Decca Navigator System

The Decca Navigator System was a hyperbolic radio navigation system and first deployed during World War 2 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. The system was deployed extensively in the North Sea and was used by helicopters operating to oil platforms. It has now been superceded by systems such as the American GPS system and the planned European Galileo positioning system.

It was deployed in the United Kingdom after World War 2 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.

Principles of Operation

Overview

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. 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 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.

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. Rather than all stations transmitting at the same frequency, each chain was allocated a nominal frequency, 1f, and the stations 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.

Decca receivers multiplied the signal received from the Master and each Slave by different values to arrive at a common frequency for each Master/Slave pair, as follows:

It was 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.

Lanes and Zones

Early Decca receivers were fitted with 3 rotating Decometers that indicated the phase difference for each pattern. The Decometers 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, and 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 patterns. Typical lane and zone widths (for chain 5B) are shown in the table below:

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.

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 4 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 and 8.2f signals allowed a 0.2f signal to be derived and so resulted in a hyperbolic pattern in which one phase difference equates to 5 zones.

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

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 vessel is further from land, the reduced accuracy at long ranges was not a great problem.

Other Applications

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