Your TV can see through walls

A digital television broadcast can act as a motion sensor because radio waves reach a receiver both directly and after reflecting off nearby objects. In the demonstration, a receiver tuned to a local TV station records the signal’s changing strength and frequency content. Moving the antenna slightly creates “selective fading”: a strong reflection cancels parts of the direct signal, with the cancellation varying across frequencies. This makes the signal unusually sensitive to movement. When the experimenter stands up or walks in a neighboring room, the reflections shift and produce visible stripes in the signal display, even though the person does not cross the direct path between transmitter and receiver.

The spacing between the stripes reveals how quickly the reflected path changes. With a display updating at 150 times per second, stripes about 70 pixels apart indicate a repeating change around 2.1 times per second. At a broadcast frequency of 490 MHz, that corresponds to an estimated walking speed of about 1.3 meters per second, or 4.6 kilometers per hour. The result shows that a basic radio receiver can detect motion and estimate speed without a camera or direct line of sight. WiFi and LTE signals can provide similar information, though their more complicated signal patterns require additional processing.