August 3, 2012 weblog
See-through-wall surveillance with WiFi shown at UCL
(Phys.org) -- A surveillance device that uses WiFi radio waves has been devised to see through walls to detect, in military and surveillance parlance, moving personnel targets. The device serves as a radar prototype designed by two UK scientists at the University College London (UCL). The scientists devised the radar prototype as a way to track Wi-Fi signals in order to spy through walls. Their device identifies frequency changes to detect the moving objects. An important feature of their work is that since the device itself does not emit radio waves, it cannot be detected; it operates in stealth.
Karl Woodbridge and Kevin Chetty, researchers at University College London, designed their detector to be able to use these signals. Chetty is a lecturer in the Department of Security and Crime Science at University College London; Woodbridge is with the schools Department of Electronic and Electrical Engineering. The two demonstrate what is called a passive radar system that can see through walls using WiFi signals generated by wireless routers and access points. Passive radar systems detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals.
The scientists paper, Through-the-Wall Sensing of Personnel Using Passive Bistatic WiFi Radar at Standoff Distances" coauthored with G.E. Smith, appeared in the April issue of Geoscience and Remote Sensing, IEEE Transactions.
The prototype is about the size of a suitcase and it carries two antennae and signal processing unit, to monitor baseline WiFi frequency in an area for any change that would indicate movement. The device in tests successfully determined a persons location, speed and direction through a brick wall that was one-foot-thick.
See Through The Wall (STTW) technologies are of great interest to law enforcement and military agencies; this particular device has the UK Military of Defense exploring whether it might be used in urban warfare, for scanning buildings. Other more benign applications might range from monitoring children to monitoring the elderly.
According to Woodbridge, some challenges remain which he and others involved in the research will attempt to resolve. The UCL team hopes to raise system sensitivity so that their system can pick up and detect not only people who are moving but also people who are standing or sitting still. The device, he said, may be made to be sensitive enough to pick up on subtle motions that the ribcage makes in breathing in and out.
Wi-Fi radio signals are found in homes worldwide. Strategy Analytics, a market intelligence company, found in its recent study that 439 million households by the end of 2011 worldwide had WiFi network setups, or about 25 percent of all households. The same report predicts that the worldwide number of Wi-Fi households will reach nearly 800 million in 2016, a penetration rate of 42 percent.
In this paper, we investigate the feasibility of uncooperatively and covertly detecting people moving behind walls using passive bistatic WiFi radar at standoff distances. A series of experiments was conducted which involved personnel targets moving inside a building within the coverage area of a WiFi access point. These targets were monitored from outside the building using a 2.4-GHz passive multistatic receiver, and the data were processed offline to yield range and Doppler information. The results presented show the first through-the-wall (TTW) detections of moving personnel using passive WiFi radar. The measured Doppler shifts agree with those predicted by bistatic theory. Further analysis of the data revealed that the system is limited by the signal-to-interference ratio (SIR), and not the signal-to-noise ratio. We have also shown that a new interference suppression technique based on the CLEAN algorithm can improve the SIR by approximately 19 dB. These encouraging initial findings demonstrate the potential for using passive WiFi radar as a low-cost TTW detection sensor with widespread applicability.
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