# Detecting Human Movement Using Ambient Radio Signals Radio frequency interference reveals that standard TV signals can track indoor human motion. - **Source:** [Hacker News (news.ycombinator.com)](https://brifing.si/source/13c923cd-6d99-442b-9b8b-1181f4d8cd91) - **Published:** 2026-11-10 - **Topic:** [technology](https://brifing.si/c/technology) - **Original article:** - **On Brifing:** https://brifing.si/s/57486dc2-be7d-4591-8d08-99885a0eb53c ## Radio Signal Interference Analysis A standard ATSC digital television signal can function as a motion sensor through selective fading. When a person moves between a receiver and a transmitter, the direct signal experiences interference from reflections. This phenomenon creates measurable disturbances in the signal phase, allowing for the detection of human presence within a room. ## Selective Fading Mechanism Selective fading occurs when a direct radio signal is canceled out by a strong reflection. Because the phase of this reflection depends on frequency, the effect is not uniform across the entire signal bandwidth. This state makes the radio environment extremely sensitive to any physical movement occurring near the receiver antenna. ## Quantifying Human Walking Speed By analyzing the gaps between stripes on a 150 Hz waterfall display, one can calculate movement speed. Each gap represents a path length change of one wavelength. Using a 490 MHz signal, the observed movement was calculated at 1.3 meters per second, which is approximately 4.6 kilometers per hour for the walker. ## Technical Setup and Equipment The demonstration utilized an Ettus USRP B200 software-defined radio to capture the ATSC signal. By recording the spectrogram, the researcher observed how even minor movements, such as standing up from a chair or walking in a neighboring room, caused distinct, identifiable patterns in the signal data captured by the receiver. ## Broadband Signal Versatility This motion detection capability is not limited to television broadcasts. Any wideband radio signal, including WiFi or LTE, can be exploited for similar purposes. While these signals often produce messier waterfall displays compared to TV signals, the underlying physics allows for the extraction of motion data with sufficient signal processing effort. ## Geometry and Speed Calculations The accuracy of speed estimation depends on the geometry of the movement relative to the signal path. In this experiment, the subject walked perpendicular to the direct signal path, eliminating the need for complex geometric adjustments. This simplified the calculation, directly linking the frequency shift to the physical velocity of the subject. ## Signal Sensitivity Limitations While effective, the method relies on the sensitivity of the receiver to phase changes. The gaps in the signal represent moments where the reflection perfectly cancels the residual signal. This requires a stable, continuous signal source to maintain the integrity of the data, as fluctuations in the source signal could introduce errors. ## Implications for Passive Sensing The ability to track movement via ambient radio signals highlights the pervasive nature of electromagnetic interference. This technique demonstrates that existing infrastructure, such as TV or cellular networks, can be repurposed for passive sensing. It raises questions about privacy and the potential for non-intrusive monitoring using signals already present in homes. ## Sources covering this story - https://maurycyz.com/projects/fading/ — --- _Summary written by Brifing from the original reporting. Copyright in the original article belongs to its publisher; read the full piece at the link above._