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Uns científics anglesos han aplicat els principis del radar per tal de poder utilitzar el senyal dels routers WiFi per detectar la presència de persones dins d'un edifici («Seeing Through Walls With a Wireless Router»)

Wi-Fi radio signals are found in 61 percent of homes in the U.S. and 25 percent worldwide, so Karl Woodbridge and Kevin Chetty, researchers at University College London, designed their detector to use these ubiquitous signals. When a radio wave reflects off a moving object, its frequency changes—a phenomenon called the Doppler effect. Their radar prototype identifies frequency changes to detect moving objects. It’s about the size of a suitcase and contains a radio receiver composed of two antennas ­and a signal-processing unit. In tests, they have used it to determine a person’s location, speed and direction—even through a one-foot-thick brick wall. Because the device itself doesn’t emit any radio waves, it can’t be detected.

Wi-Fi radar could have domestic applications ranging from spotting intruders to unobtrusively monitoring children or the elderly. It could also have military uses: The U.K. Ministry of Defence has funded a study to determine whether it could be used to scan buildings during urban warfare. With improvements, Woodbridge says, the device could become sensitive enough to pick up on subtle motions the ribcage makes during breathing, which would allow the radar to detect people who are standing or sitting still.

El funcionament s'explica en aquest gràfic:

WiFi Radar

1. El router WiFi emet senyals que topen amb els objectes en moviment, canviant la seva freqüència
2. El router WiFi omple el recinte on es troba amb ones de ràdio en una freqüència determinada (2 o 5 GHz) en funció de la variant de 802.11 utilitzada
3. El radar detecta d'una banda la freqüència base
4. La segona antena del radar detecta els objectes en moviment
5. Un ordinador compara els dos senyals i els utilitza per detectar la posició i la velocitat de moviment de l'objecte en moviment

Pels interessats, he trobat dues referències sobre aquest mètode (les quals no semblen estar disponibles online de forma gratuïta):

• Development and Demonstration of a Software Radio Based Wireless Passive Radar. K. Woodbridge, K. Chetty, L. Young, N. Harley, G. Woodward. IET Electronics Letters, Vol. 48, No. 2, pp 120–121. 19^th January 2012.
• Through-The-Wall Sensing of Personnel using Passive Bistatic WiFi Radar at Standoff Distances. K. Chetty, G.E. Smith and K. Woodbridge. IEEE Transactions on Geoscience and Remote Sensing. Vol. 50 issue 4, pp 1218 — 1226, april 2012.