Movement disrupts radio paths each time an individual alters the physical signal route by a single wavelength, generating readable interference patterns. In one practical demonstration utilizing a 490 MHz broadcast, activity in a neighboring room produced distinct stripes on a spectrogram display updating at 150 Hz. Analysts were able to use the pixel spacing between these stripes to calculate a perpendicular walking speed of 1.3 meters per second. The same detection principles apply broadly across modern wireless infrastructure, including commercial Wi-Fi and LTE transmissions.
Key points
Standard television transmissions can be captured by basic radio receivers to detect huma…: Standard television transmissions can be captured by basic radio receivers to detect human motion occurring behind solid walls.
Calculating the gaps between cancellation stripes on a waterfall display allows observers…: Calculating the gaps between cancellation stripes on a waterfall display allows observers to accurately derive a subject's walking speed.
Wideband radio transmissions like Wi-Fi and LTE possess identical tracking capabilities,…: Wideband radio transmissions like Wi-Fi and LTE possess identical tracking capabilities, though their complex displays require additional processing effort.
Sources checked by our analysis pipeline: 1
Your TV can see through walls:
—This is a spectogram of the signal from a local TV station:
Walking between the receiver and TV station causes a slight disturbance to the signal, but it's nothing to write home about:
For this capture, I moved the antenna a few centimeters:
This is selective fading: The direct signal is being canceled out by a strong reflection. Because the phase of the reflection depends on frequency, this effect is not uniform over the whole signal. Apart from being annoying if you're trying to watch TV, this state is extremely sensitive to any nearby movement.
Here's what happened to the signal when I got up from my chair:
... and here's me walking around in a neighboring room:
The gaps between the stripes happen when my reflection perfectly cancels out the residual signal, which happens each time the path length changes by one wavelength. Let's use this to figure out how fast I was walking:
The gap between each stripe is around 70 pixels, and the waterfall has an update rate of 150 Hz...
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