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