Hardware: Radio navigation

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Radio navigation or radionavigation is the application of radio frequencies to determining a position on the Earth. Like radiolocation, it is a type of radiodetermination.
Contents
1 Radio Direction Finding
2 Lorenz
3 VOR
4 Hyperbolic systems
4.1 GEE
4.2 LORAN
4.3 Other hyperbolic systems
5 GPS
6 See also
6.1 Radio navigation systems
7 External links
7.1 History
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Radio Direction Finding
The first system of radio navigation was the Radio Direction Finder, or RDF. By tuning in a radio station and then using a directional antenna to find the direction to the broadcasting antenna, radio sources replaced the stars and planets of celestial navigation with a system that could be used in all weather and times of day. By using triangulation, two such measurements can be plotted on a map where their intersection is the position. Commercial AM radio stations can be used for this task due to their long range and high power, but strings of low-power radio beacons were also set up specifically for this task. Early systems used a loop antenna that was rotated by hand to find the angle to the signal, while modern systems use a much more directional solenoid that is rotated rapidly by a motor, with electronics calculating the angle. These later systems were also called Automatic Direction Finders, or ADF.
Lorenz
Main article: Lorenz (navigation)
In the 1930s German radio engineers developed a new system, called the "Ultrakurzwellen-Landefunkfeuer" (LFF), or simply "Leitstrahl" (guiding beam) but referred to outside Germany as Lorenz, the name of the company manufacturing the equipment. In Lorenz two signals were broadcast on the same frequencies from highly directional antennas with beams a few degrees wide. One was pointed slightly to the left of the other, with a small angle in the middle where they overlapped. The signals were chosen as dots and dashes, timed so that when the aircraft was in the small area in the middle the sound was continuous. Planes would fly into the beams by listening to the signal to identify which side of middle they were on, and then corrected until they were in the center.
Originally developed as a night and bad-weather landing system, in the late 1930s they also started developing long-range versions for night bombing. In this case a second set of signals were broadcast at right angles to the first, and indicated the point at which to drop the bombs. The system was highly accurate and a battle of the beams broke out when United Kingdom intelligence services attempted, and then succeeded, in rendering the system useless.
In the post-war era similar systems were widely deployed, notably in the United States where a system of long range "airways" was created spanning the country with stations about 200 miles (320km) apart. The signals were chosen as the A and N letters from morse code, dot-dash and dash-dot respectively. However, new developments soon rendered these systems obsolete.
VOR

VOR transmitter station
Main article: VHF omnidirectional range
The next major advance in "beam based" navigation system was the use of two signals that varied not in sound, but in phase. In these systems, known as VHF omnidirectional range, or VOR, a single master signal is sent out continually from the station, and a highly directional second signal is sent out that varies in phase 30 times a second compared to the master. This signal is timed so that the phase varies as the secondary antenna spins, such that when the antenna is 90 degrees from north, the signal is 90 degrees out of phase of the master. By comparing the phase of the secondary signal to the master, the angle can be determined without any physical motion in the receiver. This angle is then displayed in the cockpit of the aircraft, and can be used to take a fix just like the earlier RDF systems, although it is, in theory, easier to use and more accurate.
Hyperbolic systems
Systems based on the measurement of the difference of signal arrival times from two or more locations are called hyperbolic systems due to the shape of the lines of position on the chart. These include:
GEE
The British GEE system was developed during World War II. GEE used a series of transmitters sending out precisely timed signals, and the aircraft using GEE, RAF Bomber Command's heavy bombers, 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. By making similar measurements with other stations, additional lines of position can be produced, leading to a fix. GEE was accurate to about 165 yards (150 m)...(and so on)