Tracking cellphone signals is much, much easier than you think. This demo from Danish IT pro Keld Normal uses a $7 USB device to snoop in on cell signals, essentially sniffing out any cellphones connected to a tower nearby.

There’s a lot of Ubuntu and Python stuff going on under the hood, but it’s not as complicated as it looks. The USB doohickey is basically an antenna, picking up signals as they pass between phones and cell towers. This won’t get everything, but it’ll get more data the closer you are to a tower (here’s a good database), and the programs mentioned in the video are pretty good at sorting through the incoming data.

The end result is a database of every phone that connected in the area, along with the unique phone ID, called an IMSI (International Mobile Subscriber Identity) number.

Fun with cell towers

To be clear, this isn’t quite as powerful as the Stingray devices that police and other agencies use, but it also won’t get you in quite as much trouble. Those devices are actively sending out signals to nearby phones, masquerading as cell towers. That lets you get more data (along with more privacy concerns), but it also disrupts cell traffic and could get you in trouble with the FCC.

This is a simpler, safer version: it receives signals, but it doesn’t send them out. In a practical sense, that means the device can track which phones connect to a given tower, as long as it’s active and within range when the connection is made. If you manage to log a specific person’s IMSI number, you could even set an alert when that person connected to a given cell tower — although it would take a permanent setup near the cell tower and a little more programming work. And since you’re not disrupting any signals, I don’t think any of that would actually be illegal, although I am very much not a lawyer.

Anyway, this is just one more weird thing you can do with the omnipresent tracking system that is cellphone infrastructure.