E-Loran

LORAN, short for long range navigation, was a hyperbolic radio navigation system developed in the United States during World War II. It was similar to the UK's Gee system but operated at lower frequencies in order to provide improved range up to 1,500 miles (2,400 km) with accuracy of tens of miles. It was first used for ship convoys crossing the Atlantic Ocean, and then by long-range patrol aircraft, but found its main use on the ships and aircraft operating in the Pacific theatre. LORAN, in its original form, was an expensive system to implement, requiring a cathode ray tube (CRT) display. This limited use to the military and large commercial users. Use was never widespread, and by the time new receivers were available in the 1950s, the same improved electronics led to new systems with higher accuracy. 

eLoran is a low frequency terrestrial navigation system based on a number of transmission stations, which emit precisely timed and shaped radio pulses centred at 100 kHz radio frequency. Each station emits a sequence of 8 pulses spaced 1000 microseconds apart. The stations are grouped into chains, which each consists of a single master station and two or more secondary stations. The master station transmits first, followed by successive transmissions from each of the secondary stations of the chain. The master/secondary transmission sequence is repeated periodically, with the period between repetitions called the Group Repetition Interval (GRI).

eLoran represents a move away from the hyperbolic Loran-C of old. Today, modern receivers can measure the "time of arrival" of signals from many stations (and from multiple chains) at once. eLoran is derived from the Loran-C system, but uses solid-state transmitters, precise timing (using atomic clocks) and a data channel to provide correction and integrity messages. The use of built-in microprocessors means that the receiver is also able to output latitude and longitude directly. Modern eLoran works in much the same way as GPS but it is an independent and complementary system, offering a navigation system with no failure modes in common with GPS or any other satellite based system.

eLoran, together with a future maritime service of differential-Loran covering approaches to major UK and Irish ports, can offer the increased accuracy, integrity, and continuity of service modern navigators demand.