Scientists have now applied that same principle to make the first device that tracks existing Wi-Fi signals to spy on people through walls.
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.