INTRODUCTION

DECTRA (Decca Track) was a radio position-fixing system designed to cover specific air route segments and, in particular, long trans-ocean crossings across the Atlantic Ocean. The system was based largely on the existing Decca Navigator technique and a considerable proportion of the airborne installation was common to both systems. It was experimentally installed on aboard aircraft of several airlines, including the short-lived DC-7's of British Overseas Airway Corporation and Pan-American.

DESIGN INTENTIONS

Dectra was installed for aviation use following the RTCA requirement for a long range transatlantic complement to Decca Navigator. Its main difference from the Delrac system was that it used the normal Decca Navigator frequency band of 70-130 kHz and existing transmitters of the Scottish and Newfoundland Decca Navigator chains  which had been equipped with aerials twice as high as usual (600 ft) and transmitters of four times normal power. The consequent radiated power was some 20 times greater than normal, sufficient to provide usable signals in daylight out to 750 miles, more than halfway across the Atlantic. At that range, since the transmitters were still on the normal, rather short, Decca Navigator baselines of 50 to 60 miles, the system was a bearing system rather than a hyperbolic one and was intended only to provide track guidance. A transmitter was installed in Iceland and another proposed for the Azores to give distance-to-go information. Dectra was a much more economical proposition than Delrac and in theory did not require the aircraft to carry a special receiver provided it already had normal Decca Navigator. In practice, a special receiver was in fact needed. In the design proposal, it was stated that the Dectra receiver including a meter presentation shall weigh in the order of 70 pounds. Adding a Flight Log would grow the weight by an additional 50 to 60 pounds. By integrating a Navigator receiver with the Dectra receiver it would add about 20 more pounds to the base system.

TRACKS, COVERAGE and ACCURACY

The transmitters used were the master and purple slaves of the East Newfoundland and Scottish Decca chains transmitting at about 70 kHz, their baselines fortuitously being aligned so that their right bisectors ran parallel and thus formed a continuous track along the great circle path across the Atlantic. The intention was that aircraft would navigate using standard Decca while over land, use the Dectra tracks over the ocean, and return to standard Decca on the other side.
 

Tracks: This drawing illustrates the Dectra tracks across the Atlantic.

 

This map details the locations of the Dectra stations in Newfoundland. (From Decca Navigator News November 1956)

 

Coverage: Unfortunately the clarity of this chart is not the best, but it does give the reader some idea of the Dectra coverage area and the expected accuracy. Within the area of the two small ovals adjoining Gander, Newfoundland and Prestwick, Scotland, the system would provide an accuracy of 5 miles. Elsewhere, and within the confines of the large oval, the best resolution would be 10 miles.

 

POSITION PRESENTATION

A Flight Log, mounted in the cockpit, was used for presentation, On this, ranging information would be displayed on a suitably scaled chart and tracking would be shown on a scale variable in steps from 2 miles to the inch in terminal areas and 40 miles to the inch at mid route. Normally one chart would be sufficient for a particular route so that no in-flight re-setting would have to be done. Alternatively, meter presentation was employed in those cases where it is desired to use the system primarily as tracking facility.
 

TEST MODE: The Decca Standard Flight Log with three decometers and a control unit arranged as a test installation on an aircraft owned by British European Airways. This photo is believed to be from the mid 1950's. Normally the Flight Log is in view of the pilot. (Photo courtesy of Meccano Magazine, March 1957).

 
 
 

Mounted at the forward part of the cockpit and above the instruments was the special lightweight version of the Decca Flight Log suitable for short flights or for use in helicopters. This photo is believed to be from the mid 1950's. (Photo via Meccano Magazine, March 1957)

This is the first Dectra receiver fitted aboard a ship - the Canadian Pacific liner Empress of England. (From Decca Navigator News July 1957)

EXPANSION

The development of the Dectra System on the North Atlantic was taken to a further stage in June 1967 with the opening of an additional Ranging Station in Iceland. With transmissions from the existing Ranging Stations in Canada and Scotland, the new station produced two new hyperbolic ranging patterns for use in the aircraft's navigation computer.

The decision to install an additional station in Iceland was taken to safeguard the system against extremes of adverse propagation effects, and is in alignment with modern concepts.  The installation and commissioning of the Icelandic Dectra Station was completed quickly despite the rigorous weather conditions which made the erection of the 300 ft antenna difficult. Transmissions commenced only 71 days after the date on which Her Majesty's Ambassador to Iceland approached the Icelandic Government on behalf of The Decca Navigator Company Limited.

This rapid progress was brought on by three factors:  the speed with which the Icelandic Government
agreed to the station's being located in Iceland; the wholehearted and efficient co-operation of the Icelandic Posts and Telegraph Administration and the prefabrication in the U. K. of the transmitter building with all its equipment.

The whole station, including the four diesel generating units, were designed to run on a semi-unattended basis estimated to be 7 to 10 man/hours per week. After an initial period of a few months, the Icelandic P.T.T. assumed responsibility for the operation of the equipment.

The U.K. Board of Trade, with the co-operation of the FAA, sponsored an operational evaluation of Dectra during the next 12 months. The Aeroplane and Armament Experimental Establishment, Boscombe Down, the FAA itself, and a number of major airlines also took part.

The first airline to standardize its navigation on Decca/ Dectra was be Seaboard World Airlines Inc., the only all-cargo airline operating on the North Atlantic air routes at the time. The decision to install Decca was taken after a comprehensive program of flight evaluation and analysis of all the alternative systems available. At the conclusion of these tests, Seaboard World Airlines said of Decca/ Dectra "the system goes further than any other navigation system in providing effective and accurate cockpit automatic operation".

Dectra is also under evaluation by BOAC who equipped a Super VC-10 with one of the most comprehensive navigation systems ever installed. The system was based on a Decca Omnitrac computer which accepts inputs from Dectra 2, Decca, Doppler, Loran C, VOR/DME, and Air Data. The Omnitrac computer operated a pictorial map display and digital readout of Time, ETA, Distance and Bearing and also provided autopilot coupling.  BOAC evaluated the system for possible fleet installation.

Off-loading the operations building at Reykjavik .(From Decca Navigator News, October 1967)

Transporting the prefabricated and prefitted operations building from Reykjavik to site. (From Decca Navigator News, October 1967)

SUMMARY

At the time there was continuous Decca coverage down the Canadian East Coast as well as over the UK, and the scheme might have met with success had Decca been adopted for aircraft use.  Dectra was discontinued in the late 1960's when inertial navigation systems became the standard airline long range navaid.

REFERENCES:

1) The Journal Of Navigation - Chapter 4. W.F. Blanchard, Royal Institute of
    Navigation; Vol 44, No. 3; Sept 1991. Used with permission.

2) The Dectra System For Long-Range Air Navigation. The Decca Navigator Company
    Ltd. Issue 3 - February 1955.

3) Decca Navigator News , October 1967. Dectra in Iceland

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