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Questions about aircraft navigation systems using radio signals, such as VOR and NDB.

Radio navigation, or radionavigation, refers to any method of using radio signals to provide positional information. Aircraft have been using radionavigation for almost as long as aircraft capable of controlled flight have existed; without it, aircraft in would be flying completely blind, with no idea of their location.

Some radionavigation systems provide information only on one's direction relative to a fixed ; some provide information only on one's distance from a navaid; and some provide information on one's exact (more or less) position in the sky.

There are many (many many) types of radionavigation that are used (or have been used in the past) by aircraft, falling into five main categories:

##Bearing-measurement systems##

These are the oldest radionavigation systems, dating back as far as 1907; they work by broadcasting a signal from a fixed navaid, allowing aircraft within range of the signal to use it to determine their direction from the navaid (the "bearing to/from the [navaid]", in navigator-speak). These systems do not provide distance information; determining the aircraft's exact location requires that fixes be taken relative to several navaids (enabling the aircraft's location to be determined by triangulation), or else the use of a second method of navigation to pin down the aircraft's location.

  • Radio direction finder (RDF) systems (and the newer, fully-automatic version, the automatic direction finder []) use radio beacons (known nowadays as non-directional beacons [⁠s], to distinguish them from the newer VOR system [see below]) that broadcast a simple omnidirectional signal; an aircraft can then use a directional , or multiples thereof, to figure out what direction the signal came from. NDBs transmit using a low-frequency signal, which allows them to be picked up from well beyond line-of-sight distance (meaning that one can still take a fix from an NDB even if it's below the horizon, as long as it isn't too far below), and has allowed them to stay (somewhat) relevant despite the advent of the VOR.
  • Reverse RDF, instead of transmitting a signal in every direction at once, uses a navaid which transmits a two-part signal: one part is a non-directional identification signal which momentarily broadcasts once every second (or whatever time period the navaid uses; it's typically one second, however), and the second part is a narrow beam that rotates about the navaid's location. An aircraft can determine its direction from the navaid by measuring the time offset between the identification signal and the rotating-beam signal. (This is much easier to demonstrate than it is to explain; here is an animation of one such navaid in operation, courtesy of Orion 8 at Wikimedia Commons.) Most reverse-RDF systems nowadays transmit in the VHF (very high frequency) range, using so-called VHF omnidirectional [radio] range () stations, although older reverse-RDF systems used other frequency bands. (TACtical Air Navigation) is a higher-precision form of VOR (and, unlike an ordinary VOR, also includes a DME system [see below]), used primarily by military aircraft (and often colocated with a civilian VOR, to form a ).

##Beam systems##

These systems broadcast in a set of stationary beams; aircraft stay on course by flying down the boundary line between two adjacent beams.

  • The most common beam system nowadays is the , or localiser if you're British (usually used as part of an instrument landing system, or , but also sometimes seen on its own), which uses a pair of adjacent beams (the one to the right of the desired course transmitting a "fly-left" signal; the one to the left, a "fly-right" signal) which guide an aircraft down to a . The localizer was developed from the earlier Lorenz beam system (invented in post-World-War-I Germany), which worked on essentially the same principle.
  • The second part of an ILS is the ; like the localizer, it broadcasts two beams which bracket the desired flightpath, but, this time, the beams are stacked vertically rather than horizontally, with the lower one sending a "fly-up" signal, and the upper one a "fly-down" signal.
  • Another beam-navigation system, this one virtually extinct, is the (LFR) system. This system used radiobeacons which broadcast in two hourglass-shaped patterns; aircraft could hold an accurate course by flying down the boundary between two adjacent half-hourglasses (there were four such boundaries per navaid, providing this system with its other common name - the four-course navigation system).

##Transponder systems##

These systems use an aircraft-mounted , and determine an aircraft's distance from a navaid by transmitting a signal from the aircraft to the navaid (or vice versa), and measuring how long it takes for the navaid's (or aircraft's) response to arrive back at the aircraft (or navaid).

  • With distance-measuring equipment (), the aircraft sends the signals, and the navaid responds; the aircraft knows how far it is from the ground station, but not vice versa. Most DMEs are colocated with VOR stations, to form what is known as a "VOR/DME".
  • Modern s (known as secondary surveillance radar, or SSR for short) use a form of "reverse DME", where the ground station pings the aircraft, and the aircraft's transponder responds. This lets the radar know how far away it is from the aircraft, which helps the radar figure out where, exactly, the aircraft is.

##Terrestrial hyperbolic systems##

These systems evolved from the transponder systems. Instead of a single ground-based navaid and an aircraft-mounted transponder, they consist of a network of ground-based transmitters, which transmit a time-varying signal in perfect synchrony with one another; by comparing the signal received from one ground station with that received from another, it is possible to determine how much further one's aircraft is from ground station A than from ground station B. A single such fix locates the aircraft somewhere along a particular hyperbolic curve (hence the name); taking a second fix with a different pair of ground stations generates a different hyperbola, and the point where the two hyperbolae intersect is the location of the aircraft. (If the two hyperbolae intersect at more than one point, the aircraft could be located at either one of the intersections; if the identity of the correct intersection is not already obvious from other clues [for instance, if the aircraft is flying over and one of the intersections is in the middle of Antarctica], the ambiguity can be resolved by taking a third fix.) Several different hyperbolic navigation systems exist or have existed, differing mainly in the frequency ranges used and in the methods of varying the signal with time:

  • Gee was the first such system, being developed in the during World War II; it provided accurate positional information, but had a fairly short range (no more than approximately 560 kilometers [350 miles]).
  • (LOng-RAnge Navigation) -A was developed shortly after Gee; it had a longer range (enough to provide guidance for flights), but was considerably less precise. The more advanced LORAN-C, offering both greater range and greater accuracy than LORAN-A, was developed in the 1950s. Both LORAN generations were originally reserved for use, and only later opened to civilian users.
  • CHAYKA was developed in the Soviet Union; it is very similar to LORAN-C (although not similar enough for navigation equipment to be interchangeable between the two systems).
  • Decca, another British system, was developed late in World War II; it had accuracy comparable to Gee, but used a different method of varying the transmitted signal, allowing the use of simpler, cheaper receivers. Decca was used mostly by ships, but also saw some use in aviation.
  • and Alpha were two systems that used very-low-frequency signals to provide truly global coverage, with ranges of many thousands of kilometers (or miles; it's true for both) from the ground stations; this allowed the use of a very small number of ground stations, at the expense of requiring transmitters of enormous size. The main difference between the two was their developers; Omega was an American system, Alpha a Soviet one.

Of the terrestrial hyperbolic systems, only Alpha, CHAYKA, and LORAN-C remain operational (and LORAN-C only in Asia and some parts of Europe); the others have all been shut down. The first-generation hyperbolic systems (Gee, Decca, and LORAN-A) were superseded by the second-generation hyperbolic systems (LORAN-C, CHAYKA, Omega, and Alpha), which, in turn, have been superseded by satellite-based systems (see below).

##Satellite [hyperbolic] systems##

These, too, are hyperbolic systems; however, instead of using ground-based transmitters, their signals are transmitted from satellites orbiting the earth. As orbiting satellites are constantly in motion, this requires the use of some very complicated math to figure out which satellite is where when. This is more than made up for, however, by their very high precision (due to a given aircraft, at any given moment, having a direct line of sight to a number of satellites that is considerably greater than the minimum needed for a fix; a fix can be gotten with as few as three or four satellites in view, but the number visible from an aircraft at a given time is usually more like seven or eight, and each additional visible satellite on top of the basic three adds more and more precision to the calculations), down to the meter in some cases, and their truly global coverage. The common name for each of these systems is a global positioning system (GPS); a more-recent, less-common, and more-clunky term is a global navigation satellite system (). The first GPS network was the NAVSTAR system set up by the U.S. military in the 1970s (becoming available for civilian use in the 1980s); other GPS networks (such as the Russian GLONASS, the European Galileo, and the Chinese Baidu) came online later.

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