During the Vietnam War, the U.S. military developed sensors that detect the presence of ammonia from mammalian waste. These sensors, which could be inserted by helicopter teams or dropped by aircraft into the jungle, could not distinguish between humans and other mammals, so their correct use required an accompanying terrain analysis. For example, knowing at which elevation an elephant, predator cat, or primate would be very unlikely to appear meant the detection of ammonia was much more likely to correlate with human activity. However, the North Vietnamese and their allies learned to defeat these sensors by spraying any they could find with animal urine, thus hiding their own presence amid the false-positive alarms.
However, this limitation, while important in warfare, may not be as important in biosurveillance. If we could use such monitoring to establish a baseline activity map of a heavily wooded area, which is difficult to survey by other means, then sudden, unexpected changes in such activity could in theory signal an event worthy of greater scrutiny by other appropriate means. For example, if a system detected a sudden absence of activity in the region, this could possibly signify a catastrophic event. In less extreme circumstances, its best use is likely to occur when paired with other types of biosurveillance.
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