British engineers from University College London have developed a passive radar system that can see through walls using the WiFi signals generated by wireless routers and access points.
The system, devised by Karl Woodbridge and Kevin Chetty, requires two antennae and a signal processing unit (i.e. computer), and is no larger than a suitcase. Unlike normal radar, which emits radio waves and then measures any reflected signals, this new system operates in complete stealth.
The passive radar process is actually quite simple. In any space that has WiFi, you are constantly being bombarded by 2.4GHz and 5GHz radio waves. When these waves hit a moving object, their frequency is altered (the Doppler effect). By carefully “sniffing” the WiFi signals, Woodbridge and Chetty are able to reconstruct an image any objects or humans that are moving on the other side of the wall.