The first line of defense against intruders at airports, prisons, military bases, and nuclear power plants is often nothing more than a chain-link fence.
At some high-risk areas, such as nuclear weapons storage facilities, complex sensors that detect an intruder's movement, body heat, or footsteps can augment perimeter security.
But adding these detection capabilities also adds as much as $165 per foot of perimeter fence, making them prohibitively expensive for large areas such as an airport or Navy base, which may have miles of fencing to monitor.
However, a team of researchers at Pennsylvania State University's Applied Research Laboratory (ARL) has developed an extremely simple — and inexpensive — solution to make almost any fence smarter at detecting, locating and classifying trespassers.
Secured by a String of Wire
The experimental fence monitoring system uses a single high-tension metal wire that can be embedded within the fence's chain links or strung along the top of fence structures. Any disturbance to the fence — a person attempting to climb it or a piece of trash brushing up against it — will cause the tightly wound wire to vibrate like a guitar string.
Geophones — electronic sensors commonly used to monitor earthquake activity — or other sensors strung along the wire translate these vibrations into signals that are sent back to a central computer. Software created by ARL's researchers then compares the strength and pattern of the signals to determine the type and location of the "threat" against the fence.
Sorting the Bad Vibes From Good
A tree branch that is blown against the fence, for example, produces only a momentary pattern of vibrations. But a person cutting the fence and the sensor wire creates a very brief pattern of intense signals. Even careful climbing by an intruder would stress the fence and create a heavy, sustained vibration pattern indicating a human invader was present rather than a squirrel.
The software can also help locate the source of the disturbance by monitoring the vibrations in the fence and precisely detecting the time of arrival of signals from two or more locations. Measured differences between the signal's arrival times indicate the point at which the intrusion occurs.
Heard It Through the Power Line
David Swanson, associate professor of acoustics and lead research associate on the project, says the idea for the new fence monitoring system came from his experience with how sound travels along the wire. "I distinctly remember hearing sounds on a high-tension power line miles away from a railroad crossing and thought, 'Hmmm, I bet this will work.'"
A prototype system installed at Penn State has produced excellent results says Nicholas C. Nicholas, a physicist and senior research associate at ARL who worked on the fence monitor.
"One of the advantages is that it's hard to see the thing," says Nicholas. Since the tension wire is woven in among the fence wire, it's virtually unnoticeable from distances of about 15 feet. Thinner wires and better sensors would make the system even more difficult to detect — and more formidable against intruders.