LORAN

LORAN (Long Range Navigation) was a WW-II-era low frequency radio wave transmission network, which had usable range of 1,600 km over land and 2,400 km on the sea. And for LORAN triangulation to work there must be a clear Line of Sight between the Transmitters & a Receiver, which is only possible on a flat earth plane.

If the earth were a globe, assuming LORAN antenna height of 300 m and receiver located at 2400 km, there would be a earth bulge of whopping 415 km in between them obstructing the required LoS. And if one were to argue that low frequency radio signals can follow the curvature of the earth from ionosphere skywave reflections (btw that don't exist), then that signal would be useless for triangulation.

LORAN never became obsolete and is still in active development and being deployed. The U.S. DOT awarded contracts to 11 companies to provide backup solution for GPS (which is ground transmissions based as well). Of these companies, Hellen Systems, UrsaNav, and Serco—are working on eLoran projects.