I've recently come across the concept of Ground Direction Finding. My (limited) understanding of it is that it is a system that allows you to see where a VHF transmission is coming from. I gather it was used in the Earhart-era of aviation for navigating, and could be a rudimentary air-traffic control system where there is no radar.
I think it's still used these days in boats but what about aviation? And is my understanding of it essentially correct?