Even better:
1. We figure out what kind of radio modem the police in a locality are using.
2. We set up radio receivers at 3 or more points in the city which are tuned to the frequency of the radios. These locations contain the radio receiver which streams data real time into a PC. Atomic clocks may also be needed on each site.
3. The three sites send data into to a centralized server over the internet, which includes the packet, and the timestamps which the listening location heard the packet.
4. The server triangulates the location of the packet, and marks it on a publicly viewable map.
Hey RevDisk - wanna do a project with me?
Uhm... Hey, why ask solely me to help conduct electronic/information warfare against an oppressive govt entity? My name ain't Oswald, you know.
And besides, mobile data terminals are easier to trace. You just find out the vendor (there's a few major MDT mfg) and you know the frequency band. Just need a spectrum analyzer to lookie at said band for a while and you can probably find a control frequency that's nice n' steady. Couple, not many, use conventional cell phone modems. That'd make stuff problematic, but not impossible.
Once you find a nice single frequency to watch, you set up clusters of dual or triple antennas. You'll need to do some testing to optimize the locations of the antenna clusters. Hook them up to an internet or packet radio rig. No need to precision timing, you're just using direction finding. Can't realistically just do three really senstive really big antennas, you need clusters of smaller antennas. The antenna module can do localized calculations (very simple to do) and just report direction in degrees and distance. You just need fairly precise coords for the base station, then have the base station mod that coord by the info from the antenna array module.
It's really easy to do. We made our own Early Warning Detection system back in college. Needed them for protection.