There are other surveillance technologies out there that can “see through walls”, but they all rely on transmitting something. This new idea is purely a receiver, so the subject has no way of telling you are watching them.
All it takes is for the subject to have WiFi in their home – presumably the subject needs to be within range of the WiFi. The article says that 25% of homes worldwide have WiFi, but of course in the western world it would be substantially higher.
Wi-Fi radio 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 30cm thick brick wall. Because the device itself doesn’t emit any radio waves, it can’t be detected.
