Adventures in cellular location services

Summary

Preface: I build cellular networks for a job. We support a network in Alaska, and one of the guys we work with there – John – has a story (which I’ll steal here) where he gets a phone call late at night from someone saying they’re in the US Air Force, and uh, they’ve, uh, lost a plane. And since John works for the phone company, he wouldn’t have any idea where it is would you? They ask him. As a matter of fact, John could see the last cell the SIM the pilot was carrying was attached to, they sent a helicopter out and found the pilot, who survived. This was a long time ago, and he was able to pin the location down to a cell (sector), and lookup which direction the sectors were pointing for that cell and the location of it, to give a pretty good idea of the general search area. Now that everyone carries a GPS in their pockets, the level of accuracy here is a lot more than just which cell are you served by (although that’s a lot of accuracy anyway, and not to be ignored). There’s significant privacy implications here and a lot of misinformation about pinging cell towers and “zoom enhance” stuff. I figured I’d actually share how this works IRL – There’s nothing ‘secret‘ here – All of this stuff is in the 3GPP standards which outline how mobile networks should behave. I’ve written a precursor to this a few years ago – And the call was coming from… INSIDE THE HOUSE. A look at finding UE Locations in LTE. Location Sources & Accuracy There’s roughly 4 levels of accuracy in cell phone networks, we’ll cover each one, and how the network treats it. (I’m talking 4G/5G here as most of the world has moved on or is already moving on from 2G/3G) Tracking Area Level Accuracy Cell sites get grouped into tracking areas, they’re kinda like broadcast domains in TCP/IP networking, when you need to “page” (find) a phone that’s “idle” (sleeping) you page the tracking area. Tracking Area sizing has sweet spots, you want more than a few cells, commonly about a dozen or so in the same geograp...