The hyperbolic navigators
In the first instalment of our two-part deep dive, we explore the World War II origins and post-war development of the first electronic navigation aids, Decca, LORAN and OMEGA.
It is a sad fact that war is one of the great drivers of technical progress, and World War II is a prime example. One area where dramatic progress was made during the conflict was in navigation, both for planes and ships. The most obvious example is radar, though this was initially developed for military reasons rather than navigation. Also developed at this time were Decca Navigator and LORAN, while OMEGA came many years later.
All were hyperbolic radio systems that produced position lines by measuring the difference in times of reception, or in phase difference between radio signals, from two or more synchronised transmitters.
The concept of a hyperbolic navigation system goes back to 1931 when Dr. Meint Harms proposed such a system, but the first practical application was the Royal Air Force’s bombing navigation system, Gee, in 1941.
A Decca Navigator Mk 12 display (Credit: archipel-img.com)
Decca Navigator allowed ships to determine their position by using radio signals from a dedicated system of static radio transmitters. The system was initially designed in the USA during the war, but was later developed in the UK, firstly by the Royal Navy. It used phase comparison of two low frequency signals between 70 and 129 kHz. It was first used to help clear German minefields prior to the D-Day Normandy landings in 1944.
Further developed and commercialised by Decca, it consisted of individual groups of land-based radio transmitters organised into chains of three or four stations. Each comprised a master station and three (or occasionally two) secondary stations, termed Red, Green and Purple. During daylight, ranges of around 400nm could be obtained, reducing at night to 200 - 250nm, depending on propagation conditions.
At its peak there were some 42 chains and 180 transmitting stations, and its high accuracy though short range made it particularly useful in coastal navigation. Fishing vessels were major post-war users, plotting the best areas to find their catch and making their charts invaluable possessions.
Worldwide coverage of the original LORAN system in 1945, amounting to 30% of the globe (Credit: si.edu)
LORAN (LOnge RAnge Navigation) was developed in the United States during World War II, with the first chain of two stations going live in June 1942. It was similar to the UK's Gee system but operated at the lower frequencies – around 200 kHz in order to provide an improved range up to 1,500 miles, with an accuracy of tens of miles – and was first used for convoys crossing the Atlantic Ocean. The new concept relied on the pulses being bounced off the ionosphere to allow them to be received over very long ranges.
Subsequent development led to LORAN-C which was introduced in 1957, and which operated at 90 to 110 kHz with a range of some 2,000 miles, giving an accuracy of 185 to 463m. As with the other systems, the introduction of satellite navigation led to reduced use, but its value remains as a back up if GPS suffers problems.
OMEGA required very tall towers for its eight transmitters. This is the Tsushima Island tower, station H of the system. Built in 1973, its stood 389m tall (Credit: based on [https://mapps.gsi.go.jp/maplibSearch.do#1 National Land Image Information (Color Aerial Photographs)], Japan’s Ministry of Land, Infrastructure, Transport and Tourism)
OMEGA was the first radio navigation system with a global reach. Although conceived in the 1940s, it was only approved for development in 1968, becoming operational around 1971. It was operated by the United States in cooperation with six partner nations and enabled ships (and aircraft) to determine their position with a 4-mile accuracy by receiving very low frequency (VLF) radio signals in the range 10 to 14 kHz, transmitted by a global network of eight fixed terrestrial radio beacons.
Each station transmitted a sequence of three VLF signals of 10.2 kHz, 13.6 kHz, 11.333 kHz in that order, plus a fourth frequency which was unique to each of the eight stations. The duration of each pulse (ranging from 0.9 to 1.2 seconds, with a 0.2 second blank interval between each pulse) differed in a fixed pattern and was repeated every 10 seconds; the 10-second pattern was common to all eight stations and synchronised with the carrier phase angle, which itself was synchronised with the local master atomic clock.
Using hyperbolic geometry and radio navigation principles, a position fix with an accuracy of the order of 3.1 to 6.2 miles was achievable over the whole world at any time of the day.
It was shut down in 1997 in favour of the satellite-based Global Positioning System (GPS) which also saw Decca Navigator being switched off in 2000. The emergence of the satellite, starting with the Transit, will be the subject of the next article.
John Barnes is a journalist and author and former editor of Marine Engineers Review.