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Aviation History
1955
1955 - 0703.PDF
FLIGHT, 20 May 1955 Civil Aviation 701 DECCA EVALUATED Findings of the 1951-53 M.T.C.A. Trials—and a Decca Postscript ISSUED recently by the Ministry of Transport and CivilAviation, An Evaluation of the Decca Navigator Systemis nothing if not comprehensive. It covers—in 298 pages of words, photographs, charts, drawings, diagrams and tables —an extensive programme of study and trials made during 1951,1952 and 1953. This programme was itself the outcome of a recommendation by the Communications Division of I.C.A.O. The report was prepared by the Civil Aviation Navigational Services Group of the Ministry (R. L. Gillham; A. Coates, M.A.; F. Kitchen, B.Sc; G. H. Trow, D.F.C.; D. D. Lipman, D.F.C.; H. Keeling; and E. Jenkins, D.F.C., D.F.M.). Its subject is a controversial one, and it may be that some of its findings will be questioned both by protagonists and by opponents of the Decca system. The only cause for complaint by the neutral observer, however, appears to be the length of the interval between conclusion of the trials and publication of the report. As might be expected, certain changes have been made in the Decca equipment during that period, and these changes are described in the Decca company's official comment on the M.T.C.A. report. Both report and comment are briefly summarized below. The Ministry's evaluation begins with a technical descriptionof the Decca system, in which basic principles, ground equipment and airborne equipment are described in detail. The variousfactors which influence the performance of the system in practice, and so cause departures from the theoretically postulated perform-ance, are next discussed. Examples of such factors are phenomena associated with the propagation of low-frequency radio waves andthe effects of radio-frequency noise on the operation of the system. The operational part of the report begins by describing eachphase of the flight-trials programme, in which the standard of E.T.A.-keeping, track-keeping, ease and accuracy of flying ter-minal-area procedures, and other performance characteristics of various items of Decca equipment were examined. The last twosections of the evaluation give the M.T.C.A. conclusions on the operational capabilities and limitations of the system, and anassessment of the extent to which the system meets the functional requirements of a short-range navigation aid. Among the conclusions quoted in the evaluation are thefollowing: — "Cover: The ultimate range of a chain is governed by theeffects of errors introduced by skywaves, but within this limitation the useful cover naturally depends upon the operational use to bemade of the system. Although a chain is usable at a range of at least 250 n.m. from its Master station at night, and at ranges wellin excess of this in the superior conditions of daytime, a further limitation must be applied if very high reliability is requiredwhen the system is used as an aid to Air Traffic Control. The cover given by a chain should men be considered as being limitedto the reliable range of lane identification, and at night this also happens to coincide broadly with the range beyond which accuracyand Flight Log chart scales deteriorate (150-200 n.m.) for the more elaborate uses of the system. Cover has been checked up to30-40,000ft during trials. "Operational accuracy: The basic accuracy of the system in theair has been found to be better than that predicted for ground level, and the improvement may be of the order of 20 per cent."The 95 per cent operational accuracy of track-keeping with the Flight Log at various ranges from the Master station of atypical Decca chain when in front cover, using optimum chart scales, under night conditions, and following the worst directionof tracking in relation to basic accuracy is:— ±1 n.m. at 120 n.m. range±2 n.m. at 200 n.m. range ±3 n.m. at 250 n.m. range In all other directions of tracking considerably better accuracycan be achieved by night, and the accuracy is gready improved in all tracking directions by day, particularly at the longer ranges. "The 95 per cent accuracy of E.T.A.-keeping with the FlightLog, even for a slow aircraft operating at night at a range of up to about 200 n.m. from the Master station of a Decca chain, isbetter than:— (a) ± 3 minutes when no attempt is made to meet E.T.A. byadjustment of airspeed, and (b) ± 2 minutes when the pilot attempts to meet E.T.A. byadjustment of airspeed within a range of ± 10 per cent, provided the change in wind component along track is less than10 per cent of the ground speed on 95 per cent of occasions. "Use as an aid to Air Traffic Control: Improved utilization ofairspaces and aerodromes may be achieved by use of the Flight Log, as follows:— (1) Up to approximately 200 n.m.(a) Ensuring that an aircraft stays within an airway 10 n.m. wide. (b) Reducing present longitudinal separation standardssignificantly. (c) Descent through cloud.(d) Holding an aircraft continuously within 8 n.m. of a desired point.(2) Up to approximately 150 n.m. (a) Parallel track flying within an airway 10 n.m. wide. (b) Aerodrome approach and feeding on to a runway approachaid to facilitate 'straight in' approaches. (c) Holding an aircraft continuously within 4 n.m. of a desiredpoint in such a manner that the pilot knows his position in the holding pattern at all times.(d) Sequencing aircraft for rapid landings, nominally at 3 minute intervals, when used with an aid to approach andlanding, without requiring detailed guidance from Air Traffic Control."Frequency problems: The problems likely to arise in frequency planning fall into two main categories:—"(1) The number of frequencies required to cover a given area or air route is obviously dependent upon the area or route coverto be expected in practice from one chain. The evaluation shows that the reliable range of a chain may be 150-200 n.m. when usingDecca for Air Traffic Control purposes at night, and therefore more frequencies may be required than were originally estimated."(2) The second consideration is that of the range at which it is possible to repeat frequencies, and, although theoretical con-siderations show this range to be of the order of approximately 1,500 n.m., no practical confirmation is so far available."Serviceability: Serviceability of Decca chains has attained a very high standard. Several types of airborne equipment havebeen used during the course of the evaluation. Various models of receiver and Flight Log have been used, and changes in aerialsystems and equipment layout have been made. However, the trials have been chiefly carried out using prototype equipment andserviceability has not reached the high level to be expected from production models."Vulnerability to precipitation static: Theoretical evaluation indicates that with the latest equipment, properly designed andpositioned aerials, and satisfactory installations, reliability should not be gready affected by precipitation static. However, in theseries of trials for this evaluation, it has not been possible to con- firm theoretical estimates of the degree of reliability, and it maywell be that full evidence on this problem can only be obtained after the system has been in use by air operators for a considerableperiod. "Terrain effects: During tests carried out for this evaluation,terrain effects within the normal system coverage did not appear to affect reliability and contributed little to errors of the system."Performance in high-speed, high-altitude aircraft and in heli- copters: Tests have been carried out using Decca in aircraft flyingat ground speeds of 400 kt, and altitudes of between 30,000 and 40,000ft, and also in helicopters flying at very low altitudes. Noundue difficulties have been encountered, and the tests showed that the Decca system was quite satisfactory for these operations."Operation of the equipment: The trials for this evaluation have shown that manipulation in the air of the latest type of receiverand Flight Log can be accomplished without undue difficulty, under normal conditions, once a certain degree of skill has beenacquired. "Extent to which the system meets the functional requirementsof a short-range navigation aid: The Decca system almost com- pletely meets the functional requirements in providing theaccuracy needed for optimum longitudinal and lateral separation of aircraft and maximum utilization of airspace; in providing cover
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