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Aviation History
1956
1956 - 0386.PDF
384 FLIGHT, 6 April 1956 DEVELOPMENTS IN DECCA Recent Progress Described in Lectures at the Institute of Navigation LAST week we briefly summarized three papers given in Lon-don on March 21st under the auspices of the Institute ofJ Navigation. The general heading of the Symposium was Developments of the Decca Navigator System, and the first paper,Experiences with Decca on Viscount Operations in Europe, by Capt. E. R. Wright, a senior pilot of B.E.A.'s Viscount Flight,dealt with the practical handling of the system. The lecturer took his audience on a European flight from London Airport with theaid of actual Flight Log chart illustrations, emphasizing the sim- plicity with which the trained pilot changed his charts whennecessary, the accuracy which ground-speed checks with Decca imparted to a predicted E.T.A., and the aid the airline pilotderived from the system as a whole. We hope to give abstracts from this paper in a subsequent issue. The developments and forthcoming trials of Dectra were dis-cussed by Mr. H. G. Hawker (Decca Navigator Co.) in his paper entitled The Development of Dectra as a Long-range NavigationalAid. He recalled that Dectra was one of the two British navaids [Delrac, another Decca proposal, was the second] recommendedfor evaluation at the March 1954 meeting of the I.C.A.O. Com- munications Division. Later this year, or possibly early in 1957,the system is to be tested under M.o.S. and M.T.C.A. auspices with the support of the Canadian Department of Transport. The speaker then outlined the principles of Dectra, whichused two master/slave pairs of ground stations, one pair at each end of the route to be covered. Each pair produced, by methodsbroadly similar to those of the basic Decca system, a hyperbolic pattern giving "tracking" information which spread out fanwisefrom the midpoint of the pair and thus gave increasing accuracy as the terminal area was approached. A "ranging" pattern, whoselanes crossed the route at regular intervals and gave distance-to-go along it, was generated by the two master stations, this facilitybeing supplemented by a means of determining independently the distance from the aircraft to one of the masters. Information obtained by the Dectra system would be displayedto the pilot by means of the familiar Flight Log, and slides were shown depicting typical Flight Log charts for the en route andapproach phases of the north Atlantic crossing. Dectra and the Decca System A point emphasized by Mr. Hawker was the "integration" ofDectra with the basic Decca Navigator system, not only so far as the receiver (which would be of the new Mk 10 type) and theFlight Log were concerned, but also in relation to the ground stations. The master and the "purple" slave station of the NorthBritish Decca chain, respectively at Kidsdale and a site near Stirling, would form the eastern Dectra pair. The power of thestations would be raised to 5 kW. The actual Decca transmissions would provide the Dectra service, although for the latter thepurple slave signal would act as the master. This signal was continuous except during lane identification, when it was switchedoff and a purple transmission of identical frequency was radiated momentarily from the master; thus the Dectra tracking patternwould be produced by phase-comparing master and slave signals at their carrier frequencies. The speaker added that this purplefrequency, being at the low end of the Decca band, was particu- larly suitable for long-range transmission. The western pair of stations would be at Comfort Cove (master)on the north coast of Newfoundland and Thornburn Lake (slave), five miles south of Port Blandford. It was remarked that thesesites in Newfoundland were in ideal positions to form part of a Decca chain covering eastern Newfoundland, requiring only theaddition of red and green slave stations. In conclusion, the lec- turer referred to the large amount of practical data it was hopedthe trials would provide, mentioning as a specific point of inves- tigation the question of whether the proposed radiated power ofthe transmissions would prove adequate for 24-hour use all the year round. The Development of Decca for Helicopter Operations, by Mr.J. G. Adam, was read by Mr. G. Hinchliffe (Decca Navigator Co.). Speaking of the coverage of the present Decca Navigator servicein relation to the needs of helicopter operators, Mr. Hinchliffe said that so far as the U.K. was concerned the Navigator system pro-vided solid area coverage over the entire country with the excep- tion of the northern tip of Scotland and the western side ofIreland. In this area the fixing accuracy was better than li nauti- cal miles on 95 per cent of occasions at all times of the day andnight throughout the year. Within the next few months the opening of the North Scottish chain would extend this cover north- wards to beyond the Shetland Isles. Similar coverage was avail-able over much of western Europe. It would seem that this accuracy would be adequate for the en route phase of helicopterflight between cities. It must be emphasized, however, that the accuracy figure quoted was the worst error which would be met atthe edge of the area having mis accuracy rating; throughout most of that area the fixing accuracy was very much better. If the Flight Log were used, the relatively high-speed patternfluctuations could be visually integrated so that only the slow drift remained. In these circumstances the 75 per cent probabilityfigure could generally be taken as the fixing accuracy which would be achieved. In the London area the system was capable of givinga fixing accuracy of better than ±100 yd. Where the accuracy available from existing Decca chains wasnot sufficient for A.T.C. or final approach, said the lecturer, the requisite high-accuracy local coverage could be supplied by a two-slave chain of low power costing about £10,000. With baselines five n.m. long at an angle of 120 deg the accuracy would be betterthan ±50ft over an area of some 20 square miles. Automatic equipment enabling the stations to run unaided would be used. All the error figures quoted referred to random variations in theposition-lines, and these did not contribute towards the collision risk when two Decca-equipped aircraft were in close proximity,since the two receivers would show the same error with respect to the ground. The main errors to be considered were those arisingfrom incorrect adjustment of the receivers or from the inability of the pilot to fly the desired track. The Flight Log's pictorial pre-sentation would seem to be the best solution to the helicopter- navigation problem, particularly when close lateral and longi-tudinal separation were involved. The total installed weight of the equipment (receiver plus Flight Log) was 40-100 lb. Referring to developments now in progress, the speakerdescribed a new technique whereby a route could be plotted on a Flight Log chart in the form of a long roll, any changes in patternalong the route being effected automatically. A self-setting Flight Log was under longer-term development, and here the pilot wouldhave only to select the required chart and move the pen to within ± lin of the correct position, the pen thereafter positioning itselfautomatically on the correct spot within ±0.01in. Complete auto- maticity was possible but considerations of weight and equipment-complexity led to the conclusion that it was better to compromise. Both these developments would permit an accuracy of track-keep-ing in conformity with the I.A.T.A. requirement of ± 500ft at 95 per cent probability. In conclusion, the lecturer referred to pre-sent investigations into the use of Decca for final approach. For this special lattice or Flight Log charts were employed showingthe heights at which the aircraft should be at successive points along the approach path. Points from the Discussion . .. • • The chairman, Capt. A. M. A. Majendie, then threw the meet-ing open to discussion. The first question concerned the varia- tions in accuracy by day and night and in winter and summerof the Decca system. The questioner asked if trials of Dectra would be carried out by night as well as day, and inquired howthe accuracy of the system could be measured, where no yardstick was available for checking. In reply, Mr. Hawker said that so faras the tracking accuracy was concerned a lot of evidence had been derived from die Danish chain by ships travelling to Iceland.It was known that the stability of the tracking lines would be very high, even in winter. It was expected that not more thantwo or three revolutions of the ranging Decometer would be lost in the course of a transatlantic flight, and the accuracy predictionstook account of this. Another question concerning Dectra referred to possible co-operation with Atlantic weather ships and U.S. Coastguard ser- vices. Mr. Hawker replied that co-operation did indeed exist andmuch help was expected from these organizations. It was hoped to place receivers on the weather ships. Capt. Wright was asked if he made use of aids other than Deccaon Viscount flights. An airline pilot, he replied, would always use all the available aids, if they were serviceable—that was theway to preclude errors. However, if he had to choose to fly on one aid, he would choose Decca. Finally, Mr. Hawker was asked for information as to how staticelectricity might affect the Dectra system. He said that diis prob- lem was very prominent in the forthcoming trials, but theextremely narrow band width employed was a major advantage in this connection.
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