A TV signal can detect your motion and measure how fast you walk
Your TV can see through walls
Using an Ettus USRP B200, the author captured a local ATSC TV signal and found that selective fading—where a strong reflection cancels the direct signal—makes the reception extremely sensitive to nearby movement. Walking in a neighboring room produced clear stripes in the spectrogram, and the spacing between them revealed a walking speed of 1.3 m/s. Any wideband signal, including WiFi or LTE, could do the same.
So, a simple radio receiver is able to detect motion and provide an estimated speed.
- numlock86
Wait for people to learn about that you can listen in virtually anywhere through fiber-optic cables, as sound waves are enough to redirect the light in the cable to a measurable degree by "bending" the cable itself. Special devices to filter these variances out and transform them back to audio signals are a thing. It's only per line though, as any repeating device will just output a new cleaned up signal again.
Oh well, there have always been ways to snoop on someone like that. Back in the day it was CRTs emitting measurable waves through the neighborhood, with some papers even going as far as restoring the displayed image through a wall. Nowadays it's fiber and WiFi, and those seem to be much easier to work with even. Plus everyone carries a device that is tracked by their mobile provider's cell network. shrug
- armcat
Several years back we actually made use of the principle mentioned in this post, to more systematically count traffic on the roads [1]. In our case we used LTE/4G radio, and we could even estimate the direction of travel, as well as the type of traffic (pedestrian, small vehicles, large vehicles). It was more accurate than Google Maps because it was independent of the moving phone, i.e. it was "sensing the obstacles in the environment".
[1] "Traffic Flow Estimation using LTE Radio Frequency Counters and Machine Learning", https://arxiv.org/abs/2101.09143
- swerner
As anyone observed who had a TV with just basic analog terrestrial antennas, before cable and satellite dishes - it does make a difference to the signal, when and where you stand and countless pieces of physical comedy in TV shows incorporated that.
- 1e1a
See these for a very low cost (<$10) method for observing WiFi and some LTE spectra:
- doginasuit
I thought this article was going to be about putting a webcam on the other side of the wall and streaming through the TV, which somehow had never occurred to me and I want to try it now.
- fzeindl
Amazing book about similar topics: “Silence on the wire” by Michal Zalewski
https://www.amazon.de/Silence-Wire-Passive-Reconnaissance-In...
- reactordev
I threw away all TVs in my house after the proliferation of ads on the Home Screen. Sorry, the only value a TV has is displaying images and audio at large sizes. Not getting me to shop. So I decided to get rid of them all and now I use large monitors and projectors. Make TVs dumb again.
- nubinetwork
I know this is written to sound scary, but it's been like this since the invention of the radio/telegraph (late 1800s?). You're getting blasted by rf almost continuously, unless you live in a bunker.