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Decca Navigationssystem

EINARBEITEN:

• [1]

• [2]
• [3]

GEE (Navigation)

Decca also ran other high precision radionav systems in addition to MainChain, such as Shoran, HiFix, Pulse/8 and Decca Trisponder.

• Decca Navigator Main Chain System - [4]

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. - because Decca was not obligated to publish the transmission format, this arrangement effectively prohibited other potential competitors from entering the market. - challenge in the European Court in the 1990’s that Decca, then part of the Racal Group, was abusing it’s 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. -

changes in the transmission formats

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[5]

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only two companies providing precise nav services for rigmoves and pipelay barges: Decca Survey and Kelvin Hughes. The nav guys were invariably referred to as "the Decca Man", even if they actually worked for Kelvin Hughes.

The General Manager of Decca Survey worldwide was neither a surveyor nor a navigator. He was the guy who, when he ran Decca Records, turned down The Beatles. Nobody ever got fired from Decca, so they shunted the guy sideways into Decca Survey.

Decca Survey was later taken over by Racal, but for some years one of the Decca Survey companies retained its old name when all the others around the world had been subsumed into the Racal brand. That was the Bahrain company, Decca Survey Overseas Ltd (DSOL). It was known as Deezol. Racal spotted the obvious trap of changing DSOL into RSOL as Deezol would have become Arzole.

The first operational use of Decca was June 5th 1944 to sweep the corridors to the Normandy Beaches.

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Decca worked on somewhat different principles from Gee; describing its operation is a bit dodgy because it evolved considerably over time. In its more or less original form, it was a very longwave system, operating from 4,285 to 2,307 meters (70 to 130 kHz). Such low frequencies could propagate over very long distances, particularly through ionospheric bounce at night. The drawback was limited accuracy, and so Decca was focused on maritime navigation and not for military targeting.

In addition, it proved easier to implement a continuous broadcast scheme for the longwave signals used by Decca, in contrast to the pulsed broadcast scheme used by Gee. Decca's continuous broadcast scheme meant that distances had to be determined by comparison of signal phase, not by comparison of pulse timing. The same phase relationships would be repeated after certain distance intervals, and so, at least initially, Decca would give the same distance indication on intervals, or "lanes" that were separated by "nulls" where all the signals were in phase.

Decca featured a single master and three slaves, with the slaves designated "Red", "Green", and "Purple", matching the colors of hyperbola sets on the Decca map. They were separated by about 100 kilometers (60 miles) or so and transmitted on frequencies that were a multiple of a base frequency "F" in the pattern:

  Master:        6F at 85 kHz, providing the phase reference
  Red slave:     8F at 112 kHz
  Green slave:   9F at 197 kHz
  Purple slave:  5F at 71 kHz

All the transmitters were phase-locked into a common cycle. The three slaves transmitted for a half-second each in a one minute sequence, with the Red beginning the cycle; the Green broadcasting 15 seconds into the cycle; the Purple broadcasting 30 seconds into the cycle; and then all three going idle for 30 seconds until the red broadcast again. The phase differences for the Red-Green-Purple signals were displayed on a set of clock-style dials known as "decometers" that could be used to obtain location.

As noted, this scheme would give the position of the vessel within a lane, but it could not show which lane the vessel was in. Early on, the receiver system itself kept track of how many lanes had been traversed -- something equivalent to an "odometer" that incremented or decremented (as controlled by a navigator) every time the decometers indicated a null. Later a "multipulse" scheme was introduced in which each transmitter would generate a signal carrying all four Decca frequencies for a short part of the cycle, with a fifth 8.2f signal being transmitted continuously. This allowed the receiver to obtain the 1F signal, giving a unique location.

Decca saw widespread service after the war. It led to a specialized high-resolution system operating in the 187.5 meter (1.6 MHz) range, known as "Hi-Fix", used for oil-drilling and the like; as well as an improved system, named "Decca Track" or "Dectra", that never caught on.

Decca itself remained in service to the year 2000, though it had been in decline for some time due to lapse of patent protection on the technology. The Decca navigation system was unique among major radio navigation systems in that it was developed, implemented, and maintained by a commercial company through its lifetime, instead of a government organization. Employees liked to joke that Decca stood for "Dedicated Englishmen Causing Chaos Abroad".

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Radio direction finding had been widely used at sea from the time of the sinking of the Titanic in 1912, although even with a skilled operator its accuracy was limited.

Radio Direction Finder loop aerials are affected by their surroundings, 'night effect' changed the angle at which the signal arrived, and refraction at the coastline all gave problems. It also depended on the operator's ability to measure the angle the signal arrives at. Thus a bearing was inherently less accurate as distance increased.

WW2 changed everything as the Royal Navy needed to position its ships to within about 20 yds, even at night or in thick fog. In 1941 a radio method of accurate position fixing was proposed by the Decca Company in which a phase tracking receiver on the ship could determine its position relative to a chain of four special land based transmitters. The system was conceived by an American, William J O'Brien.

It was used initially during the invasion of Europe in 1944 and may be described as a medium range hyperbolic system operating between 70 and 130 kHz. The frequency of all the transmitters in each chain are locked together in a coherent way giving rise to hyperbolic lines of position. There were over fifty eight chains covering the UK, mainland Europe and other parts of the world.

The receiver presents the lane identifier and the relative phase of four channels on the 'Decometers' shown in the display unit above. The navigator obtains a fix by referring these values to a special chart on which a hyperbolic lattice is overprinted; a quick and simple operation.

After the war the system came into civilian use and it was adopted quickly by the fishing industry, it revolutionised every aspect of navigation. The above example shows the indicator unit of the Decca Navigator equipment as widely fitted to fishing vessels. Although now superseded by satellite navigation, it gave 50 years of good service to navigators around the long coastline of the UK and mainland Europe; it closed down in March 2000 leaving a fine mark on our maritime history of technology. More on the history of the Decca navigation system can be found at www.jproc.ca/hyperbolic/decca.html and in the Proceedings of the Royal Institute of Navigation, see www.rin.org.uk .

The picture below shows the complicated Mk 12 receiver opened as it would have been for adjustment or repair. The three circuit modules are hinged at the bottom to allow access by the service engineer, the left hand module has been lowered revealing the four receiver channels. The two other modules contain the timing and phase comparison circuits.

Wheelhouse Project - Decca Navigator

Radio direction finding had been widely used at sea from the time of the sinking of the Titanic in 1912, although even with a skilled operator its accuracy was limited.

Radio Direction Finder loop aerials are affected by their surroundings, 'night effect' changed the angle at which the signal arrived, and refraction at the coastline all gave problems. It also depended on the operator's ability to measure the angle the signal arrives at. Thus a bearing was inherently less accurate as distance increased.

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die PRIS war ein DECCA-Empfangsgerät basierend auf dem Prinzip der Hyperbelnavigation. Die Gerätefamilie der PIRS-Reihe (in der VM u.a. PIRS-1, PIRS-1M und PIRS-1D (auf S-61) arbeiteten damit ähnlich wie der Hyperbelnavigatiosempfänger des militärischen Systems BRAS-GALS.

'ziviler' DECCA-Peiler Typ PIRS

'militärische' GALS-Empfänger

dass PIRS k e i n DECCA Gerät war. Es war eine Kreiselkompssanlage. - PIRS-1D (Anmerkung: DECCA-Empfänger); eine "Kreiselkompaßanlage des Typs PIRS" - Decca Anlage Pirs 1M

Das Verfahren hieß LORAN C -funktionierte auch als Funkpeilgerät wie DECCA bzw. BRAS/ GALS. War internatioal üblich, nachdem die DECA-Ketten, nach dem Ende des WK II kommerziealsiert wurden und die Betreibergesellschaft für den Betrieb dieser ein "Schwxxnx-Beitrag" wollte

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The company was eventually bought out by Racal, who made some wonderful radio equipment.

www.nva-flieger.de navigationssysteme/hyperbel.html

A B C 1 2

Decca-Ketten in Nordwest-Australien