384:
432:
31:
624:
122:
788:
828:
708:
748:
208:
in 1906 which were later experimented with by the US Bureau of
Standards and Army Signal Corps in the early 1920s. The technology was quickly adopted by the U.S. Commerce Department, who set up a demonstration range on June 30, 1928, and the first series of stations entered service later that year. But the loop antenna design generated excessive horizontally polarized skywaves that could interfere with the signals, especially at night. By 1932, the
235:, representing military aviators, preferred a solution based on a stream of audio navigation signals, constantly fed into the pilots' ears via a headset. Civilian pilots on the other hand, who were mostly airmail pilots flying cross-country to deliver the mail, felt the audio signals would be annoying and difficult to use over long flights, and preferred a visual solution, with an indicator in the instrument panel.
522:
137:
403:(AM) sets—were tuned to the frequency of the low-frequency radio ground transmitters, and the Morse code audio was detected and amplified into speakers, typically in headsets worn by the pilots. The pilots would constantly listen to the audio signal, and attempt to fly the aircraft along the course lines ("flying the beam"), where a uniform tone would be heard. If the signal of a single letter (
344:
501:"on the beam", i.e., on one of the low-frequency legs, with the holding fix (key turning point) over the low-frequency radio station, in the cone of silence or over one of the fan markers. The holds were used either during the en route portion of a flight or as part of the approach procedure near the terminal airport. Low-frequency radio holds were more accurate than
685:—would preempt the navigational signals every 30 seconds to transmit its Morse code identifier ("RL"). The station identification would be heard once or twice, possibly with different relative amplitudes, depending on the aircraft location. Pilots would listen to and navigate by these sounds for hours while flying. Actual sounds contained
197:, and a specially designed directional radio system to navigate to and from the airport. Doolittle's experimental equipment was purpose-built for his demonstration flights; for instrument flying to become practical, the technology had to be reliable, mass-produced and widely deployed, both on the ground and in the aircraft fleet.
539:(NDBs). While the low-frequency radio required a complex ground installation and only a simple AM receiver on board the aircraft, NDB ground installations were simple single-antenna transmitters requiring somewhat more complex equipment on board the aircraft. The NDB's radio emission pattern was uniform in all directions in the
482:
268:
224:
109:. 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.
422:
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
207:
were used initially. The Ford Motor
Company developed the first commercially workable application of a loop-based, low-frequency radio range. They installed it at their Dearborn and Chicago fields in 1926 and filed the patent for it in 1928. Earlier concepts for the system were developed in Germany
664:
navigation system, had many advantages. The VOR was virtually immune to interference, had 360 available course directions, had a visual "on course" display (with no listening needed), and was far easier to use. Consequently, when the VOR system became available in the early 1950s its acceptance was
279:
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
590:
and night flying, it had some well-known limitations and drawbacks. Pilots had to listen to the signals, often for hours, through the early generation headsets of the day. The course lines, which were a result of a balance between the radiation patterns from different transmitters, would fluctuate
238:
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
112:
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
651:
to low minimums. On the downside, however, it had only four course directions per station, was sensitive to atmospheric and other types of interference and aberrations, and required pilots to listen for hours to an annoying monotonous beep, or a faint stream of Morse codes, often embedded in
246:
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
446:
were normally flown near the range station, which ensured increased accuracy. When the aircraft was over the station, the audio signal disappeared, since there was no modulation signal directly above the transmitting towers. This quiet zone, called the
368:, to ensure coverage in all quadrants. 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.
570:, to supplement the low-frequency radio system. 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
359:
modulation signal, each transmitting station would also transmit its Morse code identifier (typically 2 or 3 letters) once every thirty seconds for positive identification. The station identification would be sent twice: first on the
152:
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
947:
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
411:) became audibly distinct, the aircraft would be turned as needed so that the modulation of the two letters would overlap again, and the Morse code audio would become a steady tone. The "on course" region, where the
165:
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.
287:
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
336:
signals were of equal intensity, with their combined Morse codes merging into a steady 1,020 Hz audio tone. These course lines (also called "legs"), where only a tone could be heard, defined the
547:(RDF). The NDB-RDF combination allowed pilots to determine the direction of the NDB ground station relative to the direction the airplane was pointing. When used in conjunction with the on board
375:, which were used as a visual backup, especially for night flights. Additional "marker beacons" (low power VHF radio transmitters) were sometimes included as supplementary orientation points.
689:, interference and other distortions, not reproduced by the simulation. Adjusting the volume would affect the effective course width. For example, in the simulated sound for "twilight"
1582:
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.
909:". Since LFR frequencies "straddled" the dividing line between the two bands, they were technically called "Low Frequency/Medium Frequency (LF/MF) Radio Range" stations.
200:
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.
1751:
562:
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
451:", signified to the pilots that the aircraft was directly overhead the station, serving as a positive ground reference point for the approach procedure.
328:). The intersections between the quadrants defined four course lines emanating from the transmitting station, along four compass directions, where the
665:
rapid, and within a decade the low-frequency radio was mostly phased out. VOR itself is gradually being phased out today in favor of the far superior
173:, U.S. Army, demonstrated the first "blind" flight, performed exclusively by reference to instruments and without outside visibility, and proved that
1939:
1944:
1653:
250:
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
240:
1363:
644:
587:
162:
1060:
966:
Every 30 seconds, Silver Lake station's Morse code identifier, "di-dah-dit di-dah-di-dit" (R-L), would preempt the navigation signals.
391:). Aircraft at location 1 would hear: "dah-dit, dah-dit, ...", at 2: "di-dah, di-dah, ...", at 3: a steady tone, and at 4: nothing (
1861:
1851:
1700:
1632:
1542:
1515:
1318:
1031:
419:
audibly merged, was approximately 3° wide, which translated into a course width of ±2.6 miles when 100 miles away from the station.
1716:
1558:
1237:"BLIND FLYING ON THE BEAM: AERONAUTICAL COMMUNICATION, NAVIGATION AND SURVEILLANCE: ITS ORIGINS AND THE POLITICS OF TECHNOLOGY"
804:
513:, whereas the low-frequency radio hold was as accurate as the low-frequency radio leg, with an approximate course width of 3°.
217:
1337:
1236:
1267:
803:
220:
referred to the Adcock solution as the "T-L Antenna" (for "Transmission Line") and did not initially mention Adcock's name.
1744:
1168:
574:
navigation systems, although it is gradually being phased out. All questions regarding NDB/ADF operation were removed from
847:
846:
1454:
766:
726:
608:
121:
1394:
767:
727:
1419:
304:, into four quadrants. The radiation of one opposing quadrant pair was modulated (at an audio frequency of 1,020
254:-based VOR technology, starting in the late 1940s. The VOR, still used today, includes a visual left-right indicator.
1426:. Vol. 1, no. 6. Washington, D.C.: Aeronautics Branch, Department of Commerce. 15 September 1929. p. 9
227:
The vibrating reed, developed in the 1920s, was a simple, panel-mounted instrument with "turn left-right" indicator.
161:, was constructed for the airmail pilots. But the beacons were useful mostly at night and in good weather, while in
231:
In the air, there were also two competing designs, originating from groups of different backgrounds and needs. The
1949:
666:
661:
632:
571:
455:
293:
83:
678:
586:
Although the low-frequency radio system was used for decades as the main aeronautical navigation method during
213:
1886:
1313:
591:
depending on weather conditions, vegetation or snow cover near the station, and even the airborne receiver's
458:
would begin over the range station, with a turn to a specific course. The pilot would descend to a specified
1109:
885:
448:
435:
392:
174:
129:
79:
865:
239:
preferred the visual type, according to a published report. The reed-based solution was passed over by the
976:
592:
552:
544:
536:
530:
502:
289:
212:
array eliminated this problem by only having vertical antennae and it became the preferred solution. The
105:
in the sky. Pilots navigated using low-frequency radio by listening to a stream of automated "A" and "N"
1765:
1485:
247:
virtually all weather conditions, helping to make consistent and reliable flight schedules a reality.
1912:
1802:
1489:
640:
400:
648:
628:
567:
506:
494:
459:
443:
87:
1294:
603:
quadrant (or vice versa), causing a false "virtual course" away from any real course line. Also,
498:
475:
182:
91:
535:
From its beginning in the early 1930s, the low-frequency radio was augmented with low-frequency
1371:
795:
474:, low-frequency radio approach procedure, minimum descent altitude could be as low as 300 feet
203:
On the ground, to obtain directional radio beams with a well-defined navigable course, crossed
1847:
1830:
1696:
1628:
1602:
1597:
1538:
1511:
1064:
1027:
682:
337:
232:
190:
186:
102:
98:
1868:
1820:
1810:
957:
Pilots had to request to stop the weather report if they were using the LFR for an approach.
906:
540:
194:
75:
271:
Early low-frequency radio station based on crossed loop antennas; later installations used
1778:
898:
835:
486:
467:
170:
125:
347:
Adcock range station. The central fifth tower was typically used for voice transmissions.
17:
1806:
755:
715:
281:
272:
209:
1825:
1271:
1933:
902:
686:
653:
612:
604:
372:
154:
1562:
136:
1667:
1243:
1142:
463:
204:
86:(VOR), beginning in the late 1940s. It was used for en route navigation as well as
1022:
Lawrence, Harry (2004). "Airways—from
Lighted Beacons to Radio Navigation".
1787:
845:
802:
765:
725:
918:
In the U.S. the quadrant which included the true north radial was designated as
639:
The low-frequency radio navigation system required, at a minimum, only a simple
145:
922:(if a course leg was exactly on true north, then the northwest quadrant became
1922:
471:
309:
178:
158:
106:
1917:
1606:
1461:
681:
low-frequency radio. The range station—located about 10 miles north of
462:(MDA), and if the airport was not in sight within a specified time (based on
623:
1834:
1815:
939:
Since the station identification was transmitted in sequence, first on the
148:, aviation began to expand its role into the civilian arena, starting with
1844:
Blind landings: low-visibility operations in
American aviation, 1918-1958
563:
693:
below, where the aircraft is nearly on the beam but slightly inside the
478:, and required minimum visibility one mile, depending on aircraft type.
30:
1908:
Flying the Beams, History of the Low
Frequency Radio Range (with maps)
1000:
548:
510:
149:
787:
343:
243:, however, and the audio signals became standard for decades to come.
481:
383:
267:
223:
1268:"Flying Blind: A Brief History of Aviation Advancements, 1918-1930"
431:
827:
622:
520:
480:
430:
382:
342:
305:
297:
266:
222:
135:
120:
29:
888:
instrument which was relatively common by the time of his flight.
1907:
1194:
747:
707:
454:
In a typical low-frequency radio instrument approach procedure,
301:
643:
on board the aircraft to accurately navigate the airways under
521:
1292:"'BLIND' PLANE FLIES 15 MILES AND LANDS; FOG PERIL OVERCOME".
657:
575:
388:
251:
578:
pilot certification testing materials prior to
October 2017.
371:
The low-frequency radio range was originally accompanied by
1745:"Development of the Visual-Type Airway Radio-Beacon System"
1601:. Hearst Magazines: 402–404, 138A, 140A, 142A. March 1936.
1061:"Four-Course Radio Range (Low-Frequency Radio Range (LFR))"
525:
NDB ground installations are simple single tower antennas.
635:
technology superseded low-frequency radio by the 1960s.
169:
On
September 24, 1929, then-Lieutenant (later General)
1561:. Virtual Institute of Applied Science. Archived from
1506:
Nagaraja (2001). "The LF/MF Four-Course Radio Range".
1788:"A Radio System for Blind Landing of Aircraft in Fog"
1242:. Journal of Air Transportation. 2003. Archived from
930:
was the quadrant which included the 045° true radial.
868:
if you encounter problems playing these sound files.)
97:
Based on a network of radio towers which transmitted
611:to disrupt the range signals and produce crackling
1786:Diamond, H.; Dunmore, F. W. (September 19, 1930).
701:sound, whereas a loud one makes it more distinct.
1625:Flight Check!: The Story Of Faa Flight Inspection
438:low-frequency radio instrument approach procedure
697:quadrant, a low volume almost obscures the weak
607:and other atmospheric disturbances would create
101:radio signals, the radio range defined specific
82:in the 1930s and 1940s, until the advent of the
1913:Fort Chimo, Quebec low-frequency radio approach
1795:Proceedings of the National Academy of Sciences
1063:. Museum of Air Traffic Control. Archived from
988:Basic flight instruments would still be needed.
509:are predicated on the accuracy of the on-board
442:Final approach segments of low-frequency radio
284:for improved performance, especially at night.
1752:National Institute of Standards and Technology
1695:. McGraw-Hill Professional. pp. 110–111.
979:(AGC) when assessing relative signal strength.
595:. Under some conditions, the signals from the
470:procedure would be initiated. In the depicted
399:The airborne radio receivers—initially simple
387:Chart of Silver Lake low-frequency radio (269
1717:""What's New and Upcoming in Airman Testing""
423:tone volume) was getting stronger or weaker.
177:was feasible. Doolittle used newly developed
8:
901:, the frequency band below 300 kHz is "
1648:U.S. Dept. of Commerce (January 16, 1957).
1618:
1616:
1501:
1499:
1449:
1447:
1445:
1443:
1441:
677:The following are simulated sounds for the
543:. The NDB's on-board receiver was called a
493:The low-frequency radio range also allowed
27:Navigation system formerly used by aircraft
1686:
1684:
1590:
1588:
1529:
1527:
559:chosen course radiating from the station.
1923:Video clip explaining low-frequency radio
1824:
1814:
1368:United States Patent and Trademark Office
34:Low-frequency radio range audio signals:
1137:
1135:
1133:
1131:
1129:
1127:
1125:
1123:
1121:
1119:
1055:
1053:
1051:
1049:
1047:
1045:
1043:
1654:United States Coast and Geodetic Survey
1650:Joliet Airport Approach Procedure (CAA)
1163:
1161:
1159:
1011:
877:
1774:
1763:
1537:. Diane Publishing. 1982. p. 28.
1535:Airport and air traffic control system
1455:"FAA HISTORICAL CHRONOLOGY, 1926-1996"
1362:Donovan, Eugene S. (5 December 1933).
1307:
1305:
1099:
1097:
1095:
1093:
1091:
1089:
1087:
1085:
1083:
1081:
1017:
1015:
320:), and the other pair with the letter
1867:. Columbia University. Archived from
1395:"Drahtloser Kursweiser und Telegraph"
1270:. Columbia University. Archived from
1231:
1229:
1227:
1225:
1223:
1221:
1219:
1217:
1215:
1195:"Low Frequency Radio Range Locations"
1112:. January 20, 1966. pp. 14/1–17.
7:
1393:Scheller, Otto (11 September 1908).
975:The pilot would have to disable the
645:instrument meteorological conditions
485:Diagram showing low-frequency radio
1862:"Interview with James H. Doolittle"
1106:Instrument Flying - AF Manual 51-37
1024:Aviation and the Role of Government
292:. The stations emitted directional
1492:. November 27, 1931. p. 1177.
364:pair of transmitters, then on the
42:stream and combined uniform tone (
25:
1508:Elements of Electronic Navigation
1486:"New Wireless Beacon for Croydon"
1319:Invention and Technology Magazine
1627:. DIANE Publishing. p. 46.
1420:"Radio Range and Marker Beacons"
1298:. September 25, 1929. p. 1.
843:
826:
800:
786:
763:
746:
723:
706:
656:. Its eventual replacement, the
189:—to help him maintain his
1940:Aeronautical navigation systems
884:Doolittle also used a standard
599:quadrant would "skip" into the
553:navigate to or from the station
505:(NDB) holds, since NDB holding
1945:History of air traffic control
497:to instruct pilots to enter a
1:
351:In addition to the repeating
282:Adcock vertical antenna array
60:LF/MF four-course radio range
1312:Heppenheimer, T. A. (1995).
1026:. Kendall Hunt. p. 92.
609:electromagnetic interference
140:Doolittle's instrument panel
1623:Thompson, Scott A. (1990).
1338:"Ford Radio Beacon Station"
1110:Department of the Air Force
43:
1966:
1887:"Visual-Aural Radio Range"
1169:"Four-Course Radio Ranges"
866:Knowledge (XXG):Media help
528:
300:and from 50 to 1,500
171:James H. "Jimmy" Doolittle
163:poor visibility conditions
128:demonstrated in 1929 that
1722:. October 2017. p. 5
667:Global Positioning System
615:in the pilots' headsets.
294:electromagnetic radiation
275:for improved performance.
84:VHF omnidirectional range
52:low-frequency radio range
18:Low frequency radio range
1842:Conway, Erik M. (2006).
1108:. Air Training Command,
460:minimum descent altitude
214:U.S. Commerce Department
113:Continental U.S. alone.
1918:The Adcock Range System
1145:. www.navfltsm.addr.com
899:international standards
537:non-directional beacons
517:Non-directional beacons
56:four-course radio range
1816:10.1073/pnas.16.11.678
1773:Cite journal requires
1693:Aviator's guide to GPS
977:Automatic Gain Control
647:, and even execute an
636:
545:radio direction finder
531:Non-directional beacon
526:
503:non-directional beacon
490:
439:
396:
348:
276:
228:
157:, similar to maritime
141:
133:
47:
1691:Clarke, Bill (1998).
1424:Air Commerce Bulletin
853:Slightly off beam in
626:
524:
484:
444:instrument approaches
434:
386:
346:
270:
226:
139:
124:
88:instrument approaches
78:used by aircraft for
33:
1889:. The Airways Museum
1595:"Flying the Beams".
1510:. Tata McGraw-Hill.
427:Approaches and holds
401:Amplitude Modulation
54:, also known as the
1807:1930PNAS...16..678D
1670:. September 6, 1998
1104:"The Radio Range".
905:", and above that "
649:instrument approach
495:air traffic control
191:aircraft's attitude
1295:The New York Times
637:
619:Replacement by VOR
551:, the pilot could
527:
491:
440:
397:
349:
277:
229:
218:Aeronautics Branch
183:attitude indicator
181:instruments—
142:
134:
68:Adcock radio range
48:
1598:Popular Mechanics
1374:on 19 August 2022
869:
848:
805:
768:
728:
683:Baker, California
641:AM radio receiver
233:Army Signal Corps
175:instrument flying
130:instrument flying
80:instrument flying
76:navigation system
16:(Redirected from
1957:
1950:Radio navigation
1897:
1895:
1894:
1882:
1880:
1879:
1873:
1866:
1857:
1838:
1828:
1818:
1792:
1782:
1776:
1771:
1769:
1761:
1759:
1758:
1749:
1731:
1730:
1728:
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1721:
1713:
1707:
1706:
1688:
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1678:
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1675:
1664:
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1639:
1638:
1620:
1611:
1610:
1592:
1583:
1580:
1574:
1573:
1571:
1570:
1559:"Adcock Antenna"
1555:
1549:
1548:
1531:
1522:
1521:
1503:
1494:
1493:
1482:
1476:
1475:
1473:
1472:
1466:
1460:. Archived from
1459:
1451:
1436:
1435:
1433:
1431:
1416:
1410:
1409:
1407:
1405:
1399:
1390:
1384:
1383:
1381:
1379:
1370:. Archived from
1359:
1353:
1352:
1350:
1348:
1334:
1328:
1327:
1309:
1300:
1299:
1289:
1283:
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1264:
1258:
1257:
1255:
1254:
1248:
1241:
1233:
1210:
1209:
1207:
1205:
1199:Flying the Beams
1190:
1184:
1183:
1181:
1180:
1165:
1154:
1153:
1151:
1150:
1139:
1114:
1113:
1101:
1076:
1075:
1073:
1072:
1067:on June 22, 2009
1057:
1038:
1037:
1019:
989:
986:
980:
973:
967:
964:
958:
955:
949:
937:
931:
916:
910:
907:Medium Frequency
895:
889:
882:
863:
850:
849:
830:
807:
806:
790:
770:
769:
750:
730:
729:
710:
549:magnetic compass
541:horizontal plane
511:magnetic compass
487:holding patterns
74:", was the main
44:simulated sounds
21:
1965:
1964:
1960:
1959:
1958:
1956:
1955:
1954:
1930:
1929:
1904:
1892:
1890:
1885:
1877:
1875:
1871:
1864:
1860:
1854:
1841:
1801:(11): 678–685.
1790:
1785:
1772:
1762:
1756:
1754:
1747:
1742:
1739:
1737:Further reading
1734:
1725:
1723:
1719:
1715:
1714:
1710:
1703:
1690:
1689:
1682:
1673:
1671:
1666:
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1647:
1646:
1642:
1635:
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1614:
1594:
1593:
1586:
1581:
1577:
1568:
1566:
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1556:
1552:
1545:
1533:
1532:
1525:
1518:
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1497:
1484:
1483:
1479:
1470:
1468:
1464:
1457:
1453:
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499:holding pattern
468:missed approach
449:cone of silence
429:
393:cone of silence
381:
312:for the letter
273:Adcock antennas
265:
260:
241:U.S. government
155:lighted beacons
126:Jimmy Doolittle
119:
70:, or commonly "
64:A-N radio range
28:
23:
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1775:|journal=
1743:Oser, Hans J.
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529:Main article:
518:
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456:final approach
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373:airway beacons
296:at 190 to 535
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210:Adcock antenna
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1874:on 2010-01-15
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1853:0-8018-8449-7
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1846:. JHU Press.
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1193:Davis, Doug.
1189:
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1143:"On the Beam"
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903:Low Frequency
900:
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881:
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1891:. Retrieved
1876:. Retrieved
1869:the original
1843:
1798:
1794:
1766:cite journal
1755:. Retrieved
1724:. Retrieved
1711:
1692:
1672:. Retrieved
1668:"ADF Basics"
1662:
1649:
1643:
1624:
1596:
1578:
1567:. Retrieved
1563:the original
1553:
1534:
1507:
1480:
1469:. Retrieved
1462:the original
1428:. Retrieved
1423:
1414:
1402:. Retrieved
1388:
1376:. Retrieved
1372:the original
1367:
1357:
1345:. Retrieved
1341:
1332:
1323:
1317:
1293:
1287:
1276:. Retrieved
1272:the original
1262:
1251:. Retrieved
1244:the original
1202:. Retrieved
1198:
1188:
1177:. Retrieved
1175:. 1997-05-10
1173:www.aopa.org
1172:
1147:. Retrieved
1105:
1069:. Retrieved
1065:the original
1023:
984:
971:
962:
953:
944:
940:
935:
927:
923:
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893:
880:
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854:
837:
817:intersection
814:
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774:
757:
734:
717:
698:
694:
690:
676:
638:
600:
596:
585:
561:
556:
534:
492:
464:ground speed
453:
441:
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143:
132:is feasible.
111:
96:
71:
67:
63:
59:
55:
51:
49:
39:
35:
1400:(in German)
836:"Twilight"
756:"Distinct"
716:"Distinct"
679:Silver Lake
652:background
582:Limitations
187:gyrocompass
159:lighthouses
146:World War I
107:Morse codes
99:directional
1934:Categories
1893:2010-05-19
1878:2009-08-02
1757:2009-08-02
1726:2020-07-24
1674:2009-07-30
1569:2009-07-22
1471:2009-07-30
1278:2009-07-24
1253:2009-07-31
1179:2022-04-05
1149:2009-07-21
1071:2009-07-21
1007:References
568:approaches
472:Joliet, IL
436:Joliet, IL
310:Morse code
258:Technology
179:gyroscopic
1607:0032-4558
948:quadrant.
564:waypoints
308:) with a
280:used the
72:the range
1835:16577291
995:See also
777:quadrant
737:quadrant
687:"static"
654:"static"
613:"static"
326:— ·
318:· —
38:stream,
1803:Bibcode
1430:7 March
1404:7 March
1378:7 March
1347:7 March
1204:7 March
1001:SCR-277
773:Inside
733:Inside
669:(GPS).
631:-based
507:courses
338:airways
195:heading
150:airmail
117:History
103:airways
1850:
1833:
1826:526716
1823:
1699:
1631:
1605:
1541:
1514:
1490:Flight
1030:
673:Sounds
555:along
290:homing
263:Ground
144:After
1872:(PDF)
1865:(PDF)
1791:(PDF)
1748:(PDF)
1720:(PDF)
1465:(PDF)
1458:(PDF)
1398:(PDF)
1247:(PDF)
1240:(PDF)
873:Notes
864:(See
660:band
466:), a
302:watts
92:holds
1848:ISBN
1831:PMID
1779:help
1697:ISBN
1629:ISBN
1603:ISSN
1539:ISBN
1512:ISBN
1432:2021
1406:2021
1380:2021
1349:2021
1326:(4).
1206:2021
1028:ISBN
627:The
566:and
415:and
332:and
193:and
185:and
90:and
50:The
1821:PMC
1811:doi
662:VOR
658:VHF
633:VOR
629:VHF
576:FAA
572:GPS
557:any
476:AGL
407:or
389:kHz
379:Air
355:or
298:kHz
252:VHF
216:'s
1936::
1829:.
1819:.
1809:.
1799:16
1797:.
1793:.
1770::
1768:}}
1764:{{
1750:.
1683:^
1652:.
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1316:.
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1197:.
1171:.
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1080:^
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1014:^
395:).
340:.
306:Hz
94:.
66:,
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1896:.
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1777:(
1760:.
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1408:.
1382:.
1351:.
1281:.
1256:.
1208:.
1182:.
1152:.
1074:.
1036:.
945:A
941:N
928:N
924:N
920:N
855:A
838:A
815:N
813:+
811:A
775:N
758:N
735:A
718:A
699:A
695:A
691:A
601:N
597:A
489:.
447:"
417:N
413:A
409:N
405:A
366:A
362:N
357:N
353:A
334:N
330:A
324:(
322:N
316:(
314:A
46:)
40:A
36:N
20:)
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