Lightning detection. You have a couple of ground stations with known positions that wait for certain electromagnetic puses, and which record the timestamps of such pulses. With enough stations you can triangulate the location of the source of each pulse. Also a great way to detect nuclear detonations.

There is a german club that builds and distrubutes such stations (using GPS for location and timing), with a quite impressive global coverage by now:

https://www.blitzortung.org