Editing Radio navigation (section)

===Low-frequency radio range===
{{Main|Low frequency radio range}}
[[File:LFR-photo.jpg|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 flight rules|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 approach]]es.{{fact|date=July 2022}}

The ground stations consisted of a set of four antennas that projected two overlapping directional figure-eight signal patterns at a 90-degree angle to each other. One of these patterns was "keyed" with the Morse code signal "A", dit-dah, and the second pattern "N", dah-dit.  This created two opposed "A" quadrants and two opposed "N" quadrants around the station.  The borders between these quadrants created four course legs or "beams" and if the pilot flew down these lines, the "A" and "N" signal merged into a steady "on course" tone and the pilot was "on the beam". If the pilot deviated to either side the "A" or "N" tone would become louder and the pilot knew to make a correction. The beams were typically aligned with other stations to produce a set of [[Airway (aviation)|airways]], allowing an aircraft to travel from airport to airport by following a selected set of stations. Effective course accuracy was about three degrees, which near the station provided sufficient safety margins for [[instrument approach]]es down to low minimums. At its peak deployment, there were over 400 LFR stations in the US.<ref>{{Cite web |url=https://flyingthebeams.com/ |title=Low Frequency Radio Range, Flying the Beam |access-date=2021-02-01 |archive-date=2021-01-16 |archive-url=https://web.archive.org/web/20210116050317/https://flyingthebeams.com/ |url-status=dead }}</ref>