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It is only intended to give a very brief technical description of the principles of the Decca Navigator Main Chain System as a number of good references are available elsewhere.
Over the history of the Decca System, a number of improvements and transmission format changes took place. As the Decca Marine and Aircraft Receivers were only available on a rental basis, the changes in transmission formats did not cause problems to the end users the company simply upgraded the receivers free of charge as part of the rental contract. But because Decca was not obligated to publish the transmission format, this arrangement effectively prohibited other potential competitors from entering the market. This did result in a challenge in the European Court in the 1990s that Decca, then part of the Racal Group, was abusing its position in the market place by effectively creating a monopoly which was detrimental to the user. However, it was the introduction of GPS, and the availability of low cost receivers which finally caused the Decca System to pass into history. Paradoxically, Satellite Position fixing was used to determine the true locations of the masts for the Decca Navigator Lagos Chain.
Due to the changes in the transmission formats, a number of references are very out of date. On the Internet, Jerry Proc VE3FAB has a website http://www.jproc.ca/hyperbolic/decca.html giving some useful background information and a good historical description of the Decca System, but much of the technical information and descriptions relate to the system as operational in the 1950s/early 60s, which bares very little resemblance to the system in its final form.
My recommendation is a book entitled Radar and Electronic Navigation by G.J. Sonnenberg 5th Edition published by Newnes-Butterworth ISBN 0-408-00272-7. This gives a very good primer for anyone interested in hyperbolic and other navigation systems and includes sections on Loran, Omega, Console and, of course, Decca, together with sections on radar and the Transit Satellite System as used before the deployment of GPS. The Decca chapter is particularly well written and is an excellent source for a technical description of the operation of the Decca System.
A Brief Technical Overview
The Decca Navigator Main Chain
The classic Decca Chain comprised of 4 stations consisting of a Master Station and three Slave Stations. The Slave Station were referred to by a Colour Code, namely Red, Green and Purple which was used for drawing distinctive navigation position lines on Marine Charts.
Each Station in the chain would normally transmit a particular unmodulated phase stable carrier wave. These carriers were all harmonically related to an internal station reference which was about 14.2kHz, referred to as f.
The Master Station normally transmitting a 6f unmodulated carrier wave signal in the 85kHz band, a Red Slave Station transmitting a 8f signal in the 112kHz Band, a Green Slave transmitting a 9f signal in the 127kHz band, and the Purple Slave transmitting a 5f signal in the 71kHz band. The Slave stations received and phase locked their station reference oscillators to the Master 6f Transmission.
Although for most of the time the stations only transmitted their single carrier, during part of the transmission cycle, each station would transmit what was termed a Multi-pulse.
The Multi-pulse was transmitted by each of the stations in turn during the 20 second transmission cycle to provide a coarse reading, or a Zone reading, and was generated by all 5 transmitters at the given station briefly transmitting simultaneously. During transmission of the Multi-pulse by, say the Red Slave, all transmissions from the other stations in the Chain would be suppressed.
The only omission from virtually every publication which Ive seen is that there is no reference to the Orange 8.2f transmissions which were used to provide a extra coarse Zone indication on certain models of receiver. Also in the Nigerian Chains the 8.2f frequency was used to provide the signaling for the monitoring and control of the slave stations in the chain.
Other Hyperbolic Decca Systems
Other Decca Systems worthy of mention were Hi-Fix, Dectra and Delrac.
Hi-Fix was operated by another company within the Decca Group, namely Decca Survey, of Leatherhead, Surrey. A number of Hi-Fix Systems operated at about 1.9MHz and produced very distinctive signals in the 160 metre Amateur Band in the UK up until late 80s. Hi-Fix, as can be ascertained from the name was a high accuracy surveying tool and not intended for general navigation, and although permanent chains did exist, one being for navigation on the lower reaches of the River Thames, most were temporary and set up for a particular surveying exercise.
The advent of the Jet Airliner in the form of the Comet and Boeing 707 in the late 50s and the attendant increase of Transatlantic Air traffic necessitated improvements over the dead reckoning navigation used by the pilots of the slower propeller driven aircraft.
A solution was sought and Decca created a system called Dectra. I have heard three explanations for the term Dectra one being Decca Tracks, another being Decca Tracking and Ranging, and the third Decca Transatlantic. A good description of the system is available on the Jerry Proc (VE3FAB) Dectra Page.
One of the Dectra Stations here in the UK was at Earl's Hill near Sterling, Scotland. This Dectra Station used 10 kw transmitters into a 700ft guyed mast as opposed to the standard Chain which used 1kw nominal output transmitters with a 300ft (100m) mast. I have seen the Dectra Antenna Tuning Coils which were still in situe in the late 70s and in use as part of the Sctoish Chain of which the Sterling Station was part, the only difference between these and an normal set were their size about twice the normal size. The coil tressle stood at about 6ft high, and the capacitors were similarly of an increased size.
However, Dectra never was adopted by the airline industry, the aircraft operators opting for the inertial navigation systems then becoming available.
Delrac Decca Long Range Area Coverage - only existed as a proposal although it was patented. The proposal was to set up VLF Stations operating in the 10kHz band where the ionosphere acts like a waveguide to provide a global navigation system. See the Jerry Proc (VE3FAB) Delrac Page for a description.
As mentioned on the VE3FAB website, the Delrac frequencies were eventually used by the US Government for their Omega System. I recall being told stories by Decca Staff of the patent infringement actions brought by Decca against the US Government David vs. Goliath!
The Nigerian Chains
The Nigerian Decca Navigator Main Chains probably were the last Decca Navigator Main Chains to be constructed. A total of four chains were planned, namely:-
Mid Western 3A/MP
South Eastern 2B/MP
Where in the first chain Lagos is the name of the chain each chain had a geographically identifiable name; 8F was the channel number; and MP the type of transmission format, MP meaning Multi-pulse.
These chains when completed would give contiguous coverage along the Nigerian Coastline.
I was only involved with the Construction of the Lagos Chain over the period 1979/80. The Master Station was located at Ikorodu to the North of Lagos, the Red Slave at Abigi along the Eastern Shoreline of the Lagos Lagoon; the Green Slave at Badagary near the Benin Boarder, and the Purple Slave at Abeokuta, about 60km North of the Master Station. On the Lagos Chain the Slave stations were to be unmanned with planned visits of only once a month, and diesel generator service/overhaul at three monthly intervals.
A monitoring station was located at Agbowa, about 7km North of the Master Station which was used to monitor the Chain Stability and other transmission parameters. This simply received the raw off-air Chain signals which base-band modulated a analogue microwave link transmitter in the 900MHz band. The link receiver was located at the Master/Control Station where the signal was processed and displayed on a monitoring receiver and a series of pen recorders.
The Master station was the only manned site on the Chain, with the Control and monitoring equipment co-located. One of the Nigerian Chains was planned to have a remote Control Station, with the Master and three slaves totally unmanned.
The Lagos Chain was commissioned and Marine Charts with the Decca Overlay were produced, albeit for internal Decca reference only, and although the Lagos Chain operated for a period of time, from my understanding, it never went into public service. It is understood that the Rivers chain was also built and did operate, but again never entered service. The status of the Mid Western and the South Eastern Chains is unknown.
A Virtual tour of a Decca Main Chain
All the pictures of the Lagos Chain and were taken during the period 1979/1980. I apologise for the snap shot nature of the photos, but they simply taken as that and were never intended to be published for a wide audience. However, they do give an impression of a typical Main Chain Station.
Also please forgive me for any technical errors, it was over 20 years ago since I last was involved with a Decca Chain and some details have faded from my memory. I would very much appreciate to hear from anyone who can complete or add to the prictures and decriptions - particular the fate of the chain.
Antenna System Photos
Other Decca Stations
Last update 30/7/2006