Wi-Vi system uses Wi-Fi to see through walls...

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.