A waterfall display captured with an Ettus USRP B200 reveals stripe patterns when the author walks in a neighboring room, illustrating motion‑induced selective fading.
TV SIGNALS CAN DETECT MOVEMENT THROUGH WALLS
*A hobbyist using a cheap software‑defined radio proved that ordinary ATSC TV broadcasts act as motion sensors. By capturing the signal with an Ettus USRP B200, he showed that nearby movement creates distinct fading patterns, exposing a hidden surveillance vector.*
A hobbyist using a cheap software‑defined radio has shown that an ordinary ATSC digital TV broadcast can act as a motion sensor. By pointing an Ettus USRP B200 antenna at a local TV station, he recorded the signal’s spectrogram and observed that any nearby movement disturbs the pattern. The effect, known as selective fading, occurs when a strong reflection cancels the direct line‑of‑sight signal, leaving the composite highly sensitive to changes in path length.
When the researcher stood up or walked in an adjacent room, the waterfall display flashed distinct stripes. Each stripe gap corresponded to a path‑length shift of one wavelength, producing a periodic cancellation. By measuring the pixel distance between gaps (≈70 pixels) and the display’s refresh rate (150 Hz), he derived a 0.47‑second interval, or 2.1 Hz. Multiplying by the wavelength of the 490 MHz carrier yields a speed of about 1.3 m/s (4.6 km/h). The calculation required no extra hardware beyond the radio front‑end.
Can a TV broadcast act as a motion detector?
The experiment used an Ettus USRP B200 to capture the ATSC digital TV signal from a local station. The spectrogram showed a stable carrier when the environment was static. Introducing a person between the antenna and the transmitter created a slight disturbance, visible as a ripple in the waterfall. Moving the antenna a few centimeters produced a similar effect, confirming that the signal’s phase is highly responsive to path‑length changes. The phenomenon, called selective fading, arises when a reflected wave interferes destructively with the direct wave. Because the phase shift varies with frequency, the cancellation is not uniform across the entire bandwidth, making the display a sensitive motion map.
How does selective fading reveal movement?
Selective fading occurs when a strong reflection arrives with a phase offset that cancels the line‑of‑sight component. The cancellation depth depends on the exact path‑length difference, which changes by one wavelength for each stripe gap in the spectrogram. When the researcher rose from his chair, the signal dipped; when he walked in a neighboring room, the display filled with alternating bright and dark bands. Each band represents a moment when the reflected path perfectly negates the residual signal. The pattern’s regularity shows that even modest motions—on the order of a few centimeters—produce measurable spectral shifts.
Can the technique estimate walking speed?
The researcher measured the pixel distance between consecutive dark bands at roughly 70 pixels. The waterfall updates at 150 Hz, so each 70‑pixel interval spans 0.47 seconds, or 2.1 Hz. Using the speed‑of‑light relationship for the 490 MHz carrier, the calculation yields a velocity of about 1.3 m/s, equivalent to 4.6 km/h. Because the subject walked perpendicular to the direct signal path, no geometric correction was needed. The result demonstrates that a simple radio receiver can infer speed without dedicated motion sensors, relying solely on phase‑induced fading.
Do other wireless signals work the same way?
The author notes that any wideband radio signal—Wi‑Fi, LTE, or similar—can produce comparable fading patterns. Those bands generate messier waterfalls due to channel hopping and modulation, but with sufficient processing the same stripe analysis can extract motion data. The principle remains the interference between direct and reflected paths, which is inherent to all radio propagation. Thus, everyday communications infrastructure unintentionally doubles as a covert motion‑tracking system, a fact that raises privacy concerns for anyone within range of a broadcast.
Frequently asked questions
How was motion detected using a TV broadcast?
By recording the ATSC digital TV signal with an Ettus USRP B200 and observing stripe patterns in the spectrogram that appear when a person moves, indicating phase‑induced fading.
What calculation gave the walking speed?
The gap between stripes (~70 pixels) divided by the 150 Hz update rate gave a 0.47 second interval (2.1 Hz); applying the wavelength of the 490 MHz carrier produced an estimated speed of about 1.3 m/s (4.6 km/h).
Can other wireless signals be used similarly?
The author states that any wideband radio like Wi‑Fi or LTE can produce similar fading, though the waterfalls are messier, and the same analysis can extract motion data.
The demonstration proves that everyday broadcast infrastructure leaks more than video—it leaks motion data. No additional hardware, no cameras, just the physics of radio interference. As Wi‑Fi and LTE networks proliferate, the same principle expands the surveillance surface. Citizens and policymakers must reckon with a world where a TV tower or a router can map movement through walls, turning ordinary signals into unintended espionage tools.
Sources: Hacker News (https://maurycyz.com/projects/fading/)