Radio Waves See Through Walls

University of Utah engineers showed that a wireless network of radio transmitters can track people moving behind solid walls. The system could help police, firefighters and others nab intruders, and rescue hostages, fire victims and elderly people who fall in their homes. It also might help retail marketing and border control. By showing the locations of people within a building during hostage situations, fires or other emergencies, radio tomography can help law enforcement and emergency responders to know where they should focus their attention. Their method uses radio tomographic imaging (RTI), which can see, locate and track moving people or objects in an area surrounded by inexpensive radio transceivers that send and receive signals. People don't need to wear radio-transmitting ID tags. The study involved placing a wireless network of 28 inexpensive radio transceivers - called nodes - around a square-shaped portion of the atrium and a similar part of the lawn. In the atrium, each side of the square was almost 14 feet long and had eight nodes spaced 2 feet apart. On the lawn, the square was about 21 feet on each side and nodes were 3 feet apart.

The transceivers were placed on 4-foot-tall stands made of plastic pipe so they would make measurements at human torso level. Radio signal strengths between all nodes were measured as a person walked in each area. Processed radio signal strength data were displayed on a computer screen, producing a bird's-eye-view, blob-like image of the person. A second study detailed a test of an improved method that allows ‘tracking through walls’. That study has been placed on arXiv.org, an online archive for preprints of scientific papers. The study details how variations in radio signal strength within a wireless network of 34 nodes allowed tracking of moving people behind a brick wall. The wireless system used in the experiments was not a Wi-Fi network like those that link home computers, printers and other devices. Researchers used Zigbee network - the kind of network often used by wireless home thermostats and other home or factory automation.

More information:

http://www.unews.utah.edu/p/?r=100509-1

http://span.ece.utah.edu/radio-tomographic-imaging