• Two terminals should be chosen from those designated as having specialized operations (e.g. FAA designated Category X) due to their locations.

• Two should include small (phase three) airport terminals having differing baggage check-in facilities and operating conditions.

• One should involve a terminal operation that functions under extreme cold weather conditions; one that functions under extreme moist heat conditions; and one other that operates under extreme dry heat conditions.

• One terminal complex should have a mix of international and domestic passengers and be within five years of age. Another should have a similar passenger mix, but be older, (i.e., 20 or more years old). For the case of a large international airport, distinction between domestic and international air carrier operators and screening procedures for passengers' checked baggage and carry-on baggage should be made in the data.

• A terminal complex with one or more centralized operations within a single operational zone serving more than one carrier should be selected.

• Selection of the candidate terminals should include those facilities being considered for remodeling and expansion.

The effort to gather existing and new data will require site visits to each of the selected terminal sites. Discussions with airport, air carrier, and FAA representatives will also be required. It can be anticipated that degrees of cooperation and sharing of available data and information about the checked baggage systems and baggage inspection processes of selected airports will vary, and that gaps in the data may have to be filled by visiting additional airports.

The simulation model should be validated incrementally as key modules are completed. The modules could be validated by comparing the results of the model for a particular airport (using current airport operations) to actual performance at that airport. Performance factors such as passenger and baggage flow rates associated with peak hour, average day, and peak month actuals versus simulated should be compared. Results from EDD and EDS demonstration tests, if well-controlled and statistically valid, could also aid validation of the models.

The modeling should allow analysis of the implications on space requirements and air carrier operations associated with new federal regulations pertaining to checked baggage explosive detection and expansion of FAA screening requirements having to do with passenger and baggage explosive detection and screening. For example, it would be of interest to simulate the impact of the following scenarios:

• the explosive detection screening of all checked baggage for all international departures;

• the explosive detection screening of all checked baggage for all domestic departures;

• application of positive passenger/baggage matching procedures for use throughout the checked baggage explosive detection process and up to the time of aircraft departure;

• elimination of carry-on baggage, other than essential items, for departures;

• opening bags for insertion of test probes; and

• restricting carry-on luggage size and contents (segregation into bags containing only clothing versus metal and other objects).

The expected impact of the implementation of the foregoing scenarios could be simulated for at least some of the selected study terminal sites.

The objective of these simulations would be to identify the extent and cost associated with appropriate solutions. The analyses should be capable of being expanded to include the identification and evaluation of alternative conceptual layouts and screening equipment to fit into available space. The simulation should include an assessment of costs required to implement each potential alternative. Results of these efforts could serve as a basis for identifying guidelines for the appropriate deployment of equipment and development of responsive passenger/baggage flow patterns to meet potential alternative levels of passenger and baggage requirements. Output of the simulations should be presented in a manner that will facilitate a clear understanding of the findings, including dynamic graphic displays. The output could also be presented so that relationships and activity levels should be capable of further study and evaluation. Findings of the study should highlight the advantages and disadvantages associated with certain design solutions under differing levels of activity, including generalized conceptual equipment deployment and flow patterns, and the resulting sensitivity and specificity for explosive detection.

Potential outputs resulting from a comprehensive simulation modeling activity are:

• confidence level that an EDS would meet the FAA-mandated requirements, including threat detection, baggage processing rate, and false alarm rate, in the context of overall airport operations;

• number of EDS required to service passenger traffic;

• costs and operating impacts on air carriers and airports;

• optimum locations for screening equipment;

• Airport Terminal Architectural Considerations

  • space requirements for baggage processing;

  • spatial relationships within the baggage and passenger handling areas of the terminals between explosive detection and other activity centers within the terminal itself;

  • location(s) of entrance and exit areas for passengers;

  • structural loading implications;

  • partition locations;

  • stacking areas for baggage make-up and passenger handling; and

• other considerations relating to the passenger and baggage screening processes.