The LORAN (Long Range Navigation) radio navigation system is enjoying a new lease of life with E-LORAN promising improvements in accuracy and resistance to electronic attack.
The US pioneered LORAN during the Second World War to aid maritime navigation, particularly for Allied convoys crossing the Atlantic bringing troops, materiel and supplies to Europe. It uses hyperbolic techniques to provide ships or aircraft with information on their position.
The Maths Part
LORAN exploits the speed of light which travels at 161,595 nautical miles-per-second (299,274 kilometres-per-second). At least two radio transmitters are needed for LORAN to work. Imagine two radio transmitters, ‘A’ and “B’ on a coastline. Out at sea is the MV Ted Rogers, a fictious ship equipped with a LORAN receiver. Our two transmitters send out a regular pulse every half a second. These are sent on a frequency of 100 kilohertz/KHz, a standard LORAN waveband allotted by the International Telecommunications Union.
The MV Ted Rogers is positioned at the exact mid-point between both transmitters and will receive the pulses of transmitters A and B every half a second. If the ship is nearer transmitter A, its LORAN receiver will get the pulse from transmitter B slightly later and vice versa if it is closer to B.
The clever thing is that although transmitter A and B are sending out their pulses at the same rate, pulses from the transmitter which the ship is furthest from will take longer to arrive. By knowing the physical distance between transmitter A and B, and the ship’s relative proximity to each, the LORAN receiver can calculate the ships position relative to the coastline.
LORAN gradually fell out of favour from the late 1980s onwards as advances in GNSS (Global Navigation Satellite System) technology gained momentum in the 1990s.
However it may be about to enjoy a new lease of life in the guise of Enhanced LORAN, better known as E-LORAN. Examples of GNSS jamming have been noted in the eastern Mediterranean during Russia’s intervention in the Syrian Civil War from September 2015. While GNSS signals are weak and easy to jam, they can also be ‘spoofed’; manipulated to provide false information. Meanwhile, research and development of anti-satellite weapons is continuing in India, the People’s Republic of China, Russia and the United States. All these factors are prompting interest around the world for GNSS alternatives to provide PNT (Position Navigation and Timing) services in the event of GNSS systems being jammed, spoofed or physically damaged.
Hellen Systems has developed an E-LORAN system which is a major overhaul of the existing LORAN architecture. The latter’s vacuum tube transmitter and analogue signal technology has been replaced by solid state architecture and digital signals, says Bridge Littleton who alongside Daniel Olmes, is a founder and president of the firm. Hellen Systems’ approach makes no change to the standard 90KHz to 100KHz waveform used by LORAN but adds a major enhancement in signal quality.
Mr. Littleton says that factors like the weather can degrade the LORAN signal as it travels through the air. This in turn affects the quality of the navigation information the transmitters can provide. Hellen Systems’ approach is to use a proven approach for improving accuracy via a Differential Correction Station (DCS). A DCS is about the size of a fridge which can run autonomously. It is positioned within range of a users receiver. The DCS samples the transmitted signal and compares this to what the signal should be like if there was no impedance. These corrections to the signal are then sent back to the E-LORAN broadcast tower and transmitted in the E-LORAN signal. Any E-LORAN receiver within range of the transmitter will receive the E-LORAN signals from the transmitter, and any corrections to that signal provided by the DCS. Mr. Littleton adds that the DCS has completed testing and validation and is ready for service for any E-LORAN deployment.
He says that much of the infrastructure needed by E-LORAN still exists in the form of legacy transmitters and equipment at LORAN sites around the world. These would need to be upgraded with a solid state transmitter and timing equipment. The DCS stations can be placed anywhere inside the arch of coverage provided by a LORAN transmitter which typically reaches between 1,043nm (1931km) to 1,216nm (2,233km) in range: “Compared to a GNSS system, the E-LORAN system is exponentially less expensive to implement,” argues Mr Littleton.
He says that E-LORAN has good resistance to electronic attack. The signal is typically five million times stronger than the circa 50 watt signals the US GPS constellation transmits. Anyone wishing to do so would have to build a large power-hungry system with a tower at least 213 metres (700 feet) tall if they wanted to seriously upset LORAN transmissions in a particular area. Mr. Littleton says that digital coding is built into the E-LORAN signal to make it harder to spoof. LORAN receivers will be able to recognise potentially false signals bereft of this coding.
Both the American and British governments are looking to introduce modernised LORAN systems in their countries to aid maritime navigation and provide resilient PNT services nationwide. Mr. Littleton says that the firm could have a fully operational E-LORAN system up and running in the UK within two years and in the US within five years of a contract signature.