MOBILE PHONE DETECTOR
Everyone (except me) is carrying mobile (cell) phones now. Moreover
these days most people use smartphones which are continuously
connected to the internet in order to exchange information with
various web servers which feed the 'apps' that the user is running.
What this means is that everyone's walking around with a little RF
beacon in their pocket. It's well known that the operator of the
phone or WiFi network that their phone connects to can use this
beacon to track them as they move around within the network. But
what about an individual device passively detecting the actual
transmissions from phones nearby?
This device wouldn't just be a mobile phone detector, today it
would effectively be a person detector. You could set some of them
up around a site and detect whether anyone entered it. Moreover,
with a very advanced spectrum analyser it might be possible to
identify specific frequency characteristics of individual phones,
corresponding to particular models or even variations of the
transmitter characteristics of the same model. Therefore with
enough work you might be able to passively identify the actual
phone from its RF signature, and therefore its owner.
OK, at first I was drowning in "find my phone app" results, but
after writing all that I've worked out the web search terms that
bring up relevent results and it turns out this sort of thing does
already exist after all:
https://landginternational.net/solutions/mobile-phone-detection/
https://www.homelandsecuritynewswire.com/wolfhound-sniffs-out-contraband-cell-phones
https://en.wikipedia.org/wiki/Radio_fingerprinting
Anyway it might be interesting to see whether a simple DIY solution
might work using RTL-SDR USB TV tuners with some spectrum analyser
software and an antenna:
https://hackaday.com/2014/11/19/rtl-sdr-as-a-spectrum-analyzer/
However ham radio software always seems to be skewed towards
Windows, and I often have trouble getting the few Linux options to
compile/run. Plus it's often designed for more powerful computers
than anything I use so I'm always running into performance
bottlenecks. So I rarely have much success in the end (including
last time I tried to find an RTL-SDR spectrum analyser program for
Linux that worked).
To combat the performance problem I could probably try the old
server that I picked up, notably at a hamfest, and wasn't worth the
trouble of selling like the other one. Power consumption would rule
out running it all the time as an actual phone detector rig for
security purposes though. SDR + low-power SBCs would be a wonderful
combination if only the free software was up to a better standard,
more like commercial stuff wich can be found working on vastly
weaker hardware. Much is even written in Python, yuck!
- The Free Thinker, 2023
