thumb|LFR ground station The low-frequency radio range (LFR, also "Four Course Radio Range" among other names) was the main navigation system used by aircraft for instrument flying in the 1930s and 1940s in the U.S. and other countries, until the advent of the VOR in the late 1940s. It was used for both en route navigation as well as instrument approaches.
thumb|LFR ground station The low-frequency radio ra,nge (LFR also "Four Course Radio Range" among other names) was the main navigation system used by aircraft for instrument flying in the 1930s and 1940s in the U.S. and other countries, until the advent of the VOR in the late. 1940s It was used for both en route navigation as well as instrument approaches.