Detecting Human Movement Using Ambient Radio Signals
Radio frequency interference patterns reveal human motion through walls using standard receivers.
1Radio Signal Interference
Ambient radio waves, such as ATSC digital TV signals, are susceptible to selective fading when reflections interfere with direct paths. By recording these signals with an Ettus USRP B200, one can observe how environmental changes disrupt the wave phase. This phenomenon creates distinct visual patterns on a frequency-based spectrogram display.
2Human Motion Detection
Human movement between a receiver and a transmitter alters the signal path length, causing periodic cancellations. Each time a person moves by one wavelength, the signal experiences a null point. These gaps in the data stream provide a reliable indicator of physical presence and movement within a monitored indoor space.
3Calculating Walking Speed
By analyzing the waterfall update rate of 150 Hz and the pixel distance between signal gaps, precise velocity can be calculated. In this specific experiment, the movement resulted in a calculated speed of 1.3 meters per second, or 4.6 kilometers per hour, while walking perpendicular to the direct signal path.
4Signal Phase Dynamics
The interference is not uniform across the entire signal spectrum because phase shifts depend heavily on frequency. When a person walks through a neighboring room, the reflection perfectly cancels out the residual signal at specific intervals. This sensitivity allows for the detection of motion even when the subject is not visible.
5Broadband Signal Versatility
This detection technique is not limited to television broadcasts; any wideband radio signal can be utilized for similar purposes. Signals like WiFi or LTE transmissions exhibit comparable characteristics, though they often present a messier waterfall display. With additional processing, these common signals can also be repurposed for tracking human activity.
6Technical Implementation Requirements
To successfully capture these disturbances, a software-defined radio receiver is necessary to process the raw signal data. The experiment demonstrates that standard digital signals contain enough environmental information to act as a passive radar system. No active transmission is required to track movement, as ambient background noise provides sufficient data.
7Geometric Considerations
The accuracy of speed estimation depends on the geometry of the movement relative to the signal path. Walking perpendicular to the direct line of sight simplifies the calculation, as no trigonometric adjustments are required. If the movement angle changes, the Doppler-like shift in the signal frequency would necessitate more complex mathematical modeling.
8Implications for Surveillance
The ability to detect motion through walls using existing radio infrastructure highlights significant privacy and security considerations. While this demonstration focuses on TV signals, the underlying physics applies to most modern wireless communication protocols. Future security systems could potentially leverage these ambient signals to monitor spaces without deploying dedicated hardware sensors.
Original article: Hacker News — Detecting Human Movement Using Ambient Radio Signals