Editing Radio navigation (section)

==Bearing-measurement systems==
These systems used some form of directional radio antenna to determine the location of a broadcast station on the ground. Conventional navigation techniques are then used to take a [[radio fix]]. These were introduced prior to World War I, and remain in use today.{{fact|date=July 2022}}

===Radio direction finding===
[[File:Earhart-electra 10.jpg|thumb|right|[[Amelia Earhart]]'s [[Lockheed Model 10 Electra|Lockheed Electra]] had a prominent RDF loop on the cockpit roof.]]
{{Main|Radio direction finder}}

The first system of radio navigation was the ''Radio Direction Finder'', or RDF.<ref name="KF116">Kayton, Fried 1977, p.116</ref>  By tuning in a [[radio station]] and then using a [[directional antenna]], one could determine the direction to the broadcasting antenna. A second measurement using another station was then taken. Using [[triangulation]], the two directions can be plotted on a map where their [[Line-line intersection|intersection]] reveals the location of the navigator.<ref name="Ecologist 2021 f124">{{cite web | title=An Introduction to Radio Direction Finding | website=Cognitive Ecology | date=October 22, 2021 | url=https://cogecog.com/radio-direction-finding/ | access-date=April 4, 2024}}</ref><ref name="Murphy_1983">{{cite report |last= Murphy |first= Charles J. |date=1983-06-01 |title=Evaluation of VHF-FM Shore-Based Direction Finding Triangulation System in Massachusetts Bay Area |url=https://rosap.ntl.bts.gov/view/dot/10712 |publisher=United States Coast Guard |access-date=2024-04-04}}</ref> Commercial [[AM radio]] stations can be used for this task due to their long range and high power, but strings of low-power [[Nondirectional beacon|radio beacon]]s were also set up specifically for this task, especially near [[airport]]s and harbours.{{fact|date=July 2022}}

Early RDF systems normally used a [[loop antenna]], a small loop of metal wire that is mounted so it can be rotated around a vertical axis.<ref name="KF116"/> At most angles the loop has a fairly flat reception pattern, but when it is aligned perpendicular to the station the signal received on one side of the loop cancels the signal in the other, producing a sharp drop in reception known as the "null". By rotating the loop and looking for the angle of the null, the relative bearing of the station can be determined. Loop antennas can be seen on most pre-1950s aircraft and ships.{{fact|date=July 2022}}

===Reverse RDF===
[[File:The 'Black Beacon', Orford Ness - geograph.org.uk - 935140.jpg|thumb|right|The Orfordness Beacon as it appears today.]]
The main problem with RDF is that it required a special antenna on the vehicle, which may not be easy to mount on smaller vehicles or single-crew aircraft. A smaller problem is that the accuracy of the system is based to a degree on the size of the antenna, but larger antennas would likewise make the installation more difficult.{{fact|date=July 2022}}

During the era between [[World War I]] and [[World War II]], a number of systems were introduced that placed the rotating antenna on the ground. As the antenna rotated through a fixed position, typically due north, the antenna was keyed with the [[morse code]] signal of the station's identification letters so the receiver could ensure they were listening to the right station. Then they waited for the signal to either peak or disappear as the antenna briefly pointed in their direction. By timing the delay between the morse signal and the peak/null, then dividing by the known rotational rate of the station, the bearing of the station could be calculated.{{fact|date=July 2022}}

The first such system was the German [[Telefunken Kompass Sender]], which began operations in 1907 and was used operationally by the [[Zeppelin]] fleet until 1918.<ref name=bauer2004>{{cite web|last=Bauer|first=Arthur O.|title=Some historical and technical aspects of radio navigation, in Germany, over the period 1907 to 1945|url=http://www.cdvandt.org/Navigati.pdf|access-date=25 July 2013|date=Dec 26, 2004}}</ref>  An improved version was introduced by the UK as the [[Orfordness Beacon]] in 1929 and used until the mid-1930s. A number of improved versions followed, replacing the mechanical motion of the antennas with phasing techniques that produced the same output pattern with no moving parts. One of the longest lasting examples was [[Sonne (navigation)|Sonne]], which went into operation just before [[World War II]] and was used operationally under the name Consol until 1991. The modern VOR system is based on the same principles (see below).{{fact|date=July 2022}}

=== ADF and NDB===
{{Main|Non-directional beacon}}
A great advance in the RDF technique was introduced in the form of phase comparisons of a signal as measured on two or more small antennas, or a single highly directional [[solenoid]]. These receivers were smaller, more accurate, and simpler to operate. Combined with the introduction of the [[transistor]] and [[integrated circuit]], RDF systems were so reduced in size and complexity that they once again became quite common during the 1960s, and were known by the new name, [[automatic direction finder]], or ADF.{{fact|date=July 2022}}

This also led to a revival in the operation of simple radio beacons for use with these RDF systems, now referred to as ''non-directional beacons'' (NDB). As the LF/MF signals used by NDBs can follow the curvature of earth, NDB has a much greater range than [[VHF Omnidirectional Range|VOR]] which travels only in ''line of sight''. NDB can be categorized as ''long range'' or ''short range'' depending on their power. The frequency band allotted to non-directional beacons is 190–1750&nbsp;kHz, but the same system can be used with any common AM-band commercial station.{{fact|date=July 2022}}

===VOR===
[[File:VOR DME BUB.JPG|right|thumb|VOR transmitter station]]
{{Main|VHF omnidirectional range}}
{{more citations needed|section|date=February 2022}}
[[VHF omnidirectional range]], or VOR, is an implementation of the reverse-RDF system, but one that is more accurate and able to be completely automated.{{fact|date=July 2022}}

The VOR station transmits two audio signals on a VHF carrier – one is [[Morse code]] at 1020&nbsp;Hz to identify the station, the other is a continuous 9960&nbsp;Hz audio modulated at 30&nbsp;Hz, with the 0-degree referenced to magnetic north. This signal is rotated mechanically or electrically at 30&nbsp;Hz, which appears as a 30&nbsp;Hz AM signal added to the previous two signals, the phasing of which is dependent on the position of the aircraft relative to the VOR station.{{fact|date=July 2022}}

The VOR signal is a single RF carrier that is demodulated into a composite audio signal composed of a 9960&nbsp;Hz reference signal frequency modulated at 30&nbsp;Hz, a 30&nbsp;Hz AM reference signal, and a 1020&nbsp;Hz 'marker' signal for station identification. Conversion from this audio signal into a usable navigation aid is done by a navigation converter, which takes the reference signal and compares the phasing with the variable signal. The phase difference in degrees is provided to navigational displays. Station identification is by listening to the audio directly, as the 9960&nbsp;Hz and 30&nbsp;Hz signals are filtered out of the aircraft internal communication system, leaving only the 1020&nbsp;Hz Morse-code station identification.{{fact|date=July 2022}}

The system may be used with a compatible glideslope and marker beacon receiver, making the aircraft ILS-capable (Instrument Landing System)}. Once the aircraft's approach is accurate (the aircraft is in the "right place"), the VOR receiver will be used on a different frequency to determine if the aircraft is pointed in the "right direction." Some  aircraft will usually employ two VOR receiver systems, one in VOR-only mode to determine "right place" and another in ILS mode in conjunction with a glideslope receiver to determine "right direction." }The combination of both allows for a precision approach in foul weather.<ref>[https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_application/application_notes/1gpan09/1GPAN09_0E.pdf VOR/ILS Testing with Signal Generator SMT]</ref>