Through the wall, clearly: Radio waves "see" through walls
Lake City home. Variations in radio waves were measured as Wilson walked around inside. The system successfully tracked Wilson’s location to within 3 feet.
The wireless system used in the experiments was not a Wi-Fi network like those that link home computers, printers and other devices. Patwari says the system is known as a Zigbee network — the kind of network often used by wireless home thermostats and other home or factory automation.
Wilson demonstrated radio tomographic imaging during a mobile communication conference last year, and won the MobiCom 2008 Student Research Demo Competition. The researchers now have a patent pending on the method.
“I have aspirations to commercialize this,” says Wilson, who has founded a spinoff company named Xandem Technology LLC in Salt Lake City.
The research was funded by the National Science Foundation.
How it works
Radio tomographic imaging (RTI) is different and much less expensive than radar, in which radar or radio signals are bounced off targets and the returning echoes or reflections provide the target’s location and speed. RTI instead measures “shadows” in radio waves created when they pass through a moving person or object.
RTI measures radio signal strengths on numerous paths as the radio waves pass through a person or other target. In that sense, it is quite similar to medical CT (computerized tomographic) scanning, which uses X-rays to make pictures of the human body, and seismic imaging, in which waves from earthquakes or explosions are used to look for oil, minerals and rock structures underground. In each method, measurements of the radio waves, X-rays or seismic waves are made along many different paths through the target, and those measurements are used to construct a computer image.
In their indoor, outdoor and through-the-wall experiments, Wilson and Patwari obtained radio signal strength measurements from all the transceivers — first when the rectangle 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.
RTI has advantages. “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.”
Even “where video cameras could work, privacy concerns may prevent their deployment,” Wilson
