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LORAN (LOng RAnge Navigation) is a terrestrial radio-navigation system using ground-based transmitters.

Hyperbolic LORAN lines of position (LOP) are formed by measuring the difference in reception times of synchronized signals. Groups of LORAN stations are used to form intersecting LOP to provide cross fixing. A LORAN net, or chain, requires a master station, initiating the pulse, and a series of slave stations. In very simplified terms, the master transmits and the slave responds. Various delays are built in for technical purposes.

In principle the signals of a pair arrive at the same time along the base line connecting the pair. It is at the base line where the apex of each hyperbolic LOP lies. Beyond the stations they flare so that no useful hyperbolic rate information is provided. This is the unusable area of the base line extension. At any point other than the base line and base line extension the difference in arrival time of the master and slave pulses form the LOP in the form of hyperbolic curves. These can be charted or given in tables.

It is the calculated difference in arrival times of master-slave pairs that form charted rates and the observed arrivals that form the navigational information. If the observation is exactly that of the charted value one is somewhere along the hyperbola of that value. That information is of little value without a second pair's hyperbola intersecting to provide a definitive fix along the first hyperbola. The station configurations thus must provide a geometry that allows such intersection. Where LORAN chains overlap one may obtain cross fixes from two different master/slave pairs.

LORAN suffers from electronic effects of weather and in particular atmospheric effects related to sunrise and sunset. The most accurate signal is the groundwave, that following the Earth's surface, preferably along a sea water path. At night the indirect skywave, taking paths bent back to the surface by the ionosphere, is a particular problem as multiple signals may arrive via different paths. The ionosphere's reaction to sunrise and sunset accounts for the particular disturbance during those periods. Magnetic storms have serious effects as with any radio based system.

LORAN-A was a less accurate system operating in the 1,750-1,950 Kc frequencies prior to deployment of the more accurate LORAN-C system. It continued in operation partly due to the economy of the receivers and widespread use in civilian recreational and commercial navigation.

LORAN-C, operating at 100-110 kHz, was initially developed under U.S. Navy sponsorship and deployed to cover large portions of the northern hemisphere for both marine and air navigation. In the early 1970s the system was placed under civil control with stations staffed by the United States Coast Guard worldwide. The coverage included North America (the entire United States) and coastal areas of Europe and much of east Asia. The North Atlantic, North Pacific and Mediterranean oceans were largely covered by the system, particularly after the ranging technique came into use. It came into widespread civilian use as the initially expensive receivers came into commercial production. It offered much greater accuracy than LORAN-A. LORAN-C, due in large part to continued widespread civilian use, survived the initial Doppler navigational satellites which provided intermittent fixes and into the current era of the GPS system. LORAN-D was a similar, but smaller, tactical navigation system apparently used by the United States Air Force.

Ranging LORAN-C, sometimes termed Range-Range or Rho-Rho, was a development that extended LORAN-C into areas not covered by hyperbolic rates or into areas where the LOP intersections were of such shallow angles as to provide seriously degraded cross fixing. As one example, the hyperbolic formed by one pair along with the ranges observed from either master or slave or both provided useful geometry for navigation in areas with only one usable hyperbolic was normally available. In effect this geometry filled in the previously unusable areas out along the base line extensions.

Ranging was dependent upon the availability of highly accurate atomic clocks (See Atomic clock) to refine LORAN station accuracy and carry aboard ships. These provided a capability to synchronize the ship with the actual master and slave pulses so that the ship's receivers "knew" when the stations transmitted their signals. The time delays from the station's known time of transmission to the receipt at the ship formed a range circle that overlay hyperbolic LOPs or in themselves intersected to provide useful cross fixes.

There are signs LORAN-C will continue in operation into the first decade of the 21st Century despite frequent questions concerning the continued operation of LORAN-C with the advent of inexpensive, portable, civilian Global Positioning System receivers. It is particularly popular in the general aviation and recreational boating community. New ideas for applications continue.

Other hyperbolic systems were in use before and during LORAN system operation. The Chayka system was of Soviet origin and is still in operation under Russian sponsorship. These were similar, but based on different configurations and technical details. LORAC (LOng RAnge Accuracy) was a comparatively short range system composed of a master and two slave stations using phase comparison of beat frequencies. This system was often used as a semi portable one for local surveys, particularly in oil exploration. British hyperbolic systems were deployed and one, the Decca Navigation System, was often deployed by private firms in a portable form for such operations as oil exploration. Another British system was named Gee. The British Consol system was an early hyperbolic system based on the German Sonne and earlier Elektra systems. Though "hyperbolic" this aspect was not generally used and it was used as a directional system based on its dot/dash sectors. A United states Version was known as Consolan was deployed for a time. Both these systems were more used for air than surface navigation. Omega was a very low frequency system intended for world wide coverage. It had low accuracy, but provided useful navigational information in areas uncovered by any other system.

See also Time transfer

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