1. DeCCa navigator system
Hyperbolic navigation system

2. The Decca Navigator System was a hyperbolic radio navigation system which allowed ships and aircraft to determine their position by receiving radio signals from fixed navigational beacons. The system used low frequencies from 70 to 129 kHz.
It was first deployed by the Royal Navy during World War II when the Allied forces needed a system which could be used to achieve accurate landings. After the war it was extensively developed around the UK and later used in many areas around the world.

3. Overview
The Decca Navigator System consisted of a number of land-based radio beacons 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.

4. Principles of Operation

5. ZONES
LANES
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 distinct location could be identified.

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

7. Range
Accuracy
During daylight ranges of around 400 nautical miles (740 km) could be obtained, reducing at night to 200 to 250 nautical miles (460 km), depending on propagation conditions.
Width of the lanes
Angle of cut of the hyperbolic lines of position
Instrumental errors
Propagation errors (for example, Skywave)

8. History
The Decca Navigator system found its origins in the United States but was later developed into an operational system by Decca Radio and Television Ltd. of London.
In 1936, it was conceived by an American, William J. O'Brien as a method of measuring the ground speed of aircraft undergoing trials and was simply named  'Aircraft Position Indicator'. 
He was unsuccessful in raising any interest in the US Army and Navy so the system lapsed until the outbreak of war in Thereupon he offered the idea to the British Air Ministry through his friend Harvey F. Schwarz, an American working for the Decca Record Company in England.

9.  Initially the idea was rejected by Watson-Watt as being prone to jamming and subject to interference (and likely due to the existing work on the Gee system, being carried out by Watt's group). However, in October 1941 the British Admiralty Signal Establishment (ASE) became interested in the system. O’Brien brought the Californian equipment 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.
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.

10. The monopoly on leased, not purchased, receivers by Decca generated great wealth for the company. This monopoly was later broken in 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.
The company Aktieselskabet Dansk Philips (The limited company Danish Philips, ap for short) started manufacturing receivers that indicated the position (longitude and latitude) of a vessel directly in degrees and minutes with two decimals. So the resolution indicated by the display was better than ±9.3 m, much better than the position accuracy of the Decca system. With these receivers onboard there was no need for the special Decca maps anymore. As users didn't pay rental for using the system, these receivers became very popular in fishing vessels and leisure crafts.

11. The Decca Navigator System provided by the general lighthouse authorities ceased to operate at midnight on 31 March 2000.

12. Disadvantage
Despite its early success Decca had a number of disadvantages compared with other land based low frequency radio navigation 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. Another serious disadvantage is a significant reduction of coverage at night caused by sky wave interference.

13. 1.
A more accurate system named Hi-Fix was developed using signalling in the 1.6 MHz range. 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. 2.
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. 3.
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 Barbados, separating the islands of Andros and New Providence. The application was for sonar studies made possible by the unique characteristics of the ocean floor.

14. The 'low frequency' signalling of the Decca system also permitted its use on submarines.5.
A long range trans North Atlantic system was in operation from the mid-1950s. It was called DECTRA. 6.