Gotcha! A radio transceiver array can track an intruder by displaying his position on a computer screen (inset).

Photo: S. Joshi and J. Wilson/University of Utah

Researchers Use Radio Waves to "See" Through Walls

Researchers have discovered that an array of radio transceivers--devices that send and receive signals--can track people's movements behind walls. Possible uses include detecting people trapped in burning buildings, controlling lighting or heating and cooling systems as people enter or exit rooms, and spotting burglars or enemy soldiers.

The technology is based on the fact that human beings absorb radio waves. The phenomenon is called multipath fading, and it causes the sudden static on an AM radio in a room where people are moving around.

Electrical engineer Neal Patwari of the University of Utah in Salt Lake City had been working on ways to overcome multipath fading when he decided to turn the problem on its head. Why not use fading for a different purpose: to locate someone or something moving? The advantage of radio waves is that, unlike visible light, they can be detected at night and in dust, smoke, or fog. Patwari and his team developed software that displays on a screen the approximate position of someone moving within a cordon of radio transceivers.

The experiments started in 2007, Patwari says, but the results were spotty because the team was "using clunky hardware and the system was not real time." But as doctoral student Joey Wilson and Patwari will report in an upcoming issue of Transactions on Mobile Computing, they were eventually able to create a prototype wireless network of pole-mounted, cell phone–sized transceivers that they tested outdoors and indoors. In both cases, the prototype system worked well and in real time. By measuring the radio-signal strength from all the transceivers while Wilson walked around inside the array, the system could calculate his location with an accuracy of about 1 meter.

One advantage of the transceiver array is that it could be set up and dismantled quickly, around a military encampment or burning house, for example. But a few bugs must be worked out before the system is ready for commercialization, Patwari says. For one thing, the prototype's frequency can be jammed, which might limit its usefulness for security systems. A solution might be to program the software to instantly switch frequencies if the signal is interrupted.

The research has yielded "an interesting way of taking a long-standing technical problem, creating a new type of technology using existing hardware, and applying it in ways that can help in emergencies," says electrical engineer Jeff Frolik of the University of Vermont, Burlington.