Demise Of Decca
AFTER MORE THAN 50 years of providing radio positioning for
mariners, 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 was
discontinued on 19 May 2000.
The Decca Navigator System was originally developed in 1937 in
the United States and it was first used in June 1944 to guide ships
leading the D-Day invasion through the mine fields off the north
coast of France.
The first chain was established on the south coast of England, and began transmitting the day before the invasion. It is widely believed that had such a service not then existed, the invasion plan might have been very different, since it was considered impossible to navigate a minesweeper in action across the English Channel and make a precise landfall at night.
A year later the service became commercially available for hire from the Decca Navigator Company. Decca was a radionavigation position fixing system based on continuous low-frequency wave signals. Transmitting stations at known locations provided hyperbolic lines of position to a receiver on board a ship. The receiver displayed the position lines either as numerical readings which could be plotted on a chart, or in later years as the position of the ship in latitude and longitude. The system operated by measuring the differences in the signals received from the transmitters, allowing users to establish their positions with a reasonable degree of accuracy and consistency.
At the peak of its service, Decca chains operated in all the major shipping areas of the world, with an estimated 200,000 users in Europe alone. The Decca company derived its income from the hire of Decca receivers to users of the system. By the early 1980s inexpensive receivers made by competing manufacturers became available for purchase, resulting in a loss of rental income to the company which, in 1979, was bought by the Racal Electronics group. The loss of rental income meant that Racal-Decca Marine Navigation Ltd could no longer finance the continued operation of the Decca system. Negotiations led to an agreement, which came into effect in February 1987, whereby the General Lighthouse Authorities assumed responsibility for the Decca system at the expense of the General Lighthouse Fund. Modernisation and automation of the system was completed by the end of 1995.
Despite its early success Decca had a number of disadvantages compared with other land based low frequency radionavigation systems. The range of the system was short compared to long range systems such as Loran-C. Decca used low power transmitters and required 24 transmitters to provide coverage for British and Irish waters, making it very expensive to operate. The termination of the system will result in a saving to the General Lighthouse Fund in the region of £3.5 million a year. Another serious disadvantage is a significant reduction of coverage at night caused by skywave interference.
In recent years the use of Decca declined rapidly. The advent of more accurate and reliable satellite positioning systems and, more recently, the availability of gps differential corrections dramatically increased this trend, with the result that national Decca chains elsewhere in the world have been withdrawn over the past number of years.
The decision of the General Lighthouse Authorities to terminate Decca was taken as a consequence of a series of studies, reviews, and recommendations, including the issue in 1996 of a consultation document Navigational Requirements into the 21st Century, and publication in 1997 of the Marine Navigation Plan to 2015, both of which endorse the closure of Decca.
It is considered that the existing mix of traditional aids, and both satellite and terrestrial radio aids, including the differential gps provided by the General Lighthouse Authorities in Ireland and Great Britain, is fully adequate for safe marine navigation.
The Editor acknowledges additional contributions to these Radionavigation features from IALA Bulletin; The Irish Skipper; Capt. Kieran O'Higgins, Deputy Inspector of Lights and Marine Superintendent; and Colin Day, Senior Electrical Engineer.
The first chain was established on the south coast of England, and began transmitting the day before the invasion. It is widely believed that had such a service not then existed, the invasion plan might have been very different, since it was considered impossible to navigate a minesweeper in action across the English Channel and make a precise landfall at night.
A year later the service became commercially available for hire from the Decca Navigator Company. Decca was a radionavigation position fixing system based on continuous low-frequency wave signals. Transmitting stations at known locations provided hyperbolic lines of position to a receiver on board a ship. The receiver displayed the position lines either as numerical readings which could be plotted on a chart, or in later years as the position of the ship in latitude and longitude. The system operated by measuring the differences in the signals received from the transmitters, allowing users to establish their positions with a reasonable degree of accuracy and consistency.
At the peak of its service, Decca chains operated in all the major shipping areas of the world, with an estimated 200,000 users in Europe alone. The Decca company derived its income from the hire of Decca receivers to users of the system. By the early 1980s inexpensive receivers made by competing manufacturers became available for purchase, resulting in a loss of rental income to the company which, in 1979, was bought by the Racal Electronics group. The loss of rental income meant that Racal-Decca Marine Navigation Ltd could no longer finance the continued operation of the Decca system. Negotiations led to an agreement, which came into effect in February 1987, whereby the General Lighthouse Authorities assumed responsibility for the Decca system at the expense of the General Lighthouse Fund. Modernisation and automation of the system was completed by the end of 1995.
Despite its early success Decca had a number of disadvantages compared with other land based low frequency radionavigation systems. The range of the system was short compared to long range systems such as Loran-C. Decca used low power transmitters and required 24 transmitters to provide coverage for British and Irish waters, making it very expensive to operate. The termination of the system will result in a saving to the General Lighthouse Fund in the region of £3.5 million a year. Another serious disadvantage is a significant reduction of coverage at night caused by skywave interference.
In recent years the use of Decca declined rapidly. The advent of more accurate and reliable satellite positioning systems and, more recently, the availability of gps differential corrections dramatically increased this trend, with the result that national Decca chains elsewhere in the world have been withdrawn over the past number of years.
The decision of the General Lighthouse Authorities to terminate Decca was taken as a consequence of a series of studies, reviews, and recommendations, including the issue in 1996 of a consultation document Navigational Requirements into the 21st Century, and publication in 1997 of the Marine Navigation Plan to 2015, both of which endorse the closure of Decca.
It is considered that the existing mix of traditional aids, and both satellite and terrestrial radio aids, including the differential gps provided by the General Lighthouse Authorities in Ireland and Great Britain, is fully adequate for safe marine navigation.
The Editor acknowledges additional contributions to these Radionavigation features from IALA Bulletin; The Irish Skipper; Capt. Kieran O'Higgins, Deputy Inspector of Lights and Marine Superintendent; and Colin Day, Senior Electrical Engineer.