MIT researchers have demonstrated the application of using Wi-Fi signals to track the movements, locations and number of people inside buildings. Dubbed Wi-Vi, the inexpensive and portable "through-the-wall" radar system could have practical applications in law enforcement, search and rescue operations and home security, even if it does present some potential privacy concerns.

While listening in on radio signals transmitted by standard Wi-Fi routers and access points, a Wi-Vi transceiver blankets the area with its own low-power, directional, wireless transmissions. As objects move within the target area, signals are absorbed and reflected differently. The device monitors these slight changes, giving Wi-Vi operators the ability to determine the movements, locations, speeds and the number of moving objects (e.g. people) behind walls.

Since Wi-Vi relies on physical radio waves instead of higher network layers, operators do not need access to a Wi-Fi network (i.e. no password is needed) in order to make use of signals emanating from a nearby Wi-Fi router or AP.

Wi-Vi can detect objects and humans moving behind opaque structural obstructions. This applies to 8" concrete walls, 6" hollow walls, and 1.75" solid wooden doors. A Wi-Vi device pointed at a closed room with 6" hollow walls supported by steel frames can distinguish between 0, 1, 2, and 3 moving humans in the room. Computed over 80 trials with 8 human subjects, Wi-Vi achieves an accuracy of 100%, 100%, 85%, and 90% respectively in each of these cases.

Source: (pdf), See Through Walls with Wi-Fi!

While Wi-Vi doesn't provide anything that resembles an actual photograph of what's behind a wall, the results are detailed enough to be used for detecting body gestures (e.g. hand waves) with a remarkable amount of accuracy.

In the same room, and given a single person sending gesture based messages, Wi-Vi correctly decodes all messages performed at distances equal to or smaller than 5 meters. The decoding accuracy decreases to 75% at distances of 8 meters, and the device stops detecting gestures beyond 9 meters. For 8 volunteers who participated in the experiment, on average, it took a person 8.8 seconds to send a message of 4 gestures.

Unfortunately for potential voyeurs (and fortunately for privacy advocates), it remains a far cry from detailed airport scanners, X-ray vision or even Batman's implausible cell phone "sonar" network, but it could prove to be an inexpensive way to gain life-saving insight for rescue operations and police busts. As with many technologies though, it's not difficult to imagine Wi-Vi being subverted by tech-savvy criminals, like burglars or kidnappers, to get the drop on unsuspecting victims.