LORAN

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LORAN (LOng RAnge Navigation) is a terrestrial radio navigation system using low frequency radio transmitters that uses multiple transmitters (multilateration) to determine location and/or speed of the receiver. The current version of LORAN in common use is LORAN-C, which operates in the low frequency portion of the EM spectrum from 90 to 110 kHz. Many nations are users of the system, including the United States, Japan, and several European countries. Russia uses a nearly identical system in the same frequency range, called CHAYKA. LORAN use is in steep decline, with GPS being the primary replacement. However, there are current attempts to enhance and re-popularize LORAN, mainly because it has proven to be a very reliable and simple system.

[edit] 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 built 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.

[edit] 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.

[edit] LORAN method

LORAN pulse

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).

[edit] LORAN chains (GRIs)

LORAN Station Malone, Malone, Florida Great Lakes chain (GRI 8970) / Southeast U.S. chain (GRI 7980)

Every LORAN chain in the world uses a unique Group Repetition Interval, the number of which, when multiplied by ten, gives how many microseconds pass between pulses from a given station in the chain (in practice, the GRI delays in many, but not all, chains 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 two 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 orthogonal 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-orthogonal grid can become a grid that is significantly skewed. 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.

[edit] LORAN charts

This nautical chart of New York Harbor includes LORAN-A TD lines. Note that the printed 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 display latitude and longitude instead of time differences, and with improved accuracy.

[edit] Transmitters and antennas

LORAN transmitter bank

LORAN-C transmitters operate at peak powers of 100 kilowatts to four megawatts, comparable to longwave broadcasting stations. Most LORAN-C transmitters use mast radiators insulated from ground with heights between 190 and 220 metres. The masts are inductively lengthened and fed by a loading coil (see: electrical lengthening). A well known-example of a station using such an antenna is LORAN-C transmitter Rantum.

Free-standing tower radiators in this height range are also used. LORAN-C transmitter Carolina Beach uses a free-standing antenna tower.

Caesium atomic clocks used for LORAN signal synchronization

LORAN-C transmitters with output powers of 1000 kW and higher sometimes use supertall mast radiators (see below).

Other high power LORAN-C stations, like LORAN-C transmitter George, use four T-antennas mounted on four guyed masts arranged in a square.

All LORAN-C antennas radiate an omnidirectional pattern. Unlike longwave broadcasting stations, LORAN-C stations cannot use backup antennas. The slightly different physical location of a backup antenna would produce Lines of Position different from those of the primary antenna.

[edit] Limitations

LORAN suffers from electronic effects of weather and the ionospheric effects of sunrise and sunset. The most accurate signal is the groundwave that follows the Earth's surface, ideally over seawater. At night the indirect skywave, bent back to the surface by the ionosphere, is a 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 uses ground based transmitters that only cover certain regions. Coverage is quite good in North America, Europe, and the Pacific Rim.

The absolute accuracy of Loran-C varies from 0.1 to 0.25 nautical miles. Repeatable accuracy is much greater, typically from 60 to 300 feet.^[1]

[edit] LORAN-A and other systems

LORAN-A was a less accurate system operating in the upper mediumwave frequency band prior to deployment of the more accurate LORAN-C system. For LORAN-A the transmission frequencies 1750 kHz, 1850 kHz, 1900 kHz and 1950 kHz were used. LORAN-A 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 USAF 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-A was used in the Vietnam War for navigation by large United States aircraft (C-124, C-130, C-97, C-123, HU-16, etc). A common airborne receiver of that era was the R-65/APN-9 which combined the receiver and cathode ray tube (CRT) indicator into a single relatively lightweight unit replacing the two larger, separate receiver and indicator units which comprised the predecessor APN-4 system. The APN-9 and APN-4 systems found wide post-World War II use on fishing vessels in the U.S. They were cheap, accurate and plentiful. The main drawback for use on boats was their need for aircraft power, 115 VAC at 400 Hz. This was solved initially by the use of rotary inverters, typically 28 VDC input and 115 VAC output at 400 Hz. The inverters were big and loud and were power hogs. In the 1960s, several firms such as Topaz and Linear Systems marketed solid state inverters specifically designed for these surplus LORAN-A sets. The availability of solid state inverters that used 12 VDC input opened up the surplus LORAN-A sets for use on much smaller vessels which typically did not have the 24-28 VDC systems found on larger vessels. The solid state inverters were very power efficient and widely replaced the more trouble prone rotary inverters.

LORAN-A saved many lives by allowing offshore boats in distress to give accurate position reports. It also guided many boats whose owners could not afford radar safely into fog bound harbors or around treacherous offshore reefs. The low price of surplus LORAN-A receivers (often under $150) meant that owners of many small fishing vessels could afford this equipment, thus greatly enhancing safety. Surplus LORAN-A equipment, which was common on commercial fishing boats, was rarely seen on yachts. The unrefined cosmetic appearance of the surplus equipment was probably a deciding factor.

Pan American World Airways used APN 9s in early Boeing 707 operations. The World War II surplus APN-9 looked out of place in the modern 707 cockpit, but was needed. There is an R65A APN-9 set displayed in the museum at SFO Airport, painted gold. It was a retirement present to an ex Pan Am captain.

An elusive final variant of the APN 9 set was the APN 9A. A USAF technical manual (with photographs and schematics) shows that it had the same case as the APN-9 but a radically different front panel and internal circuitry on the non-RF portions. The APN-9A had vacuum tube flipflop digital divider circuits so that TDs (time delays) between the master and slave signal could be selected on front panel rotary decade switches. The older APN-9 set required the user to perform a visual count of crystal oscillator timing marker pips on the CRT and add them up to get a TD. The APN 9A did not make it into widespread military use, if it was used at all, but it did exist and represented a big advance in military LORAN-A receiver technology.

In the 1970s one U.S. company, SRD Labs in Campbell, California, made modern LORAN-A sets including one that was completely automatic with a digital TD readout on the CRT, and autotracking so that TDs were continuously updated. Other SRD models required the user to manually align the master and slave signals on the CRT and then a phase locked loop would keep them lined up and provide updated TD readouts thereafter. These SRD LORAN-A sets would track only one pair of stations, giving you just one LOP (line of position). If one wanted a continuously updated position (two TDs giving intersecting LOPs) rather than just a single LOP, one needed two sets.

Long after LORAN-A broadcasts were terminated, commercial fishermen still referred to old LORAN-A TDs, e.g., "I am on the 4100 [microsecond] line in 35 fathoms", referring to a position outside of Bodega Bay. Many LORAN-C sets incorporated LORAN A TD converters so that a LORAN-C set could be used to navigate to a LORAN-A TD defined line or position.

[edit] 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.

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

[edit] The future of LORAN

Worldwide Loran coverage

As LORAN systems are government maintained and operated, their continued existence is subject to public policy. With the evolution of other electronic navigation systems, such as Global Navigation Satellite Systems (GNSS), funding for existing systems is not always assured.

Critics, who have called for the elimination of the system, state that the Loran system has too few users, lacks cost-effectiveness, and that GNSS signals are superior to Loran.^{[citation needed]} Supporters of continued and improved Loran operation note that Loran uses a strong signal, which is difficult to jam, and that Loran is an independent, dissimilar, and complementary system to other forms of electronic navigation, which helps ensure availability of navigation signals.^[2]

Recently both the US and European governments have announced political decisions to maintain and upgrade their Loran systems.

[edit] eLORAN

With the perceived vulnerability of GNSS systems, and their own propagation and reception limitations, renewed interest in LORAN applications and development has appeared. Enhanced LORAN, also known as eLORAN or E-LORAN, 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, the system becomes competitive with unenhanced GPS. eLoran also includes additional pulses which can transmit auxiliary data such as DGPS corrections. eLoran 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.

[edit] United Kingdom eLORAN implementation

On 31 May 2007, the UK Department for Transport (DfT), via the General Lighthouse Authorities (GLA), awarded a 15 year contract to provide a state-of-the-art enhanced LORAN (eLORAN) service to improve the safety of mariners in the UK and Western Europe. The service contract will operate in two phases, with development work and further focus for European agreement on eLORAN service provision from 2007 through 2010, and full operation of the eLORAN service from 2010 through 2022. The eLORAN transmitter is situated at Anthorn transmitting station Cumbria, UK, and operated by VT Communications, which is part of the VT Group PLC.^[3]

[edit] List of LORAN-C transmitters

A list of LORAN-C transmitters. Stations with an antenna tower taller than 300 metres (984 feet) are shown in bold.

[edit] 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.
• Decca Navigator System, a British system that used phase difference instead of time difference.
• SHORAN
• Oboe (navigation)
• G-H (navigation)
• GEE (navigation)
• GPS

[edit] References

• Jennet Conant, 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 on 2006-02-26.

[edit] External links

• The Case for LORAN- It is a good backup, and very accurate.
• United States Coast Guard's official site- how to use LORAN C
• 9th Pulse Modulation / LORAN Data Channel (LDC) - will provide information using pulse position modulation of the broadcast signal.
• United States National Institute of Standards and Technology Site- Using LORAN C for time-keeping.
• LORAN-C Transmitter (Rantum) in the Structurae database
• Hellissandur Transmission Tower in the Structurae database — former LORAN-C transmitter mast, now used for longwave broadcasting
• LORAN-C facility antenna (Gillette, Wyoming) in the Structurae database
• LORAN-C facility antenna (Port Clarence, Alaska) in the Structurae database
• http://www.jproc.ca/hyperbolic/loran_a.html More in-depth discussion of the Loran-A system.
• http://www.jproc.ca/hyperbolic/loran_b_d.html Histories of Lorans-B, -D, and "-F".
• Integrated GPS/Loran Prototypes for Aviation Applications
• Loran-C Introduction: eLoran
• The Migration to Enhanced or eLoran
• The Case for eLoran at GPS World
• GNSS/eLoran for Timing and Frequency by Locus, Inc.
• Loran's Capability to Mitigate the Impact of a GPS Outage on GPS Position, Navigation, and Time Applications by Locus, Inc.
• New Potential of Low-Frequency Radionavigation in the 21st Century Ph.D. dissertation
• LORAN-C chains in service
• Excel-File with information about LORAN-C transmitters
• SDR in action: The last LORAN-C receiver is a technical description of using a software-defined radio to decode LORAN-C signals.
• New UK eLORAN service provision news article News article re: UK leading the way in eLORAN service provision.