National low-frequency radio navigation system (c.1930-c.1950) replaced by an omnirange (VOR) system. It consisted of four segmented quadrants broadcasting Morse Code "A" (dot-dash) and "N" (dash-dot) signals in opposing quadrants so that pilots could orient their position relative to a "beam" broadcasting a steady tone, and a Morse Code station identifier. Using a "build-and-fade" technique, a pilot could (ideally) pinpoint his location by the strength or weakness of a signal. Read about it here.