Editing Radio navigation (section)

==Hyperbolic systems==
{{main|Hyperbolic navigation}}
Hyperbolic navigation systems are a modified form of transponder systems which eliminate the need for an airborne transponder. The name refers to the fact that they do not produce a single distance or angle, but instead indicate a location along any number of hyperbolic lines in space. Two such measurements produces a fix. As these systems are almost always used with a specific [[navigational chart]] with the hyperbolic lines plotted on it, they generally reveal the receiver's location directly, eliminating the need for manual triangulation. As these charts were digitized, they became the first true location-indication navigational systems, outputting the location of the receiver as latitude and longitude. Hyperbolic systems were introduced during World War II and remained the main long-range advanced navigation systems until GPS replaced them in the 1990s.{{fact|date=July 2022}}

===Gee===
{{main|Gee (navigation)}}
The first hyperbolic system to be developed was the British [[Gee (navigation)|Gee]] system, developed during [[World War II]]. Gee used a series of transmitters sending out precisely timed signals, with the signals leaving the stations at fixed delays. An aircraft using Gee, [[RAF Bomber Command]]'s heavy [[bomber]]s, examined the time of arrival on an [[oscilloscope]] at the navigator's station. If the signal from two stations arrived at the same time, the aircraft must be an equal distance from both transmitters, allowing the navigator to determine a line of position on his chart of all the positions at that distance from both stations. More typically, the signal from one station would be received earlier than the other. The ''difference'' in timing between the two signals would reveal them to be along a curve of possible locations. By making similar measurements with other stations, additional lines of position can be produced, leading to a fix. Gee was accurate to about 165&nbsp;yards (150&nbsp;m) at short ranges, and up to a mile (1.6&nbsp;km) at longer ranges over Germany. Gee remained in use long after World War II, and equipped RAF aircraft as late as the 1960s (approx freq was by then 68&nbsp;MHz).{{fact|date=July 2022}}

===LORAN===
{{main|LORAN}}
With Gee entering operation in 1942, similar US efforts were seen to be superfluous. They turned their development efforts towards a much longer-ranged system based on the same principles, using much lower frequencies that allowed coverage across the [[Atlantic Ocean]]. The result was [[LORAN]], for "LOng-range Aid to Navigation". The downside to the long-wavelength approach was that accuracy was greatly reduced compared to the high-frequency Gee. LORAN was widely used during convoy operations in the late war period.<ref name=JBLoran-C>{{cite web|title=The Loran-C System of Navigation|url=http://www.loran-history.info/Loran-C/Jansky%20_%20Bailey%201962.pdf|publisher=Jansky & Bailey|access-date=25 July 2013|pages=18–23|date=February 1962|archive-date=22 July 2013|archive-url=https://web.archive.org/web/20130722043053/http://www.loran-history.info/Loran-C/Jansky%20_%20Bailey%201962.pdf|url-status=dead}}</ref>

===Decca===
{{main|Decca Navigator System}}
Another British system from the same era was Decca Navigator. This differed from Gee primarily in that the signals were not pulses delayed in time, but continuous signals delayed in phase. By comparing the phase of the two signals, the time difference information as Gee was returned. However, this was far easier to display; the system could output the phase angle to a pointer on a dial removing any need for visual interpretation. As the circuitry for driving this display was quite small, Decca systems normally used three such displays, allowing quick and accurate reading of multiple fixes. Decca found its greatest use post-war on ships, and remained in use into the 1990s.{{fact|date=July 2022}}

===LORAN-C===
{{main|Loran-C}}
Almost immediately after the introduction of LORAN, in 1952 work started on a greatly improved version. LORAN-C (the original retroactively became LORAN-A) combined the techniques of pulse timing in Gee with the phase comparison of Decca.{{fact|date=July 2022}}

The resulting system (operating in the [[low frequency]] (LF) radio spectrum from 90 to 110&nbsp;kHz) that was both long-ranged (for 60&nbsp;kW stations, up to 3400 miles) and accurate. To do this, LORAN-C sent a pulsed signal, but modulated the pulses with an AM signal within it. Gross positioning was determined using the same methods as Gee, locating the receiver within a wide area. Finer accuracy was then provided by measuring the phase difference of the signals, overlaying that second measure on the first. By 1962, high-power LORAN-C was in place in at least 15 countries.<ref>Jansky & Baily 1962, pp.23–37.</ref>

LORAN-C was fairly complex to use, requiring a room of equipment to pull out the different signals. However, with the introduction of [[integrated circuit]]s, this was quickly reduced further and further. By the late 1970s, LORAN-C units were the size of a stereo amplifier and were commonly found on almost all commercial ships as well as some larger aircraft. By the 1980s, this had been further reduced to the size of a conventional radio, and it became common even on pleasure boats and personal aircraft. It was the most popular navigation system in use through the 1980s and 90s, and its popularity led to many older systems being shut down, like Gee and Decca. However, like the beam systems before it, civilian use of LORAN-C was short-lived when GPS technology drove it from the market.{{fact|date=July 2022}}

===Other hyperbolic systems===
Similar hyperbolic systems included the US global-wide [[VLF]]/[[Omega Navigation System]], and the similar [[Alpha (radio navigation)|Alpha]] deployed by the USSR. These systems determined pulse timing not by comparison of two signals, but by comparison of a single signal with a local [[atomic clock]]. The expensive-to-maintain Omega system was shut down in 1997 as the US military migrated to using [[GPS]]. Alpha is still in use.{{fact|date=July 2022}}