TECHNOLOGY FOR CHANGE

Smarter Search-and-Rescue Sees Through Walls

R. Colin Johnson | Date: 10-16-09 | Comments

First responders can use inexpensive wireless radio transmitters to locate people moving behind solid walls before commencing a rescue operation.

A smarter way to conduct search-and-rescue operations is not to blindly send in search teams or even robots, but instead to first surround the area with inexpensive wireless radio transmitters and use radio tomographic imaging (RTI) to see through the walls. Locating people moving behind solid walls before commencing a search-and-rescue operation could potentially help firefighters find victims, paramedics find elderly who have fallen, police locate illegal aliens trying to breach the border under cover of fog, and the military locate hostages inside buildings.

"Instead of charging into a building, first responders would instead throw a few dozen of these radios around the building and immediately be able to see a computer image showing where people are moving around inside," said Joey Wilson, a doctoral candidate at the University of Utah. Wilson performed the work with University of Utah Professor Neal Patwari.

Seeing through walls is already possible with sophisticated radars that the military can afford, but these University of Utah researchers wanted their system to be more affordable by using inexpensive Zigbee (IEEE 802.15.4) radios. Radar is expensive because it emits a high-frequency signal whose reflection (echo) must be analyzed by a sensitive receiver that can measure the transit time of the reflected signal. Radio tomography, on the other hand, merely measures the "shadows" created by people passing between a radio transmitter and a receiver.

"We surround an area by many inexpensive radios, and as you walk through that area you disturb the signal on each of the links," said Wilson. "We then use an algorithm that relates where you are to how you are disturbing the signal, and from that we create an image."

Each radio is a transceiver capable of both transmitting and receiving, but the algorithm only allows one transmitter to broadcast at a time, while the others listen. By going around the area round-robin style, allowing each transmitter to broadcast in turn, all the moving shadows in the area can be transformed into an image.

To prove the concept, the researchers surrounded an area with 28 Zigbee transceivers, and as a person walked through the surrounded area, the radio shadows were transformed into an image showing shades of blue for stationary objects and "blobs" of red and yellow representing the motion of people.

Wilson suspects that there are also many commercial applications, such as retail marketing systems that could count how many people congregate in different areas, making it easier for retailers to decide how to lay out product shelves in a store. Wilson has founded a spin-off company named Xandem Technology (Salt Lake City) to commercialize his implementation of radio tomography.