A navigation system from which hyperbolic lines of position are determined by measuring the difference in times of arrival of pulses from widely spaced, synchronized transmitting stations. Since radio waves travel at the speed of light, a line of position represents a constant range difference from two transmitters. Loran is used by commercial and military ships and aircraft. The name is derived from “long-range navigation.” See also: Hyperbolic navigation system
In Fig. 1, it is assumed that the master station M transmits a pulse signal at time t = 0, and that the secondary station S transmits a similar pulse signal Δ microseconds after receiving the master pulse signal. The secondary station receives the master signal at time t = β microseconds, and therefore transmits its pulse at time t = β + Δ microseconds. A receiver at R measuring the difference in the times of arrival of the signals from the secondary and master stations measures the time difference TD = (β + Δ) + tSR − tMR . The locus of all points with this common time difference is a hyperbola through the receiver position R. Similarly, every hyperbolic line of position is uniquely defined by a time difference. The intersection of two such lines of position gives a navigational fix, or location. Historically, maritime charts were printed with these lines of constant time difference and fixes were manually plotted. Since it is common for a Loran receiver to be able to receive the signals from four or more transmitters, there will exist more than two hyperbolic lines of position and they will intersect at many different locations. More recently, techniques have been developed to both automatically convert receiver observations into position and to utilize this redundant information to improve the accuracy and integrity of the fix obtained.
The Loran position-fixing system was originally developed during World War II as an aid to the navigation of Allied aircraft and the North Atlantic convoys. Following the war, its use was extended by the U.S. Coast Guard (USCG) to aid marine navigation. Operating at a frequency of about 2 MHz, this system was subsequently known as Loran A. The Loran B system, a high-accuracy cycle-matching version of Loran A, was not implemented because propagation disturbances rendered cycle selection unreliable. The Loran C system evolved from Loran A to provide greater range and more precise, repeatable, and accurate navigation. By the end of 1980, all Loran A service operated by the U.S. government had been terminated. Loran D, which evolved as a tactical (short-range transportable) version of Loran C that could be more quickly deployed, is no longer in use. In 2010, the U.S. and Canadian Governments terminated all Loran C operations in North America. Loran C operations continue throughout much of Europe and Asia, including stations in the United Kingdom, France, Norway, Saudi Arabia, Russia, South Korea, Japan, India, and China. In addition, Germany and Denmark operate stations where the operations are funded by the United Kingdom and France, respectively. Currently the French commitment to 2020 is the longest commitment to operate of these countries.
Loran C uses ground-wave transmissions at low frequencies to give an operating range in excess of 1000 nautical miles (1800 km). Sky-wave contamination is avoided by using pulse techniques. High accuracy is obtained by carrier-phase comparison, and cycle identification is accomplished by measurement of the pulse envelope. Operation is possible in poor signal-to-noise conditions by using correlation techniques and time-domain filtering. Loran C has been conservatively evaluated as providing an absolute accuracy of better than 0.25 nmi (500 m) at least 95% of the time with better than 165 ft (50 m) repeatable accuracy from day to day throughout 80% of the fishing grounds, where repeatable accuracy is desired for economic reasons. In the United Kingdom, differential Loran is being developed such that the 95% accuracy will be improved to approximately 10 m in areas that have been surveyed and are near a differential monitor site.
Loran C chains are composed of a master transmitting station and two or more secondary transmitting stations. Master and secondary stations are generally separated by about 600 mi (970 km). Peak power of up to 2 MW is generated by the transmitter. Most stations use top-loaded monopole antennas up to 720 ft (220 m) in height. Six sites have multitower antenna arrays. See also: Antenna (electromagnetism)
All Loran C transmitters operate at a fixed frequency of 100 kHz and confine 99% of their radiated energy within the 90–110-kHz band. Each radiated pulse is designed, therefore, to build up quickly and decay slowly (Fig. 2). The radio frequency within each pulse is coherent with the repetition frequency.
Each Loran C transmitting station transmits a group of these pulses at a specified group repetition interval (GRI; Fig. 3). The master pulse group consists of eight pulses spaced 1000 microseconds apart, and a ninth pulse 2000 μs after the eighth. Secondary pulse groups contain eight pulses spaced 1000 μs apart.
Multiple pulses are used so that more signal energy is available at the receiver, improving significantly the signal-to-noise ratio without an increase in the peak transmitted power capability of the transmitters. The master station transmits a ninth pulse that can be used for visual identification. In the event that a baseline is unusable for navigation, the secondary transmitter of the affected baseline notifies the navigator by blinking the first two pulses off and on.
Each pulse within a group may have its radio-frequency cycles in phase or 180° out of phase with an established reference. The phase coding identifies master or secondary transmissions so that automatic signal acquisition can be accomplished unambiguously; coding is chosen so that the effects of long sky-wave pulse trains (over 1000 μs) cancel, thus permitting ground-wave accuracy to be retained under all conditions.
High accuracy is obtained in the receiver by measuring the crossover time of individual radio-frequency (rf) cycles on the leading edge of the pulse envelope. Because of the longer propagation path of the sky wave, sky-wave contamination does not start until at least 30 μs after the beginning of the ground-wave pulse. Therefore, the first three cycles of the received signal are always stable ground waves. Tracking is accomplished on the third rf cycle of each pulse in the group to obtain the greatest measurement precision and stability. Cycle identification requires integration or filtering of measurements of the pulse envelope to permit a reliable decision as to whether the correct third cycle or an earlier or later cycle is being tracked. Once this decision is made, the correct cycle can be selected. Continuous monitoring of this cycle selection process is generally implemented, with an alarm if correction is required. See also: Radio-wave propagation
There are five major sections of a modern Loran C automatic acquisition and tracking, cycle-matching receiver (Fig. 4): the rf sensor, the analog-to-digital (A/D) converter, a Loran clock and timing circuits, a digital processor, and appropriate operator controls and displays. See also: Analog-to-digital converter
The rf sensor receives and processes the signal to provide adequate overall signal-to-interference protection, to “notch out” severe continuous-wave interferences, and, if necessary, to provide amplification to a suitable level for digital sampling. The analog-to-digital converter is used to convert the analog rf signal information from the rf sensor to digital samples associated with each Loran pulse or pulse group. The digital samples so obtained are then processed in the digital-processor. See also: Signal processing
Technology that emerged during 1972–1975 made it possible to incorporate tracking-loop algorithms, search algorithms, cycle identification algorithms, and so on, in digital software using inexpensive read-only memory for control of an inexpensive microprocessor. As a result, fully automatic microprocessor-based Loran C receivers were produced at a user cost comparable to that of the manual, and comparatively crude, predecessor Loran A equipment. See also: Microprocessor
Operator controls and displays can take many forms, depending on the application. The navigation display, in its simplest form, is time-difference line-of-position numbers in microseconds for two signal pairs. More useful navigation outputs in terms of latitude-longitude coordinates, course-to-steer and distance-to-go to a selected destination, along-track and cross-track error, and so on, are generally provided in avionics equipment configurations and are generally offered as options in marine equipment configurations. Map displays with automatic Loran C position plot are also available.
United States policy on Loran and termination
The 2010 Federal Radionavigation Plan states: “In accordance with the DHS Appropriations Act, USCG terminated the transmission of all U.S. Loran C signals on February 8, 2010. At that time, the U.S. Loran C signal became unusable and permanently discontinued. The USCG transmission of the Russian-American Loran-C/Chayka signal was terminated on August 1, 2010. The USCG transmission of the Canadian Loran-C signals was terminated on August 3, 2010. Termination of these International transmissions was delayed until August 2010 to allow time to negotiate termination of the corresponding International Agreements.”
Enhanced Loran (eLoran) and EUROFIX
The concept of Enhanced or eLoran originated within the U.S. Federal Aviation Administration Loran Evaluation program during the period 2002-2010. In February 2008, the U. S. Department of Homeland Security announced the U. S. Government intention to implement eLoran, but then this decision was reversed and the operations in North America were terminated in 2010. Currently, the effort to develop eLoran is being led by the Greater Lighthouse Authorities within the United Kingdom. eLoran adds modulation and a data channel to the Loran signal to enhance the accuracy and integrity of Loran navigation, to enable master-independent, multichain navigation and to provide absolute time. The Loran communications scheme used in the United Kingdom trials is called EUROFIX. It was developed beginning in the late 1980s, at the Delft University of Technology in the Netherlands, by a group led by Durk van Willigen. In EUROFIX, the last six navigational pulses in a group are pulse-position modulated, resulting in one seven-bit word per group. A complete message contains 30 words: 20 words are Reed-Solomon parity and 10 words or 70 bits are data. The system was originally designed to transmit differential Global Positioning System (DGPS) corrections, and message types have been added to transmit differential Loran corrections and absolute time as well. See also: Satellite navigation systems
In the late twentieth century, technological advances rapidly lowered receiver costs, and coastal coverage limitations were eliminated or reduced by system improvements and expansion; consequently, there was a high degree of user acceptance by fishing crews and other commercial marine users, by the marine recreational community, and by civil aviation. In 1990, there were over 400,000 civil maritime users and approximately 75,000 civil aviation users of Loran C worldwide.
However, since the 1990s, actual use of Loran has diminished due to the better performance of GPS. The major argument for retaining Loran has become the vulnerability of GPS and other satellite-based navigation systems to jamming or interference, both intentional and unintentional. The major issue regarding Loran facing governments is whether these vulnerabilities justify the costs of a terrestrial backup to satellite-based navigation systems; and, if so, whether Loran is the appropriate backup. Now that the United States and Canada have terminated Loran operations in North America, this issue has become even more difficult for those governments that continue to operate Loran. See also: Air navigation; Electronic navigation systems; Marine navigation