Debian developer Petter Reinholdtsen writes:
A little more than a month ago I wrote how to observe the SIM card ID (aka IMSI number) of mobile phones talking to nearby mobile phone base stations using Debian GNU/Linux and a cheap USB software defined radio, and thus being able to pinpoint the location of people and equipment (like cars and trains) with an accuracy of a few kilometer. Since then we have worked to make the procedure even simpler, and it is now possible to do this without any manual frequency tuning and without building your own packages....
The gr-gsm package is now included in Debian testing and unstable, and the IMSI-catcher code no longer require root access to fetch and decode the GSM data collected using gr-gsm.
Here is an updated recipe, using packages built by Debian and a git clone of two python scripts:
Petter Reinholdtsen: Easier recipe to observe the cell phones around you