Surveillance device uses Wi-Fi to see through walls

A new prototype stealth device can detect movement behind a thick brick wall and the UK military is looking into whether it can be used in "urban warfare" for scanning buildings.

Ministry of Defence explores Wi-Fi personnel detection.
(Credit: MOD UK)

Researchers in England created the prototype surveillance device that can be used to spy on people inside buildings and behind walls by tracking the frequency changes that Wi-Fi signals, generated by wireless routers and access points, bounce off people as they move around.

The device, which is about the size of a suitcase and has two antennae and a signal processing unit, works as a "passive radar system" that can "see" through walls, according to It was able to successfully determine the location, speed and direction of a person behind a one-foot-thick brick wall, but could not detect people standing or sitting still, the article said.

The UK Ministry of Defence is looking into whether the device — designed by Karl Woodbridge and Kevin Chetty of the University of College London — can be used in "urban warfare" for scanning buildings, PopSci reported.

The paper on the research, "Through-the-wall sensing of personnel using passive bi-static Wi-Fi radar at standoff distances", appeared in the April issue of IEEE Transactions on Geoscience and Remote Sensing.

From the abstract:

A series of experiments was conducted, which involved personnel targets moving inside a building within the coverage area of a Wi-Fi access point. These targets were monitored from outside the building using a 2.4-GHz passive multi-static 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 Wi-Fi radar. The measured Doppler shifts agree with those predicted by bi-static 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 Wi-Fi radar as a low-cost TTW detection sensor, with widespread applicability.

Given the concerns over the government's use of mobile body scanner technologies, the development of a device that allows someone to snoop on a person's movements within his or her own home will no doubt be met with public outcry.


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