Editing Radio navigation (section)

==Transponder systems==
{{main|Transponder}}
{{see also|Transponder landing system}}
Positions can be determined with any two measures of angle or distance. The introduction of [[radar]] in the 1930s provided a way to directly determine the distance to an object even at long distances. Navigation systems based on these concepts soon appeared, and remained in widespread use until recently. Today they are used primarily for aviation, although GPS has largely supplanted this role.{{fact|date=July 2022}}

===Radar and transponders===
Early [[radar]] systems, like the UK's [[Chain Home]], consisted of large transmitters and separate receivers. The transmitter periodically sends out a short pulse of a powerful radio signal, which is sent into space through broadcast antennas. When the signal reflects off a target, some of that signal is reflected back in the direction of the station, where it is received. The received signal is a tiny fraction of the broadcast power, and has to be powerfully amplified in order to be used.{{fact|date=July 2022}}

The same signals are also sent over local electrical wiring to the operator's station, which is equipped with an [[oscilloscope]]. Electronics attached to the oscilloscope provides a signal that increases in voltage over a short period of time, a few microseconds. When sent to the X input of the oscilloscope, this causes a horizontal line to be displayed on the scope. This "sweep" is triggered by a signal tapped off the broadcaster, so the sweep begins when the pulse is sent. Amplified signals from the receiver are then sent to the Y input, where any received reflection causes the beam to move upward on the display. This causes a series of "blips" to appear along the horizontal axis, indicating reflected signals. By measuring the distance from the start of the sweep to the blip, which corresponds to the time between broadcast and reception, the distance to the object can be determined.{{fact|date=July 2022}}

Soon after the introduction of radar, the radio [[transponder]] appeared. Transponders are a combination of receiver and transmitter whose operation is automated – upon reception of a particular signal, normally a pulse on a particular frequency, the transponder sends out a pulse in response, typically delayed by some very short time. Transponders were initially used as the basis for early [[Identification friend or foe|IFF]] systems; aircraft with the proper transponder would appear on the display as part of the normal radar operation, but then the signal from the transponder would cause a second blip to appear a short time later. Single blips were enemies, double blips friendly.{{fact|date=July 2022}}

Transponder-based distance-distance navigation systems have a significant advantage in terms of positional accuracy. Any radio signal spreads out over distance, forming the fan-like beams of the Lorenz signal, for instance. As the distance between the broadcaster and receiver grows, the area covered by the fan increases, decreasing the accuracy of location within it. In comparison, transponder-based systems measure the timing between two signals, and the accuracy of that measure is largely a function of the equipment and nothing else. This allows these systems to remain accurate over very long range.{{fact|date=July 2022}}

The latest transponder systems (mode S) can also provide position information, possibly derived from [[Satellite navigation|GNSS]], allowing for even more precise positioning of targets.{{fact|date=July 2022}}

===Bombing systems===
The first distance-based navigation system was the German [[Y-Gerät (navigation)|Y-Gerät]] blind-bombing system. This used a [[Lorenz beam]] for horizontal positioning, and a transponder for ranging. A ground-based system periodically sent out pulses which the airborne transponder returned. By measuring the total round-trip time on a radar's oscilloscope, the aircraft's range could be accurately determined even at very long ranges. An operator then relayed this information to the bomber crew over voice channels, and indicated when to drop the bombs.{{fact|date=July 2022}}

The British introduced similar systems, notably the [[Oboe (navigation)|Oboe]] system. This used two stations in England that operated on different frequencies and allowed the aircraft to be triangulated in space. To ease pilot workload only one of these was used for navigation – prior to the mission a circle was drawn over the target from one of the stations, and the aircraft was directed to fly along this circle on instructions from the ground operator. The second station was used, as in Y-Gerät, to time the bomb drop. Unlike Y-Gerät, Oboe was deliberately built to offer very high accuracy, as good as 35&nbsp;m, much better than even the best optical [[bombsight]]s.{{fact|date=July 2022}}

One problem with Oboe was that it allowed only one aircraft to be guided at a time. This was addressed in the later [[Gee-H (navigation)|Gee-H]] system by placing the transponder on the ground and broadcaster in the aircraft. The signals were then examined on existing [[Gee (navigation)|Gee]] display units in the aircraft (see below). Gee-H did not offer the accuracy of Oboe, but could be used by as many as 90 aircraft at once. This basic concept has formed the basis of most distance measuring navigation systems to this day.{{fact|date=July 2022}}

===Beacons===
{{main|Electric beacon}}
The key to the transponder concept is that it can be used with existing radar systems. The [[List of World War II British naval radar#ASV II|ASV]] radar introduced by [[RAF Coastal Command]] was designed to track down submarines and ships by displaying the signal from two antennas side by side and allowing the operator to compare their relative strength. Adding a ground-based transponder immediately turned the same display into a system able to guide the aircraft towards a transponder, or "beacon" in this role, with high accuracy.{{fact|date=July 2022}}

The British put this concept to use in their [[Rebecca/Eureka transponding radar|Rebecca/Eureka]] system, where battery-powered "Eureka" transponders were triggered by airborne "Rebecca" radios and then displayed on ASV Mk. II radar sets. Eureka's were provided to French resistance fighters, who used them to call in supply drops with high accuracy. The US quickly adopted the system for paratroop operations, dropping the Eureka with pathfinder forces or partisans, and then homing in on those signals to mark the drop zones.{{fact|date=July 2022}}

The beacon system was widely used in the post-war era for blind bombing systems. Of particular note were systems used by the [[US Marines]] that allowed the signal to be delayed in such a way to offset the drop point. These systems allowed the troops at the front line to direct the aircraft to points in front of them, directing fire on the enemy. Beacons were widely used for temporary or mobile navigation as well, as the transponder systems were generally small and low-powered, able to be man portable or mounted on a [[Jeep]].{{fact|date=July 2022}}

===DME===
{{main|Distance measuring equipment}}
{{see also|Tactical air navigation system}}
In the post-war era, a general navigation system using transponder-based systems was deployed as the [[distance measuring equipment]] (DME) system.{{fact|date=July 2022}}

DME was identical to Gee-H in concept, but used new electronics to automatically measure the time delay and display it as a number, rather than having the operator time the signals manually on an oscilloscope. This led to the possibility that DME interrogation pulses from different aircraft might be confused, but this was solved by having each aircraft send out a different series of pulses which the ground-based transponder repeated back.

DME is almost always used in conjunction with VOR, and is normally co-located at a VOR station. This combination allows a single VOR/DME station to provide both angle and distance, and thereby provide a single-station fix. DME is also used as the distance-measuring basis for the military [[TACAN]] system, and their DME signals can be used by civilian receivers.{{fact|date=July 2022}}