Low-frequency radio range
The low-frequency radio range, also known as the four-course radio range, LF/MF four-course radio range, A-N radio range, Adcock radio range, or commonly "the range", was the main
Based on a network of radio towers which transmitted directional radio signals, the radio range defined specific airways in the sky. Pilots navigated using low-frequency radio by listening to a stream of automated "A" and "N" Morse codes. For example, they would turn or slip the aircraft to the right when hearing an "N" stream ("dah-dit, dah-dit, ..."), to the left when hearing an "A" stream ("di-dah, di-dah, ..."), and fly straight ahead when these sounds merged to create a constant tone indicating the airplane was directly tracking the beam.[4][5]
As the VOR system was phased in around the world, low-frequency radio range was gradually phased out, mostly disappearing by the 1970s. There are no remaining operational facilities today. At its maximum deployment, there were over 400 stations exclusively using low-frequency radio range in the Continental U.S. alone.[6][2]
History
After World War I, aviation began to expand its role into the civilian arena, starting with airmail flights. It soon became apparent that for reliable mail delivery, as well as the passenger flights which were soon to follow, a solution was required for navigation at night and in poor visibility. In the U.S., a network of lighted beacons, similar to maritime lighthouses, was constructed for the airmail pilots. But the beacons were useful mostly at night and in good weather, while in poor visibility conditions they could not be seen. Scientists and engineers realized that a radio based navigation solution would allow pilots to "see" under all flight conditions, and decided a network of directional radio beams was needed.[7]
On September 24, 1929, then-Lieutenant (later General)
There were two technological approaches for both the ground and air radio navigation components, which were being evaluated during the late 1920s and early 1930s.
On the ground, to obtain directional radio beams with a well-defined navigable course, crossed
In the air, there were also two competing designs, originating from groups of different backgrounds and needs. The
A visual indicator was developed based on vibrating reeds, which provided a simple panel-mounted "turn left-right" indicator. It was reliable, easy to use and more immune to erroneous signals than the competing audio based system. Pilots who had flown with both aural and visual systems strongly preferred the visual type, according to a published report.[7][16] The reed-based solution was passed over by the U.S. government, however, and the audio signals became standard for decades to come.[7][15]
By the 1930s, the network of ground-based, low-frequency radio transmitters, coupled with affordable on-board AM radio receivers, became a vital part of instrument flying. Low-frequency radio transmitters provided navigational guidance to aircraft for en route operations and approaches under virtually all weather conditions, helping to make consistent and reliable flight schedules a reality.[4]
The radio range remained as the main radio navigation system in the U.S. and other countries until it was gradually replaced by the much-improved
Technology
Ground
The low-frequency radio ground component consisted of a network of radio transmission stations which were strategically located around the country, often near larger airports, approximately 200 miles apart. Early low-frequency stations used crossed loop antennas, but later designs for many stations used the Adcock vertical antenna array for improved performance, especially at night.[3][7]
Each Adcock range station had four 134-foot-tall (41 m) antenna towers erected on the corners of a 425 × 425 ft square, with an optional extra tower in the center for voice transmission and
In addition to the repeating A or N modulation signal, each transmitting station would also transmit its Morse code identifier (typically 2 or 3 letters) once every thirty seconds for positive identification.[20] The station identification would be sent twice: first on the N pair of transmitters, then on the A, to ensure coverage in all quadrants.[3][note 4] Also, in some installations local weather conditions were periodically broadcast in voice over the range frequency, preempting the navigational signals, but eventually this was done on the central fifth tower.[21][note 5]
The low-frequency radio range was originally accompanied by airway beacons, which were used as a visual backup, especially for night flights.[4] Additional "marker beacons" (low power VHF radio transmitters) were sometimes included as supplementary orientation points.[22]
Air
The airborne radio receivers—initially simple
Pilots had to verify that they were tuned to the correct range station frequency by comparing its Morse code identifier against the one published on their navigation charts. They would also verify they were flying towards or away from the station, by determining if the signal level (i.e., the audible tone volume) was getting stronger or weaker.[4][5][note 7]
Approaches and holds
Final approach segments of low-frequency radio
In a typical low-frequency radio instrument approach procedure,
The low-frequency radio range also allowed
Non-directional beacons
From its beginning in the early 1930s, the low-frequency radio was augmented with low-frequency
Early RDF receivers were costly, bulky and difficult to operate, but the simpler and less expensive ground installation allowed the easy addition of NDB based waypoints and approaches, to supplement the low-frequency radio system.[4] Modern RDF receivers, called "automatic direction finders" (or "ADF") are small, low cost and easy to operate. The NDB-ADF system remains today as a supplement and backup to the newer VOR and GPS navigation systems, although it is gradually being phased out.[24][25] All questions regarding NDB/ADF operation were removed from
Limitations
Although the low-frequency radio system was used for decades as the main aeronautical navigation method during
Replacement by VOR
The low-frequency radio navigation system required, at a minimum, only a simple
Sounds
The following are simulated sounds for the
(See
Notes
- turn and bank instrument which was relatively common by the time of his flight.[10]
- international standards, the frequency band below 300 kHz is "Low Frequency", and above that "Medium Frequency". Since LFR frequencies "straddled" the dividing line between the two bands, they were technically called "Low Frequency/Medium Frequency (LF/MF) Radio Range" stations.
- ^ In the U.S. the quadrant which included the true north radial was designated as N (if a course leg was exactly on true north, then the northwest quadrant became N); in Canada, N was the quadrant which included the 045° true radial.[3]
- ^ Since the station identification was transmitted in sequence, first on the N and then the A antenna pairs, it would be heard by the pilot once or twice, possibly with different relative amplitudes, depending on aircraft location. For example, it would be heard twice when on the beam, and only once when inside a quadrant.[3]
- ^ Pilots had to request to stop the weather report if they were using the LFR for an approach.[3]
- ^ Every 30 seconds, Silver Lake station's Morse code identifier, "di-dah-dit di-dah-di-dit" (R-L), would preempt the navigation signals.
- Automatic Gain Control (AGC) when assessing relative signal strength.[5]
- ^ Basic flight instruments would still be needed.
See also
References
- ^ ISBN 0-7575-0944-4.
- ^ a b c d e f g "Four-Course Radio Range (Low-Frequency Radio Range (LFR))". Museum of Air Traffic Control. Archived from the original on June 22, 2009. Retrieved 2009-07-21.
- ^ a b c d e f g h i j "The Radio Range". Instrument Flying - AF Manual 51-37. Air Training Command, Department of the Air Force. January 20, 1966. pp. 14/1–17.
- ^ a b c d e f g h i j "On the Beam". www.navfltsm.addr.com. Retrieved 2009-07-21.
- ^ a b c "Four-Course Radio Ranges". www.aopa.org. 1997-05-10. Retrieved 2022-04-05.
- ^ Davis, Doug. "Low Frequency Radio Range Locations". Flying the Beams. Retrieved 7 March 2021.
- ^ a b c d e f g h i "BLIND FLYING ON THE BEAM: AERONAUTICAL COMMUNICATION, NAVIGATION AND SURVEILLANCE: ITS ORIGINS AND THE POLITICS OF TECHNOLOGY" (PDF). Journal of Air Transportation. 2003. Archived from the original (PDF) on 2017-01-25. Retrieved 2009-07-31.
- ^ "Flying Blind: A Brief History of Aviation Advancements, 1918-1930". Columbia University. Archived from the original on 2010-01-02. Retrieved 2009-07-24.
- ^ "'BLIND' PLANE FLIES 15 MILES AND LANDS; FOG PERIL OVERCOME". The New York Times. September 25, 1929. p. 1.
- ^ Invention and Technology Magazine. 10 (4).[permanent dead link]
- ^ "Ford Radio Beacon Station". The Henry Ford. Retrieved 7 March 2021.
- ^ Donovan, Eugene S. (5 December 1933). "Radio Beacon". United States Patent and Trademark Office. Retrieved 7 March 2021.
- ^ Scheller, Otto (11 September 1908). "Drahtloser Kursweiser und Telegraph" (PDF) (in German). Retrieved 7 March 2021.
- ^ "Radio Range and Marker Beacons". Air Commerce Bulletin. Vol. 1, no. 6. Washington, D.C.: Aeronautics Branch, Department of Commerce. 15 September 1929. p. 9. Retrieved 7 March 2021.
- ^ a b c d e "FAA HISTORICAL CHRONOLOGY, 1926-1996" (PDF). Archived from the original (PDF) on 2008-06-24. Retrieved 2009-07-30.
- ^ "New Wireless Beacon for Croydon". Flight. November 27, 1931. p. 1177.
- ^ ISBN 0-07-462301-X.
- ^ ISBN 1-4289-2410-8.
- ^ "Adcock Antenna". Virtual Institute of Applied Science. Archived from the original on 2009-05-11. Retrieved 2009-07-22.
- ^ LFR station identification codes varied between one letter for early stations, two letters in the mid to late 30's and 3 letters for later stations when stations became more numerous.
- ^ ISSN 0032-4558.
- ^ ISBN 0-7881-4728-5.
- ^ U.S. Dept. of Commerce (January 16, 1957). Joliet Airport Approach Procedure (CAA). United States Coast and Geodetic Survey.
- ^ "ADF Basics". September 6, 1998. Retrieved 2009-07-30.
- ^ ISBN 0-07-009493-4.
- ^ ""What's New and Upcoming in Airman Testing"" (PDF). October 2017. p. 5. Retrieved 2020-07-24.
Further reading
- Oser, Hans J. "Development of the Visual-Type Airway Radio-Beacon System" (PDF). National Institute of Standards and Technology. Retrieved 2009-08-02.
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(help) - Diamond, H.; Dunmore, F. W. (September 19, 1930). "A Radio System for Blind Landing of Aircraft in Fog" (PDF). Proceedings of the National Academy of Sciences. 16 (11): 678–685. PMID 16577291.
- Conway, Erik M. (2006). Blind landings: low-visibility operations in American aviation, 1918-1958. JHU Press. ISBN 0-8018-8449-7.
- "Interview with James H. Doolittle" (PDF). Columbia University. Archived from the original (PDF) on 2010-01-15. Retrieved 2009-08-02.
- "Visual-Aural Radio Range". The Airways Museum. Retrieved 2010-05-19.
External links
- Flying the Beams, History of the Low Frequency Radio Range (with maps)
- Fort Chimo, Quebec low-frequency radio approach
- The Adcock Range System
- Video clip explaining low-frequency radio