Swanson decided to borrow an idea from spiders. "Every spider sits in a specific position on the web, and when prey gets caught in the web, the spider's legs pick up the vibration and the spider runs to it."
Using that model, the lab "pulled a tension wire around a fence area and placed a sensor on either end," he says. "It can pick up vibrations anywhere on the fence and mechanically transfer the information to the sensor."
The mechanism uses geophones, which are vibration sensors that pick up the vibrations, and send them along the 1/16th-inch thick wire to a computer that analyzes the wave form and transmits a wireless alert. The tricky part is programming the system to know the difference between a squirrel scurrying along a fence and, say, a man climbing that fence.
"You take knowledge of physics and acoustics and translate that into algorithms, which takes raw signals from sensors and converts it to information," Swanson explains. This means, for example, that whoever is watching the console will get an alert that says the system believes there's a 90% probability that a tank is driving alongside the fence.
The prototype fence has attracted interest from airports, industrial parks, military installations, and embassies. Says Swanson, "The phone hasn't stopped ringing off the hook."