Radio waves used to see through walls

University of Utah engineers have developed a way of tracking people moving behind solid walls using a network of radio transmitters.

The system could be useful for police, firefighters and rescue workers, they say.

The method uses radio tomographic imaging (RTI), which can 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.

Variations in radio signal strength within a wireless network of 34 nodes allowed tracking of moving people behind a brick wall. The system successfully tracked a person’s location to within three feet.

The wireless system wasn’t a standard wifi network, but a Zigbee network – the sort often used by wireless home thermostats and other home or factory automation.

“I have aspirations to commercialize this,” says researcher Joey Wilson, who has founded a spinoff company named Xandem Technology LLC in Salt Lake City.

RTI is much less expensive than radar. It measures “shadows” in radio waves created when they pass through a moving person or object. The devices transmit radio waves at powers 500 times less than a typical cell phone.

In their experiments, the team obtained radio signal strength measurements from all the transceivers – first when the rectangle of nodes was empty and then when a person walked through it. They developed math formulas and used them in a computer program to convert weaker or “attenuated” signals – which occur when someone creates “shadows” by walking through the radio signals – into a blob-like, bird’s-eye-view image of that person walking.

“RF [radio frequency] signals can travel through obstructions such as walls, trees and smoke, while optical and infrared imaging systems cannot,” the engineers wrote. “RF imaging will also work in the dark, where video cameras will fail,” says researcher Neal Patwari.

“When there is a hostage situation, for example, or some kind of event that makes it dangerous for police or firefighters to enter a building, then instead of entering the building first, they would throw dozens of these radios around the building and immediately they would be able to see a computer image showing where people are moving inside the building.”

The study has been placed on, an online archive for preprints of scientific papers.