A hobbyist experiment demonstrates how a standard TV receiver can detect motion through walls using signal analysis. By walking between the antenna and a broadcast tower, the user observed selective fading caused by signal reflections. This interference creates a visible pattern on a waterfall display, where the gaps between stripes correspond to wavelength-scale path length changes.
Analyzing the stripe frequency revealed a walking speed of approximately 1.3 meters per second (4.6 km/h). The technique relies on the phase cancellation of reflected signals, a principle applicable to any wideband radio frequency, including Wi-Fi and LTE. While TV signals provide a cleaner demonstration, the same motion-detection method can be extracted from commercial wireless signals with additional processing.
The findings highlight the surprising sensitivity of broadcast infrastructure to human movement, turning commonplace TV static into a rudimentary motion sensor capable of estimating speed without specialized hardware.
Source: Hacker News · Summarized by HeadlinesBriefing