Once a plume has been detected, the varied characteristics of BWAs make discrimination between threat and non-threat plumes even more difficult. The conventional wisdom several years ago was that fluorescence cross sections of BWAs depended on the specific agents; for example, dipicolinic acid is a fluorophore that is present in Bacillus anthracis spores but not Francisella tularensis. However, recent laboratory work has shown that interspecies differences can be obscured by other effects (Hargis et al. 2007). This critical finding suggests that a standoff capability to detect the signatures associated with those effects may be more valuable than one that can detect only specific agents. Such information is important to both developers and evaluators in assessing test conditions.
The Department of Defense (DOD) is developing biological standoff detection technology in phases. The first objective is to be able to detect an approaching plume of biological material, whether human-made or natural, and determine its size and extent. The goal is to provide longer warning times than are provided by biological point detectors, which can detect biological agents based on direct samples of agent collected at the “point” at which the detector is placed. Point and standoff detectors have different abilities to determine whether a plume contains a BWA. Standoff detectors that can detect a plume but not determine its nature might be deployed in conjunction with point detectors placed upwind from a site that is to be protected. That would provide two independent kinds of warning: one that a plume is approaching and one that a plume contains a BWA.
Standoff detection systems can be highly sensitive to various features in the atmosphere, such as clouds, dust, and the like. A military unit might react to information that a plume was approaching without knowing whether it was a natural aerosol plume, a dust storm, some other phenomenon, or an actual threat; soldiers might be ordered repeatedly to don personal protective equipment, although no biological threat is present, and this would reduce their operational effectiveness.
In order to be effective, biological standoff detection requires a capability to at least identify the approach of concentrations of biological substances. If the false-alarm rate is not too high, and in conjunction with other intelligence or warnings, it might be acceptable to act on information that some sort of biological material is approaching even if it turns out to be only natural spores from nearby fields or trees. Under such a scenario, it might still be valuable to detect an approaching plume of biological material and order troops to find protection, even if the standoff detector could not determine whether the approaching biological material is dangerous. However, it would be important to know that the standoff detection system is capable of spotting enemy-made biological agents and not only natural airborne materials. Therefore, T&E of such a system would necessarily include a demonstration that the system is not blind to enemy-made biological agents either alone or in the presence of other materials, such as contaminants or interferents.
Ultimately, DOD hopes to be able with standoff detectors to identify species in a plume, that is, to identify specific BWAs. To test and evaluate a detector with such capability, it would be necessary to demonstrate that the standoff detection system could dependably detect perhaps a dozen or more enemy-made biological agents in the presence of other atmospheric phenomena,