LORAN

LORAN (LOng RAnge Navigation) is a terrestrial navigation system using low frequency radio transmitters that use the time interval between radio signals received from two or more stations to determine the position of a ship or aircraft. Before the popularity of the satellite-based GPS system, it was primarily used in marine applications. The current version of LORAN in common use is LORAN-C, which operates in the low frequency 90 to 110 kHz band.

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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.

Principle

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Crude_loran_diagram.PNG
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 position 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

 Missing imageLORAN_Station_Malone-Outside-Large.jpg LORAN Station Malone, Malone, Florida Missing imageLORAN_Station_Malone-Transmitter_Middle-Large.jpg LORAN transmitter bank Missing imageLORAN_Station_Malone-Timers-Large.jpg Timing devices used for LORAN transmission control Missing imageLORAN_Station_Malone-Cesiums-Large.jpg Atomic cesium 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 axes 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 3 secondaries for use, to as many as 5 in one chain.

LORAN Charts

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New_York_Harbor_Nautical_Chart.jpg
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 aerials

LORAN-C transmitters uses transmission powers between 100 kilowatts and 4000 kilowatts, that mean their transmission power is comparative to longwave broadcasting stations. As aerials for stations below a transmission power of 500 kilowatts, guyed masts insulated against ground with a height of approximately 190 metres are used. These masts are electrically lengthed by guy wires spanned from basements to the top. One transmitter of this type is the LORAN-C transmitter Rantum on Sylt in Germany. For LORAN-C transmitters with transmission powers greater than 1000 kilowatts, guyed masts with heights of approximately 400 metres are used. The 412 metre high mast of the former LORAN-C station Hellissandur on Iceland is now used as aerial for longwave broadcasting of the Icelandic broadcasting company on the frequency 189 kHz. All LORAN-C stations uses aerials with an omnidirectional radiation 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-A

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.

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 0684872870) pp. 231-232.

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