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Optimising a layered port security system


Today, there are many different security inspection technologies available. These technologies may be combined in an attempt to achieve a better result. How the systems are combined strongly affects the results achieved, and different applications may require different combinations. This paper will examine several examples.

There are three major applications for screening technology today: Revenue enhancement, contraband detection, and nuclear weapons of mass destruction detection (WMD). Several technologies that can be used are: Portal monitors, gamma ray imagers, high-energy X-ray imagers, and neutron systems. Matching the application and the technology correctly is critical.

Revenue enhancement

The goal of this application is to detect undeclared or misrepresented goods in the normal flow of commerce and recover unpaid duties. Over time, in many cases, the recovered duties exceed the cost of ownership, so the initial investment in this technology is recouped and exceeded. Because the mission is not safety critical, a missed detection is not a serious problem. Only an imaging system can yield the data required to compare the container contents to the cargo manifest. Therefore the appropriate technologies here are high-energy X-ray imaging and possibly gamma imaging. The choice will depend on the type of cargo traffic at a given port. Gamma systems are best at imaging empty or lightly loaded containers while high-energy X-rays are capable of imaging all containers. However the X-ray systems are typically 1.5 to 3 times more expensive than the gamma systems. Assume for the moment that a gamma system can detect undeclared and/or misrepresented goods in containers with a detection efficiency of 100% in lightly loaded containers and 1% in very heavily loaded containers. Then the actual detection rate will depend on the distribution of container densities but in a typical cargo flow this might be less than 50%. If, however, a high-energy X-ray system can detect 100% of the undeclared or misrepresented goods it will clearly have a bigger payback. The optimal choice of technology in this case depends on the maximum enhancement dollars available.

When computing the cost of ownership of a system, it is important to compare the sum of the yearly operation plus the initial purchasing costs. Both types of systems have similar costs for manning and this turns out to be a large fraction of the cost of ownership. As an example, assume the gamma system costs $1million and the X-ray system costs $3million (these numbers are purely examples) and both require a three-person crew to operate. Although the acquisition costs differ by a factor of three, the yearly cost of ownership for the X-ray system is actually only about 50% more than the gamma system. Since the X-ray system will detect contraband or other prohibited cargo more than twice as often, the economics favour it.

Contraband detection

The goal here is to interdict goods deemed dangerous to society such as drugs or conventional weapons. The payback mechanism in this application is not easy to calculate. For drugs, interdiction increases the street prices of the drugs. If there is economic compliance it reduces drug usage and associated health and law enforcement costs. For conventional weapons, the payback is realised in terms of a reduction in the cost of fighting street crime and/or civil war. Screening for these purposes typically requires a government commitment (“war on drugs”). Often the costs of a missed detection are very high but limited in scope. The emphasis in this application depends on the political perception of the threat. In the United States’ “war on drugs” officials realised that it was virtually impossible to completely stop the flow of drugs and only partially effective technologies were deployed. These slowed the flow of drugs at a modest cost.

Weapons of mass destruction

The goal of this application is to prevent the introduction of nuclear weapons into a country. Clearly, any use of such a weapon in a port environment would cause a serious disruption of worldwide trade, with immense economic and human consequences. Any process for detecting WMD’s must accomplish two things: Prevent their ever reaching a target country, and do this successfully every time. If a WMD were actually to reach a target country, then the risk would be immense, requiring only detonation to cause unthinkable damage. This understanding is what informs the structure of the United States’ Container Security Initiative (CSI). Under CSI, the intention is to discover any threat before it is shipped to US soil by inspection at the port of embarkation. Needless to say, a single incident is unacceptable. This means that any inspection regime must guarantee 100% detection.

Nevertheless, successful inspections for WMD’s must occur in the normal stream of commerce, which cannot be disrupted. If inspection results in a significant reduction in the flow of commerce, then, like terrorism, it causes serious economic penalties. The remainder of this paper will look at how to optimise an inspection regime under these constraints.


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Dr. Paul Bjorkholm, Senior Scientist, Varian Medical Systems, Las Vegas, NV, USA
Edition: Edition 27

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