Decca Navigator System
It was first deployed by the Royal Navy during World War II for the vital task of clearing the minefields to enable the D-Day landings.
After the war, it came off the secret list and was commercially developed by the Decca Company and deployed around UK and later used in many areas around the world.
At its peak there were about 180 transmitting stations using "chains" of three or four transmitters each to allow position fixing by plotting intersecting electronic lines.
Decca's primary use was for ship navigation in coastal waters, offering much better accuracy than the competing LORAN system.
Fishing vessels were major post-war users, but it was also used on some aircraft, including a very early (1949) application of moving map displays.
The opening of the more accurate Loran-C system to civilian use in 1974 offered stiff competition, but Decca was well established by this time and continued operations to 2000.
The Decca Navigator System consisted of individual groups of land-based radio transmitters organised into chains of three or four stations.
The accuracy of this measurement was improved by choosing the set of two patterns that resulted in the lines crossing at as close to a right angle as possible.
Multipulse provided an automatic method of lane and zone identification by using the same phase comparison techniques described above on lower frequency signals.
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.
Some experiments were carried out in California in 1938, selecting frequencies with harmonic "beats" that would allow for station identification in a network of transmitters.
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.
21 minesweepers and other vessels were fitted with Admiralty Outfit QM and, on 5 June 1944, 17 of these ships used it to accurately navigate across the English Channel and to sweep the minefields in the planned areas.
The company entertained high hopes that the system could be used in aircraft, to permit much more precise navigation in the critical airspace around airports and urban centres where traffic density was highest.
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.
[3][failed verification] The first chain was installed in southwest Newfoundland in 1956 as part of a joint Canada-US Navy surveying program.
This led to commercial deployments the next year in Nova Scotia and an inland system for air traffic in the busy Quebec City-Montreal area.
When meetings in Montreal in 1958 led to VOR and DME being selected as the standard aviation navigation systems, the Montreal system was moved eastward to cover the Anticosti Island area of the Gulf of St. Lawrence, and the western Newfoundland chain was later repositioned to better cover the Cabot Strait.
Use of Decca was essential because its signals could be received down to sea level, were not subject to the line-of-sight limitations of VOR/DME and did not suffer the slant-range errors that create problems with VOR/DME close to the transmitters.
This led to the U.S. Coast Guard, under instructions from the Treasury Department to which it reported, banning the use of Decca receivers in ships entering New York harbour for fear that the system might create a de facto standard (as it had become in other areas of the world).
It also served to protect the marketing interests of the Hoffman Electronics division of ITT, a principal supplier of VOR/DME systems, that Decca might have been poised to usurp.
The purpose of this chain was to provide accurate air navigation for the corridor between Western Germany and Berlin in the event that a mass evacuation of allied personnel may be required.
The "ap" versions directly output the longitude and latitude to two decimals (originally in datum ED50 only) instead of using the "deco meter" displays, offering accuracy better than ±9.3 m[citation needed], much better than the Decca units.
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 further development, including features of the General Post Office's POPI system, was introduced in 1954, proposing 28 stations that provided worldwide coverage.
They also installed Dectra receivers with Omnitrac computers and a lightweight version of the Flight Log on a number of commercial airliners, notably a BOAC Vickers VC10.
It was used for specialised applications such as precision measurements involved with oil-drilling and by the Royal Navy for detailed mapping and surveying of coasts and harbours.
Each vehicle would report its location automatically via a conventional VHF two-way radio link, the data added to a voice channel.
Another application was developed by the Bendix Pacific division of Bendix Corporation, with offices in North Hollywood, California, but not deployed: PFNS—Personal Field Navigation System—that would enable individual soldiers to ascertain their geographic position, long before this capability was made possible by the satellite-based GPS (Global Positioning System).
A further application of the Decca system was implemented by the U.S. Navy in the late 1950s and early 1960s for use in the Tongue of the Ocean/Eleuthera Sound area near The Bahamas, separating the islands of Andros and New Providence.
The phase difference between the signals received from stations A (Master) and B (Secondary) is constant along each hyperbolic curve. The foci of the hyperbolas are at the transmitting stations, A and B.
