see people through walls

8/9/2019 See People Through Walls

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See People through walls

Lee Siegel

Radio Waves Test Wall Captures Image

University of Utah engineers showed that a wirelessnetwork of radio transmitters can track people movingbehind solid walls. The system could help police,

firefighters and others nab intruders, and rescuehostages, fire victims and elderly people who fall in theirhomes. It also might help retail marketing and bordercontrol.

"By showing the locations of people within a buildingduring hostage situations, fires or other emergencies,radio tomography can help law enforcement andemergency responders to know where they should focustheir attention," Joey Wilson and Neal Patwari wrote inone of two new studies of the method.

Both researchers are in the university's Department ofElectrical and Computer Engineering Patwari as anassistant professor and Wilson as a doctoral student.


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Their method uses radio tomographic imaging (RTI),which can "see," locate and track moving people orobjects in an area surrounded by inexpensive radiotransceivers that send and receive signals. People don't

need to wear radio-transmitting ID tags.

One of the studies which outlines the method and testsit in an indoor atrium and a grassy area with trees isawaiting publication soon in IEEE Transactions on MobileComputing, a journal of the Institute of Electrical andElectronics Engineers.

The study involved placing a wireless network of 28

inexpensive radio transceivers called nodes around asquare-shaped portion of the atrium and a similar part ofthe lawn. In the atrium, each side of the square wasalmost 14 feet long and had eight nodes spaced 2 feetapart. On the lawn, the square was about 21 feet on eachside and nodes were 3 feet apart. The transceivers wereplaced on 4-foot-tall stands made of plastic pipe so theywould make measurements at human torso level.

Radio signal strengths between all nodes were measuredas a person walked in each area. Processed radio signalstrength data were displayed on a computer screen,producing a bird's-eye-view, blob-like image of theperson.

A second study detailed a test of an improved methodthat allows "tracking through walls." The study detailshow variations in radio signal strength within a wireless

network of 34 nodes allowed tracking of moving peoplebehind a brick wall.

The method was tested around an addition to Patwari'sSalt Lake City home. Variations in radio waves were


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measured as Wilson walked around inside. The systemsuccessfully 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 isknown as a Zigbee network the kind of network oftenused by wireless home thermostats and other home orfactory automation.

Wilson demonstrated radio tomographic imaging during amobile communication conference last year, and won theMobiCom 2008 Student Research Demo Competition. The

researchers now have a patent pending on the method. "Ihave aspirations to commercialize this," says Wilson, whohas founded a spinoff company named Xandem

Technology LLC in Salt Lake City. The research wasfunded by the National Science Foundation.

How It Works

Radio tomographic imaging (RTI) is different and muchless expensive than radar, in which radar or radio signalsare bounced off targets and the returning echoes orreflections provide the target's location and speed. RTIinstead measures "shadows" in radio waves created whenthey pass through a moving person or object.

RTI measures radio signal strengths on numerous pathsas 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-raysto make pictures of the human body, and seismicimaging, in which waves from earthquakes or explosionsare used to look for oil, minerals and rock structuresunderground. In each method, measurements of the radiowaves, X-rays or seismic waves are made along many


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different paths through the target, and thosemeasurements are used to construct a computer image.

In their indoor, outdoor and through-the-wall

experiments, Wilson and Patwari obtained radio signalstrength measurements from all the transceivers firstwhen the rectangle was empty and then when a personwalked through it. They developed math formulas andused them in a computer program to convert weaker or"attenuated" signals which occur when someonecreates "shadows" by walking through the radio signals into a blob-like, bird's-eye-view image of that personwalking.

RTI has advantages. "RF [radio frequency] signals cantravel through obstructions such as walls, trees andsmoke, while optical and infrared imaging systemscannot," the engineers wrote. "RF imaging will also workin the dark, where video cameras will fail."

Even "where video cameras could work, privacy concernsmay prevent their deployment," Wilson and Patwari

wrote. "An RTI system provides current images of thelocation of people and their movements, but cannot beused to identify a person."

Would bombardment by radio waves pose a hazard?Wilson says the devices "transmit radio waves at powers500 times less than a typical cell phone." "And you don'thold it against your head," Patwari adds.

Radio 'Eyes' to the RescuePatwari says the system still needs improvements, "butthe plan is that when there is a hostage situation, forexample, or some kind of event that makes it dangerousfor police or firefighters to enter a building, then insteadof entering the building first, they would throw dozens of


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these radios around the building and immediately theywould be able to see a computer image showing wherepeople are moving inside the building." "They arereusable and you can pick them up afterwards," he says.

The technique cannot distinguish good guys from badguys, but at least will tell emergency personnel wherepeople are located, he adds.

Patwari says radio tomography probably can be improvedto detect people in a burning building, but also would"see" moving flames. "You may be able to look at theimage and say this is a spreading fire and these are

people," says Patwari.Wilson believes radio imaging also could be used in "asmarter alarm system. What if you put radios in yourhome [built into walls or plugged into outlets] and usedtomography to locate people in your home. Not onlywould your security system be triggered by an intrusion,but you could track the intruder online or over yourphone."

Radio tomography even might be used to study wherepeople spend time in stores.

"Does a certain marketing display get people to stop ordoes it not?" Wilson asks. "I'm thinking of retail stores orgrocery stores. They spend a lot of money to determine,'Where should we put the cereal, where should we putthe milk, where should we put the bread?' If I can offer

that information using radio tomographic imaging, it's abig deal."

Radio image tracking might help some elderly people liveat home. "The elderly want to stay in their homes butdon't want a camera in their face all day," Wilson says."With radio tomographic imaging, you could track where


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they are in their home, did they get up at the right time,did they go to the medicine cabinet, have they not movedtoday?"

Wilson says a computer monitoring the radio imagesmight detect an elderly person falling down the stairsbased on the unusually fast movement. He says radiotracking also might be a relatively inexpensive method ofborder security, and would work in dark and fog unlikecameras.

Another possible use: automatic control of lighting,heating and air conditioning in buildings, says Wilson.

Radio tracking might even control sound systems so thatthe best sound is aimed where people are located, as wellas noise cancellation systems which could be aimedautomatically at noise sources, Patwari says.

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