Decca Navigator System

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Decca Navigator Mk 12

The Decca Navigator System was a hyperbolic low frequency radio navigation system (also known as multilateration) that was first deployed during World War II when the Allied forces needed a system which could be used to achieve accurate landings. As was the case with Loran C, its primary use was for ship navigation in coastal waters. Fishing vessels were major post-war users, but it was also used on aircraft, including a very early (1949) application of moving-map displays. The system was deployed extensively in the North Sea and was used by helicopters operating to oil platforms. After being shut down in the spring of 2000, it has been superseded by systems such as the American GPS and the planned European Galileo positioning system.

It was deployed in the United Kingdom after World War II 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.

[edit] Principles of Operation

[edit] Overview

The Decca Navigator principle.
The phase difference between the signals received from stations A (Master) and B (Slave) is constant along each hyperbolic curve. The foci of the hyperbola are at the transmitting stations, A and B.

The Decca Navigator System consisted of a number of land-based stations organised into chains. Each chain consisted of a Master station and three (occasionally two) Slave stations, termed Red, Green and Purple. Ideally, the Slaves would be positioned at the vertices of an equilateral triangle with the Master at the centre. The baseline length, i.e. the Master-Slave distance, was typically 60~120 nautical miles. Each station transmitted a continuous wave signal that, by comparing the phase difference of the signals from the Master and one of the Slaves, resulted in a set of hyperbolic lines of position called a pattern. As there were three Slaves there were three patterns, termed Red, Green and Purple. The patterns were drawn on nautical charts as a set of hyperbolic lines in the appropriate colour. Receivers identified which hyperbola they were on and a position could be plotted at the intersection of the hyperbola from different patterns, usually by using the pair with the best angle of cut.

[edit] Detailed Principles of Operation

When two stations transmit at the same phase-locked frequency, the difference in phase between the two signals is constant along a hyperbolic path. Of course, if two stations transmit on the same frequency, it as practically impossible for the receiver to separate them; so instead of all stations transmitting at the same frequency, each chain was allocated a nominal frequency, 1f, and each station in the chain transmitted at a harmonic of this base frequency, as follows:

The frequencies given are those for Chain 5B, known as the English Chain, but all chains used similar frequencies between 70kHz and 129kHz.

Decca receivers multiplied the signals received from the Master and each Slave by different values to arrive at a common frequency (least common multiple, LCM) for each Master/Slave pair, as follows:

It was phase comparison at this common frequency that resulted in the hyperbolic lines of position. The interval between two adjacent hyperbolas on which the signals are in phase was called a lane. Since the wavelength of the common frequency was small compared with the distance between the Master and Slave stations there were many possible lines of position for a given phase difference, and so a unique position could not be arrived at by this method.

Other receivers, typically for aeronautical applications, divided the transmitted frequencies down to the basic frequency (1f) for phase comparison, rather than multiplying them up to the LCM frequency.

[edit] Lanes and Zones

Early Decca receivers were fitted with three rotating Decometers that indicated the phase difference for each pattern. Each Decometer drove a second indicator that counted the number of lanes traversed – each 360 degrees of phase difference was one lane traversed. In this way, assuming the point of departure was known, a more or less unique location could be identified.

The lanes were grouped into zones, with 18 green, 24 red, or 30 purple lanes in each zone. This meant that on the baseline (the straight line between the Master and its Slave) the zone width was the same for all three patterns of a given chain. Typical lane and zone widths on the baseline are shown in the table below (for chain 5B):

The lanes were numbered 0 to 23 for red, 30 to 47 for green and 50 to 79 for purple. The zones were labelled A to J, repeating after J. A Decca position coordinate could thus be written: Red I 16.30; Green D 35.80. Later receivers incorporated a microprocessor and displayed a position in latitude and longitude.

[edit] Multipulse

Multipulse provided an automatic method of lane and zone identification by using the same phase comparison techniques described above on lower frequency signals.

The nominally continuous wave transmissions were in fact divided into a 20 second cycle, with each station in turn simultaneously transmitting all four Decca frequencies (5f, 6f, 8f and 9f) in a phase-coherent relationship for a brief period of 0.45 seconds each cycle. This transmission, known as Multipulse, allowed the receiver to extract the 1f frequency and so to identify which lane the receiver was in (to a resolution of a zone).

As well as transmitting the Decca frequencies of 5f, 6f, 8f and 9f, an 8.2f signal, known as Orange, was also transmitted. The beat frequency between the 8.0f (Red) and 8.2f (Orange) signals allowed a 0.2f signal to be derived and so resulted in a hyperbolic pattern in which one cycle (360°) of phase difference equates to 5 zones.

Assuming that one’s position was known to this accuracy, this gave an effectively unique position.

[edit] Range and Accuracy

During daylight ranges of around 400 nautical miles could be obtained, reducing at night to 200 to 250 nautical miles, depending on propagation conditions.

The accuracy depended on:

• Width of the lanes
• Angle of cut of the hyperbolic lines of position
• Instrumental errors
• Propagation errors (e.g. Skywave)

By day these errors could range from a few meters on the baseline up to a nautical mile at the edge of coverage. At night, skywave errors were greater and on receivers without multipulse capabilities it was not unusual for the position to jump a lane, sometimes without the navigator knowing.

Although in the days of differential GPS this range and accuracy may appear poor, in its day the Decca system was one of the few, if not the only, position fixing system available to many mariners. Since the need for an accurate position is less when the vessel is further from land, the reduced accuracy at long ranges was not a great problem.

[edit] History

[edit] Origins

In 1936 William J. O'Brien, an American engineer, contracted tuberculosis which put his career on hold for a period of two years. During this period he had the idea of position fixing by means of phase comparison of continuous wave transmissions. The initial market envisaged was for aircraft and some experiments were carried out in California in 1938. However both the American Army and Navy considered the idea too complicated.

O’Brien had a friend, Harvey F. Schwarz, who was chief engineer of the Decca Record company in England, and in 1939 sent him details of the system so it could be put forward to the British military. Initially Robert Watson-Watt reviewed the system but he did not follow it up. However, in October 1941 the British Admiralty Signal Establishment (ASE) became interested in the system, which was then classified as Admiralty Outfit QM. O’Brien came over to the UK and conducted the first marine trials between Anglesey and the Isle of Man, at frequencies of 305/610 kHz, on 16 September 1942. These were successful and further trials were conducted in the northern Irish Sea in April 1943 at 70/130 kHz. A three-station trial was held in conjunction with a large-scale assault and landing exercise in the Moray Firth in February/March 1944.

The success of the trials and the relative ease of use and accuracy of the system resulted in Decca receiving an order for 27 Admiralty Outfit QM receivers. The receiver consisted of an electronics unit with two dials and was known to its operators as the "Blue Gasmeter Job". A Decca chain was set up, consisting of a master station at Chichester and slaves at Poole and Beachy Head. A fourth, decoy, transmitter was located in the Thames Estuary as part of the deception that the invasion would be focussed on the Calais area.

Twenty-one minesweepers and other vessels were fitted with Admiralty Outfit QM and on 5 June 1944 they used it to accurately navigate across the English Channel and to sweep the minefields in the planned areas. The swept areas were marked with buoys in preparation for the Normandy Landings.

[edit] Deployment

After the end of World War II the Decca Navigator Co. Ltd. was formed (1945) and the system expanded rapidly, particularly in areas of British influence; at its peak it was deployed in many of the world's major shipping areas. More than 15,000 receiving sets were in use in 1970. There were 4 chains around England, 1 in Ireland and 2 in Scotland, 12 in Scandinavia (5 each in Norway and Sweden and 1 each in Denmark and Finland), a further 4 elsewhere in northern Europe and 2 in Spain. Other chains were established in Japan (6 chains); Namibia and South Africa (5 chains); India and Bangladesh (4 chains); Canada (4 chains around Newfoundland and Nova Scotia); North-West Australia (2 chains); the Persian Gulf (1 chain with stations in Qatar and the United Arab Emirates and a second chain in the north of the Gulf with stations in Iran) and the Bahamas (1 chain). Four chains were planned for Nigeria but only 2 chains were built and these did not enter into public service. There were also two chains in Vietnam which were used during the Vietnam War for helicopter navigation, with limited success.

[edit] Decca, Racal, and the closedown

Decca Navigator was headquartered in a large residential property at Little Wymondley, Hertfordshire. There was a Decca School, at Brixham, Devon, where employees were sent on courses from time to time.

Racal, the UK weapons and communications company, acquired Decca in 1980. 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. 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 take responsibility for operating the system 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.

The Decca Navigator System provided by the General Lighthouse Authorities ceased to operate at midnight on 31 March 2000. The Irish chain provided by Bórd Iascaigh Mhara continued transmitting until 19 May 2000.

[edit] Other Applications

A more accurate system named Hi-Fix was developed using signalling in the 1.6MHz range. It 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.

[edit] Special DECCA towers

• Puckeridge DECCA tower
• Zeven DECCA-transmitter

[edit] References

• The Decca Navigator - Principles and Performance of the System, The Decca Navigator Company Limited, July 1976 [1]
• Night Passage to Normandy, Lieutenant-Commander Oliver Dawkins, R.N.V.R, Decca, 1969
• The Decca Navigator System on D-Day, 6 June 1944, An Acid Test, Commander Hugh St. A. Malleson, R.N. (Ret.)
• Hyperbolic Radionavigation Systems (Compiled by Jerry Proc VE3FAB) [2]
• Navigation Systems: A Survey of Modern Electronic Aids, ed. G.E. Beck, van Nostrand Reinhold, 1971

[edit] External links

• Decca Navigator System by Jerry Proc