GPS summary and related information.

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.^[1] Recently both the US and European governments have announced political decisions to maintain and upgrade their Loran systems.

SDR in action: The last LORAN-C receiver is a technical description of using a software-defined radio to decode LORAN-C signals.

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.

GEE (navigation)

Oboe (navigation)

OMEGA, the Western counterpart of the Alpha Navigation System, no longer in use.

SHORAN is an acronym for SHOrt RAnge Navigation, a type of electronic navigation and bombing system with a precision radar beacon used in the B-26 and B-29 bomber aircraft during the Korean...

G-H (navigation)

CHAYKA, the Russian counterpart of LORAN

LORAN-C facility antenna (Port Clarence, Alaska) in the Structurae database

LORAN (LOng RAnge Navigation) is a terrestrial radio 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 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.

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.

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.

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).

Each LORAN chain in the world uses a unique GRI (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.

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).

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.

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.

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 will be situated in Cumbria, UK, and operated by VT Communications, which is part of the VT Group PLC.

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

Alpha, the Russian counterpart of the Omega Navigation System, still in use as of 2006.

Decca Navigator System, a British system that used phase difference instead of time difference.