There's a bunch of stuff for using terrain to figure out where you are [1], generally used for missiles and planes though, not sure if works well from a ground perspective.

On the ground, you might be able to use the broadcast positioning system: more or less GPS but instead of satellites, the broadcast signal comes from existing TV towers... TV towers may well be targetted at some point in a war, but there's likely more valuable targets at the beginning of a war.

[1] https://en.wikipedia.org/wiki/TERCOM

[2] https://www.gps.gov/sites/default/files/2025-06/CGSICMeeting...