forensic investigations to determine the source of a terrorist attack or to assign responsibility.

Recent research and development, focusing most heavily on portable sensors for chemical and biological agents, has followed two basic paths. The first is a repackaging of standard laboratory-analysis techniques for field use, and it includes various methods of spectroscopy. The second basic path has been in the introduction of new affinity-based sensors, in which the chemical or biological agent is selectively bound to a surface through use of a specialized surface coating; the presence or absence of the agent on the surface is then measured by one of several mechanical, electrical, or optical transduction methods. The sensitivity, selectivity, quantification, and time response of these affinity-based sensors are functions of the specialized coatings and signal-transduction methods used.

Spectroscopy methods—the first path—tend to be more general-purpose, with a single instrument being useful for detection of a number of agents. In contrast, to use affinity-based instruments for detection of multiple agents, an array of sensors is needed where the elements of the array receive a variety of coatings, each specialized to allow detection of a specific chemical or biological agent.

Either way, to carry sensor-system performance to the level needed, homeland defense will require not only continued improvement in basic sensor performance but also a better definition and understanding of overall performance—when many sensors are networked together. A number of factors will contribute to effective functioning of sensor networks. Communications protocols will be needed, and network architecture issues associated with connectivity, bandwidth allocation, signal processing, and data fusion must also be addressed

In particular, algorithms for detection in the presence of significant clutter must be developed, with a focus on achieving excellent detection capability while minimizing false alarms. In many instances, the impact of false alarms will depend on circumstances. The trade-offs between false positives and false negatives and the consequences of each must take into account how the system can be used most effectively. Issues will include the system in which the sensors are installed (e.g., Are there backup or alternate security checks?), the users of the outputs (e.g., first responders, scientists supervising recovery efforts), and the time scales on which decisions about what to do with the results must be made.

The next important step is to address the detection of weapons of mass destruction from a systems-engineering perspective, which spans the capture/ collection of the sample, preparation of the sample, reliable delivery of the sample to the sensor, sensor interrogation (including background and metric verification), analysis of the signal, and reporting of the data from individual sensors. This perspective can be enhanced to include redundancy issues and other performance enhancements achieved from multiple networked sensors. Several other attributes will accrue from this system-design approach:

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