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Loran-C

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LORAN (LOng RAnge Navigation) is a terrestrial navigation system using low frequency radio transmitters that use the time interval between radio signals received from three or more stations to determine the position of a ship or aircraft. The current version of LORAN in common use is LORAN-C, which operates in the low frequency 90 to 110 kHz band. Many nations including the United States, Japan, and Russia operate LORAN stations. LORAN use is in deep decline, with GPS being the primary replacement, however there are current attempts to enhance and re-popularize LORAN.

History

LORAN was an American development of the British GEE radio navigation system (used during World War II). While GEE had a range of about 400 miles (644 km), early LORAN systems had a range of 1,200 miles (1,930 km). LORAN systems were up and running during World War II and were used extensively by the US Navy and Royal Navy. It was originally known as "LRN" for Loomis radio navigation, after millionaire and physicist Alfred Lee Loomis, who invented LORAN and played a crucial role in military research and development during WWII.

LORAN is a replacement for the OMEGA Navigation System. LORAN allows for smaller antennas and greater accuracy than Omega.

Principle

A crude diagram of the LORAN principle. The difference between the time of receipt of synchronized signals from radio stations A and B is constant along each hyperbolic curve.

The navigational method provided by LORAN is based on the principle of the time difference between the receipt of signals from a pair of radio transmitters. A given constant time difference between the signals from the two stations can be represented by a hyperbolic line of position (LOP). If the positions of the two synchronized stations are known, then the position of the receiver can be determined as being somewhere on a particular hyperbolic curve where the time difference between the received signals is constant. (In ideal conditions, this is proportionally equivalent to the difference of the distances from the receiver to each of the two stations.)

By itself, with only two stations, the 2-dimensional position of the receiver cannot be fixed. A second application of the same principle must be used, based on the time difference of a different pair of stations. By determining the intersection of the two hyperbolic curves identified by the application of this method, a geographic fix can be determined.

LORAN method

LORAN Station Malone, Malone, Florida
LORAN transmitter bank
File:LORAN Station Malone-Timers-Large.jpg
Timing devices used for LORAN transmission control
Cesium atomic clocks used for LORAN signal synchronization

In the case of LORAN, one station remains constant in each application of the principle, the master, being paired up separately with two other slave, or secondary, stations. Given two secondary stations, the time difference (TD) between the master and first secondary identifies one curve, and the time difference between the master and second secondary identifies another curve, the intersections of which will determine a geographic point in relation to the position of the three stations. These curves are often referred to as "TD lines."

In practice, LORAN is implemented in integrated regional arrays, or chains, consisting of one master station and at least two (but often more) secondary stations, with a uniform "group repetition interval" (GRI) defined in microseconds. The master station transmits a series of pulses, then pauses for that amount of time before transmitting the next set of pulses.

The secondary stations receive this pulse signal from the master, then wait a preset amount of milliseconds, known as the secondary coding delay, to transmit a response signal. In a given chain, each secondary's coding delay is different, allowing for separate identification of each secondary's signal (though in practice, modern LORAN receivers do not rely on this for secondary identification).

LORAN chains (GRIs)

Each LORAN chain in the world uses a unique GRI, which is designated by the number of microseconds divided by 10 (in practice the GRI delays are multiples of 100 microseconds). LORAN chains are often referred to by this designation, e.g. GRI 9960, the designation for the LORAN chain serving the Northeast U.S.

Due to the nature of hyperbolic curves, it is possible for a particular combination of a master and 2 slave stations to result in a "grid" where the axis intersect at acute angles. For ideal positional accuracy, it is desirable to operate on a navigational grid where the axes are as Cartesian as possible -- i.e., the axes are at right angles to each other. As the receiver travels through a chain, a certain selection of secondaries whose TD lines initially formed a near-Cartesian grid can become a grid that is sharply angular. As a result, the selection of one or both secondaries should be changed so that the TD lines of the new combination are closer to right angles. To allow this, nearly all chains provide at least three, and as many as five, secondaries.

LORAN charts

This nautical chart of New York Harbor includes TD lines for the 9960 GRI. Note that the printed TD lines do not extend into inland waterway areas.

Where available, common marine navigational charts include visible representations of TD lines at regular intervals over water areas. The TD lines representing a given master-slave pairing are printed with distinct colors, and include an indication of the specific time difference indicated by each line.

Due to interference and propagation issues suffered by low-frequency signals from land features, and man-made structures, the accuracy of the LORAN signal is degraded considerably in inland areas. (See Limitations.) As a result, nautical charts will not print any TD lines in those areas, to prevent reliance on LORAN for navigation in such areas.

Traditional LORAN receivers generally display the time difference between each pairing of the master and one of the two selected secondary stations. These numbers can then be found in relation to those of the TD lines printed on the chart.

Modern LORAN receivers can natively display latitude and longitude instead of signal time differences, with increasing accuracy.

Transmitters and antennas

LORAN-C transmitters operate at a power level between 100 kilowatts and four megawatts, comparable to longwave broadcasting stations. 190 meter guyed masts are used as antennas below transmitter power levels of 500 kilowatts. The mast is electrically lengthened by a massive coil called a goniometer. One transmitter of this type is the LORAN-C transmitter Rantum on Sylt in Germany. LORAN-C transmitters operating at power levels greater than 1 megawatt use 412 meter masts. The mast of the former LORAN-C station Hellissandur on Iceland is now used for longwave broadcasting by the Icelandic broadcasting company on 189 kHz. All LORAN-C antennas radiate an omnidirectional pattern.

Limitations

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 requires the reception of signals from ground based transmitters and therefore the system only works in regions with Loran transmitters. However, coverage is quite good in North America, Europe, and the Pacific Rim.

LORAN-A and other systems

LORAN-A was a less accurate system operating in the 1,750-1,950 kHz frequency band 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-B was a phase comparison variation of LORAN-A while LORAN-D was a short-range tactical system designed for Air Force bombers. The unofficial "LORAN-F" was a drone control system. None of these went much beyond the experimental stage. An external link to them is listed below.

LORAN Data Channel (LDC)

LORAN Data Channel (LDC) is a project underway between the FAA and USCG to send low bit rate data using the LORAN system. Messages to be sent include station identification, absolute time, and position correction messages. In 2001, data similar to Wide Area Augmentation System (WAAS) GPS correction messages were sent as part of a test of the Alaskan LORAN chain. As of November 2005, test messages using LDC were being broadcast from several U.S. LORAN stations.

For several years, LORAN-C has been used in Europe to send differential GPS and other messages using a similar method of transmission known as EUROFIX.

Future

Many have called for the elimination of the Loran system altogether. Critics feel that the Loran system has too few users, lacks cost-effectiveness, and that GPS is a better alternative to Loran. Supporters of the Loran system note three primary advantages of the system. First, Loran uses a very strong transmitted signal and is therefore very difficult to jam (clearly much harder than GPS). Second, Loran is an independent system and can therefore serve as a backup. Finally, Loran signals can also be combined with GPS signals to produce a better estimate of location than either system acting alone. Recently both the US and European governments have made the political decision to maintain and upgrade their Loran systems.

File:LoranCoverage.gif
Worldwide Loran coverage

The 2005 Federal Radionavigation Plan, released in February 2006, states that Loran will not be deactivated without at least six months' notification, and that an evaluation of Loran will be completed by the end of 2006. The results will determine the future of Loran.

E-LORAN

With the perceived vulnerability of the GPS system, and its own propagation and reception limitations, renewed interest in LORAN applications and development has appeared. Enhanced LORAN, aka E-LORAN or eLoran, comprises an advancement in receiver design and transmission characteristics which increase the accuracy and usefulness of traditional LORAN, with reported accuracy as high as 8m, competitive with unenhanced GPS. eLoran also includes additional pulses which can transmit auxiliary data such as DGPS corrections. E-LORAN receivers now use "all in view" reception, incorporating signals from all stations in range, not solely those from a single GRI, incorporating time signals and other data from up to 40 stations. These enhancements in LORAN make it adequate as a substitute for scenarios where GPS is unavailable or degraded.

See also

  • CHAYKA, the Russian counterpart of LORAN
  • Alpha, the Russian counterpart of the Omega Navigation System, still in use as of 2006.
  • OMEGA, the Western counterpart of the Alpha Navigation System, no longer in use.

References

  • Jennet Conan, Tuxedo Park: A Wall Street Tycoon and the Secret Palace of Science That Changed the Course of World War II (New York: Simon & Schuster, 2002, ISBN 0-684-87287-0) pp. 231-232.
  • Department of Transportation and Department of Defense (2006-02). "2005 Federal Radionavigation Plan" (PDF). Retrieved 2006-02-26. {{cite web}}: Check date values in: |year= (help); Cite has empty unknown parameter: |accessyear= (help)
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