The Decca Navigator System was an early precursor to positioning systems like the American GPS system and the planned European Galileo positioning system.

It used radio signals from three transmitters to fix a position. The receiver fixed a position by measuring the phase shift of signals received from the three transmitters. One consequence was that positions were not unique.

The system was deployed extensively in the North Sea and was used by helicopters operating to oil platforms.

As was the case with Loran C, its primary use was for ship navigation in coastal waters. Loran employed a time-domain measuring technology.

The Decca Navigator system was a hyperbolic radio navigation system and first deployed during World War 2 when the Allied forces needed a system which could be used to achieve accurate landings.

It was deployed in the United Kingdom after World War 2 and later used in many areas around the world.

Decca employees used to joke that DECCA was an acronym for Dedicated Englishmen Causing Chaos Abroad.

Basically the main chain systems comprised a Master Station complimented by 3 slave stations named Purple, Green and Red, using constant wave transmissions, that is to say unmodulated other than being switched on and off, in the 70,000Hz to 120,000Hz band. These colours originated from the overprinting applied to standard maps and charts and referred also to the frequency of a station. One colour, Orange, was a 8.2f transmission which was used to provide an extra Zone indication on certain models of receiver - for later, specialised receivers.

The Master station provided the 'master' signal which was used by its associated Slave stations to derive signal frequency and timing sequences. Loss of a Master would disable a station whilst loss of a Slave would reduce accuracy.

It operated by measuring the phase differences between continuous signals from a master and slave stations. The differences are then related to a hyperbolic lines printed on a chart. By plotting the readings from two sequential pairs of hyperbolas at any particular instant, users could plot their position.

A receiving system, leased from Decca, could display position by way of readouts, pen output and other techniques. There were systems suitable for land. sea and air use.

What was quite remarkable for its time, the signals were measured by degrees of a wave cycle, that is to say the transmission frequency divided by 360, and the inperpolation of the sets of signals determined the precise position within the transmission area. There were no integrated circuit phase-locked loops in those days.

Readers should remember the transmissions were 'circles' centred on each transmitter and, like ripples from a stone dropped into a still pool of water, these circles expanded and consequently navigation close in to a transmitter was more accurate than those obtained in points well away from transmitters. As the radiation patterns were, to a degree, circular in all planes including the vertical plane, making it possible to service airborne receivers.

Although the initial deployment provided reasonable accuracy, refinements that involved multiple switched carrier techniques provided increased accuracy.

A benefit that derived from users of the Decca navigator system was one of safety. If an inaccuracy, caused by one Slave station phase drifting, occurred all users would receive the same error and therefore would all 'drift' off true location by the same amount which, inter alia, would help in avoiding collisions.

Additionally, a 'wrong' reading on one vessel would be easily reachable by another vessel as it, too, would have an equal error.

Maps and charts were over printed with Decca readings, which was the only way a relationship between a Decca pattern and a map using an internationally recognised location standard, could be achieved.

A more accurate system was developed using signalling in the 1.6MHz range, named Hi-Fix, was used for specialised applications such as precision measurements involved with oil-drilling, etc. Other systems were used in the Middle East.

An interesting characteristic discovered on BOAC, later British Airways, test flights to Moscow, was that the carrier switching could not be detected even though the carrier could be received with sufficient strength to provide navigation. Such testing, involving civilian aircraft, is quite common and may well not be in the knowledge of a pilot.

The 'low frequency' signalling of the Decca system also permitted its use on submarines. One 'enhancement' of the Decca system was to offer the potential of keying the signal, using morse code, to signal the onset of nuclear war. This was never optioned by the UK government. Messages were clandestinely sent, however, between Decca stations thereby bypassing ancient international telephone calls, especially in non-UK chains.

Decca Navigator, headquartered in a large residential property at Little Wymondley, Hertfordshire, had several chains deployed through the UK and signal locking between the systems was possible. One chain, the Sterling Chain, provided trans-Atlantic navigation facilities using Slaves situated in Nova Scotia, Canada and Newfoundland, Canada.

There were also chains in Europe. The writer worked on the French chain, which had an associated Slave in Spain. Other Decca chains covered the Great Lakes chain (a demo), the Baltic, Bay of Biscay, the Med and the approaches to Gibraltar, too. Others were in the Far East in New Guinea, for example.

Other systems were deployed in Vietnam where were two Decca chains during the American War. The northernmost one had Master at Phu Cat (south of Da Nang), Red at Plei Ku (Central Highlands - near where this writer now resides), Green at Cu Lao Re Island (a volcanic island southeast of Hoi An) and Purple at Tuy Hoa.

The southernmost chain had Master at Van Kiep (Ba Ria-Vung Tau), Red at Tây Ninh (Cambodian border), Green at Phan Thiet (Binh Thuan) and Purple on Con Son Island (off Vung Tau used as prisoner of war camp). These were 'portable' or, more accurately, transportable systems being moved in large trucks/lorries.

The control station for these two chains was at the gate to Tân Sơn Nhất Airport (SGN), outside Saigon, now called Ho Chi Minh City.

The typical transmitter station comprised a tube/valve transmitter and receiver - switched to solid state in the 1970s - an impressive bank of rechargeable batteries, a diesel-generator set and a single mast some 200+ feet in height.

The station manager lived in a company provided 'house', not exactly luxurious accommodation, whilst assistants would live in nearby accommodation, sleeping in a 'cot' when they worked 24-hour shifts.

One site at which the writer worked was at Park Lane, East Hoathly, near Hailsham, West Sussex - now a dog kennel - on the A22 main road, spitting distance from the quaint village of Halland. It was part of the English Chain, the Master located at Puckeridge, Hertfordshire. ENGLISH CHAIN 5B comprised Puckeridge, Hertfordshire - Master - 85.000 kHz; Shotisham, near Norwich, Norfolk; Red - 113.333kHz; East Hoathley, Lewes, Sussex - Green - 127.500kHz; Wormleighton, Warwick, Warwickshire - Purple.

There was a Decca School, at Brixham, Devon - a delightful fishing town that was eventually swamped by tourism, where employees were sent on courses from time to time.

Racal, the UK weapons and communications company, acquired Decca in 1981 or thereabouts. Claiming the acquisition was to acquire Decca's radar company, rather than the avionics side of the business, it sold off parts including Decca Navigator.

The monopoly on leased, not purchased, receivers by Decca generated great wealth for the company, which was headquartered in Hertfordshire. This monopoly was later broken the early 1980s when receivers could be purchased by users, thereby reducing the cost following the lapse of the patent on the basic system technology.

A Danish company started manufacturing receivers for fishing boats which employed Decca's navigation charts, but users didn't pay rental for using the system.

In the ensuing court battle Decca lost the monopoly, and that signalled the beginning of the end. Income dwindled and eventually, the UK Ministry of Transport stepped in, having the lighthouse authorities to take us over in the early 1990s.

A ruling from the European Union forced the UK government to withdraw funding - for fishermen users - and started the process which eventually resulted in the system being closed down and the installations scrapped.

This East Hoathly station, managed by John Pratt, now retired and living in Hailsham, also served as a weather observation station, a function that has been replaced by an automatic weather station on the A22 just outside Hailsham.