LORAN explained

LORAN (LOng Range Aid to 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 to serve as a backup to GPS and other GNSS systems.

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. The RAF also used LORAN on raids beyond the range of GEE.[1] 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

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 (in practice, one of the stations in the second pair may also be—and frequently is—in the first pair). By determining the intersection of the two hyperbolic curves identified by the application of this method, a geographic fix can be determined.

LORAN method

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 (or slave) 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)

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 JAW 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 grid lines 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.

LORAN charts

Where available, common marine nautical 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.

Timing and Synchronization

Each LORAN station is equipped with a suite of specialized equipment to generate the precisely timed signals used to modulate / drive the transmitting equipment. Up to three commercial cesium atomic clocks are used to generate 5 MHz and pulse per second signals that are used by timing equipment to generate the various GRI-dependent drive signals for the transmitting equipment.

Each US-operated LORAN station is synchronized to within ±100 ns of UTC.[2]

Transmitters and antennas

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.

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.

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 (multipath interference). 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-. Repeatable accuracy is much greater, typically from 60-.[3]

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.

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.

The future of LORAN

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. 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.[4] [5]

On 26 Feb 2009 the The U.S. Office of Management and Budget released the first blueprint for the Financial Year 2010 budget[6] This document identifies the Loran-C system as “outdated” and supports its termination at an estimated savings of $36 million in 2010 and $190 million over five years.

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 8 meters, 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.

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.[7]

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.

StationCountryChainRemarks
AfifSaudi ArabiaSaudi Arabia South (GRI 7030)/Saudi Arabia North (GRI 8830)
Al KhamasinSaudi ArabiaSaudi Arabia South (GRI 7030)/Saudi Arabia North (GRI 8830)
Al MuwassamSaudi ArabiaSaudi Arabia South (GRI 7030)/Saudi Arabia North (GRI 8830)
AngissqGreenlandshutdown on December 31, 1994used until July 27, 1964 a 411.48 metre tower
AnthornUKLessay (GRI 6731)replacement for transmitter Rugby[8]
Ash ShaykSaudi ArabiaSaudi Arabia South (GRI 7030)/Saudi Arabia North (GRI 8830)
Attu, AlaskaUnited StatesNorth Pacific (GRI 9990)/Russian-American (GRI 5980)
BalasoreIndiaCalcutta (GRI 5543)
BarrigadaGuamshut down
Baudette, MinnesotaUnited StatesNorth Central U.S. (GRI 8290)/Great Lakes (GRI 8970)
BerlevågNorwayBø (GRI 7001)
BillamoraIndiaBombay (GRI 6042)
Boise City, OklahomaUnited StatesGreat Lakes (GRI 8970)/South Central U.S. (GRI 9610)
Bø, VesterålenNorwayBø (GRI 7001)/Eiði (GRI 9007)
Cambridge BayCanadashut downfree-standing lattice tower, used as NDB
Cape RaceCanadaCanadian East Coast (GRI 5930)/Newfoundland East Coast (GRI 7270)used a 411.48 metre tall tower until February 2, 1993, uses now a 260.3 metre tall tower
Caribou, MaineUnited StatesCanadian East Coast (GRI 5930) / Northeast U.S. (GRI 9960)
Carolina Beach, North CarolinaUnited StatesNortheast US (GRI 9960)/ Southeast U.S. (GRI 7980)
ChongzuoChinaChina South Sea (GRI 6780)
Comfort CoveCanadaNewfoundland East Coast (GRI 7270)
Dana, IndianaUnited StatesGreat Lakes (GRI 8970)/ Northeast US (GRI 9960)
DhrangadhraIndiaBombay (GRI 6042)
Diamond HarborIndiaCalcutta (GRI 5543)
EiðiFaroe IslandsEjde (GRI 9007)
EstartitSpainMediterranean Sea (GRI 7990); shut down
Fallon, NevadaUnited StatesU.S. West Coast (GRI 9940)
Fox HarbourCanadaNewfoundland East Coast (GRI 7270)/ Canadian East Coast (GRI 5930)
George, WashingtonUnited StatesCanadian West Coast (GRI 5990)/ U.S. West Coast (GRI 9940)
GesashiJapanEast Asia (GRI 9930)/ North West Pacific (GRI 8930)
Gillette, WyomingUnited StatesSouth Central U.S. (GRI 9610)/ North Central U.S. (GRI 8290)
Grangeville, LouisianaUnited StatesSouth Central U.S. (GRI 9610)/ Southeast U.S. (GRI 7980)
Havre, MontanaUnited StatesNorth Central U.S. (GRI 8290)
HellissandurIcelandshut down on December 31, 1994411.48 metre tall tower, now used for longwave broadcasting of RÚV on 189 kHz
HelongChinaChina North Sea (GRI 7430)
HexianChinaChina South Sea (GRI 6780)
Jan MayenNorwayBø (GRI 7001)/ Ejde (GRI 9007)
Johnston IslandUnited Statesshut-down
Iwo JimaJapanshut down in September 1993, dismantledused a 411.48 metre tall tower
Jupiter, FloridaUnited StatesSoutheast U.S. (GRI 7980)
KargaburanTurkeyMediterranean Sea (GRI 7990); shut down
Kwang JuSouth KoreaEast Asia (GRI 9930)
LampedusaItalyMediterranean Sea (GRI 7990); shut down
Las Cruces, New MexicoUnited StatesSouth Central U.S. (GRI 9610)
LessayFranceLessay (GRI 6731) / Sylt (GRI 7499)
Loop HeadIrelandwas planned (GRI 6731 and 9007), but never operational
Malone, FloridaUnited StatesGreat Lakes (GRI 8970) / Southeast U.S. (GRI 7980)
MinamitorishimaJapanNorth West Pacific (GRI 8930)used until 1985 a 411.48 metre tall tower
Nantucket, MassachusettsUnited StatesCanadian East Coast (GRI 5930) / Northeast U.S. (GRI 9960)
Narrow Cape, AlaskaUnited StatesNorth Pacific (GRI 9990) / Gulf of Alaska (GRI 7960)
NiijimaJapanNorth West Pacific (GRI 8930) / East Asia (GRI 9930)
PatpurIndiaCalcutta (GRI 5543)
PohangSouth KoreaNorth West Pacific (GRI 8930) / East Asia (GRI 9930)
Port Clarence, AlaskaUnited StatesGulf of Alaska (GRI 7960)/North Pacific (GRI 9990)uses a 411.48 metre tall tower
Port HardyCanadaCanadian West Coast (GRI 5990)
RantumGermanySylt (GRI 7499)/ Lessay (GRI 6731)
Raymondville, TexasUnited StatesSouth Central U.S. (GRI 9610)/ Southeast U.S. (GRI 7980)
RaopingChinaChina South Sea (GRI 6780)/ China East Sea (GRI 8930)
RongchengChinaChina North Sea (GRI 7430)/ China East Sea (GRI 8930)
RugbyUKexperimental (GRI 6731); shut down at the end of July 2007
Saint Paul, AlaskaUnited StatesNorth Pacific (GRI 9990)
SalwaSaudi ArabiaSaudi Arabia North (GRI 8830)/Saudi Arabia South (GRI 7030)
Searchlight, NevadaUnited StatesU.S. West Coast (GRI 9940)/South Central U.S. (GRI 9610)
Sellia MarinaItalyMediterranean Sea (GRI 7990); shut down
Seneca, New YorkUnited StatesGreat Lakes (GRI 8970)/Northeast U.S. (GRI 9960)
Shoal Cove, AlaskaUnited StatesCanadian West Coast (GRI 5990)/Gulf of Alaska (GRI 7960)
SoustonsFranceLessay (GRI 6731)
Tok, AlaskaUnited StatesGulf of Alaska (GRI 7960)
TokachibutoJapanEastern Russia Chayka (GRI 7950)/ North West Pacific (GRI 8930)
Upolo Point, HawaiiUnited Statesshut-down
VærlandetNorwaySylt (GRI 7499)/ Ejde (GRI 9007)
VeravalIndiaBombay (GRI 6042)
Williams LakeCanadaCanadian West Coast (GRI 5990)
XuanchengChinaChina North Sea (GRI 7430)/ China East Sea (GRI 8930)
YapMicronesiashut down in 1987, dismantledused a 304.8 metre tall tower

See also

References

External links

former LORAN-C transmitter mast, now used for longwave broadcasting

Notes and References

  1. Hecks, Karl. (1990) Bombing 1939-1945: the air offensive against land targets in World War Two. Robert Hale Ltd., London. ISBN 0-7090-4020-2. p.219
  2. COMDTINST M16562.4A Specification of the Transmitted LORAN-C Signal p. 2-6
  3. COMDTPUB P16562.6, "Loran-C Users Handbook," 1992
  4. USCG Navigation Center's ENHANCED LORAN (eLORAN) description
  5. MPs warned of GPS jamming risks http://news.bbc.co.uk/1/hi/uk_politics/7459213.stm
  6. Office of Management and Budget. (www.budget.gov), "A New Era of Responsibility Renewing America's Promise" The FY 2010 Budget, Department of Homeland Security Section, page 72
  7. Web site: The GLAs award a 15-year eLoran contract to VT Communications. Trinity House. 2007-05-31. 2008-01-09.
  8. Electronic Position Fixing System. Admiralty Notices to Mariners. 26/07. United Kingdom Hydrographic Office. 2007-06-28. 2008-01-19.