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Decca Navigator System
The Decca Navigator System was a hyperbolic radio navigation system that allowed ships and aircraft to determine their position by using radio signals from a dedicated system of static radio transmitters. The system used phase comparison between pairs of low frequency signals between 70 and 129 kHz, as opposed to pulse timing systems like Gee and LORAN. This made it much easier to design receivers using 1940s electronics, and operation was simplified by giving a direct readout of Decca coordinates without the complexity of a cathode-ray tube and highly skilled operator.
The system was developed by Decca in the UK. 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. The Allied forces needed an accurate system not known to the Germans and thus free of jamming. 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 system was deployed extensively in the North Sea and was used by helicopters operating to oil platforms.
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. Decca Navigator, along with Loran and similar systems, was eventually replaced by the GPS in 2000, when that became available for public use.
The Decca Navigator System consisted of individual groups of land-based radio transmitters organised into chains of three or four stations. Each chain consisted of a master station and three (occasionally two) secondary stations, termed Red, Green and Purple. Ideally, the secondaries would be positioned at the vertices of an equilateral triangle with the master at the centre. The baseline length, that is, the master–secondary distance, was typically 60–120 nautical miles (110–220 km).
Each station transmitted a continuous wave signal; comparing the relative phases of the signals from the master and one of the secondaries produced a relative phase measure that was presented on a clock-like display. The phase difference was caused by the relative distance between the stations as seen by the receiver. As the receiver moves these distances change and those changes are represented by the movement of the hands on the displays.
If one selects a particular phase difference, say 30 degrees, and plots all the locations where that phase difference occurs, the result is a set of hyperbolic lines of position called a pattern. As there were three secondaries there were three patterns, also termed Red, Green and Purple. The patterns were drawn on nautical charts as a set of hyperbolic lines in the appropriate colour.
Navigators determined their location by reading the phase difference from two or more of the patterns from the displays. They could then look at the chart to find where the two closest charted hyperbolas crossed. 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.
When two stations transmit at the phase-locked frequency, the difference in phase between the two signals is constant along a hyperbolic locus. However, if two stations transmit on the same frequency, it is impossible for the receiver to separate them. Instead, each chain was allocated a nominal frequency, known as 1f, and each station in the chain transmitted at a harmonic of this base frequency, as follows:
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Decca Navigator System
The Decca Navigator System was a hyperbolic radio navigation system that allowed ships and aircraft to determine their position by using radio signals from a dedicated system of static radio transmitters. The system used phase comparison between pairs of low frequency signals between 70 and 129 kHz, as opposed to pulse timing systems like Gee and LORAN. This made it much easier to design receivers using 1940s electronics, and operation was simplified by giving a direct readout of Decca coordinates without the complexity of a cathode-ray tube and highly skilled operator.
The system was developed by Decca in the UK. 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. The Allied forces needed an accurate system not known to the Germans and thus free of jamming. 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 system was deployed extensively in the North Sea and was used by helicopters operating to oil platforms.
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. Decca Navigator, along with Loran and similar systems, was eventually replaced by the GPS in 2000, when that became available for public use.
The Decca Navigator System consisted of individual groups of land-based radio transmitters organised into chains of three or four stations. Each chain consisted of a master station and three (occasionally two) secondary stations, termed Red, Green and Purple. Ideally, the secondaries would be positioned at the vertices of an equilateral triangle with the master at the centre. The baseline length, that is, the master–secondary distance, was typically 60–120 nautical miles (110–220 km).
Each station transmitted a continuous wave signal; comparing the relative phases of the signals from the master and one of the secondaries produced a relative phase measure that was presented on a clock-like display. The phase difference was caused by the relative distance between the stations as seen by the receiver. As the receiver moves these distances change and those changes are represented by the movement of the hands on the displays.
If one selects a particular phase difference, say 30 degrees, and plots all the locations where that phase difference occurs, the result is a set of hyperbolic lines of position called a pattern. As there were three secondaries there were three patterns, also termed Red, Green and Purple. The patterns were drawn on nautical charts as a set of hyperbolic lines in the appropriate colour.
Navigators determined their location by reading the phase difference from two or more of the patterns from the displays. They could then look at the chart to find where the two closest charted hyperbolas crossed. 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.
When two stations transmit at the phase-locked frequency, the difference in phase between the two signals is constant along a hyperbolic locus. However, if two stations transmit on the same frequency, it is impossible for the receiver to separate them. Instead, each chain was allocated a nominal frequency, known as 1f, and each station in the chain transmitted at a harmonic of this base frequency, as follows: