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Aviation History
1951
1951 - 0068.PDF
46 FLIGHT, 11 January 1951 The Decco Mark 6 airborne equipment is designed for S.BAC. standard racking. The later, Mark 7, equipment, which incorporates many refinements, is now under way. INTERNATIONAL NAVIGATION-AIDS ... base was at 700ft, and from that moment, until the ground was seen again at Farnborough, all navigation was done on the Flight Log. Once above cloud, at 6,000ft, the Smith autopilot was engaged, with the aircraft approximately on course, and thereafter corrections were made on the autopilot controller simply by watching the stylus trace. For instance, when the Flight Log showed the aircraft's position to be over Southampton and the track to be south of the direct course, the heading was changed northerly to suit By the time that the trace showed the aircraft to be flying over the south boundary of Farnborough airfield a visual fix could be obtained, and this corresponded exactly with the position shown by the Log. On landing, the trace showed the aircraft to be on the runway, and on all subsequent demonstration flights the stylus faithfully traced the orbit and the alignment of the flight path as the aircraft approached the runway, with after touch-down, the landing run, and the trace of the aircraft round the perimeter track route used by the demonstration aircraft. This report confirms the evidence of our own experience when, recently, we were invited to "sit in" during some holding-pattern trials conducted by the Ministry of Civil Aviation. The performance of the Flight Log is remark- able; as for presentation, the information is so clearly and unambiguously given that a child could understand it. The M.C.A. trials were extensive and exhaustive, and a report has recently been issued. It is enough to state here that the report, even judged by the most conservative standards, is extremely favourable. There now seems to be a reasonably firm likelihood that Display head of the Flight Log. As the aircraft flies, the chart unrolls and the cursor moves to leave a stylus-trace of the ground track. Decca will be adopted for Europe. If so, it is a sensible decision, in view of the fact that the English and Danish chains have been in operation for some time, a French chain is being built, the Swedish naval authorities are con- templating the erection of at least one chain, progress for the erection of a Spanish chain is well advanced, and the preliminary work for a German chain is well under way. It is, perhaps, too much to expect that the United States will adopt Decca; it would present an uncomfortably prickly political problem. But, as we see it, there is essentially little difference between the Americans manufacturing British gas turbines under licence for their own use and doing the same thing with a British navaid system. In the early days of the Decca Navigator system, many of the arguments advanced against it were technical and, let it be admitted, some of them were justified. The Decca Company, as well as being obstinate in their refusal to dis- continue the whole thing, had the "brains" systematically to rectify the technical shortcomings. The arguments, however, continued, changed in basis, it not in effect; and it is therefore worth while to list them, together with the facts which the Decca people can provide in answer. First, the matter of static—probably the most serious argument against the use of medium and low frequencies for aircraft navaids. A complete counter to the effects of precipitation static and the impact of charged droplets has been developed in the form of a specially shielded aerial, the design of which is such that, although the aerial measures only 30in overall, it has an efficiency as a signal collector comparable with that of a conventional long-wire system. Provision is also made for using the shield as a V.H.F. communication aeriaL Suppressed aerials which obviate aerodynamic penalty are now becoming fashionable, and aerials of this type are suitable for use with Decca. The other form of static—i.e., that associated with thunder- storm conditions, particularly in the tropics—is perhaps best referred to as atmospheric " noise." The effect of this form of interference is an inverse function of the transmission power of the ground station. It is certain that a Decca chain in the tropics would need a transmission power greater than that of a similar chain in a temperate region, but it should be remembered that neither the installation nor the operating costs increase in proportion to the requisite increase in power. The tests of Decca conducted in South Africa between October, 1949, and March, 1950—i.e., during the southern- hemisphere summer—by the South African Ministry of Transport and the South African Defence Forces are the subject of an informative report (reference ETR6), The Performance of the Decca Receiver in High Atmospheric Noise Levels, issued by the South African Council of Scientific and Industrial Research. The report makes it clear that, even with the relatively low power transmission used at the time (circa 20 watts) the "green" signals were affected scarcely at all, and the "red" signals suffered minor inter- ference. The "purple" signals were, however, very con- siderably affected. By increasing the transmission power, such interference could be reduced to negligible proportions but, in any case, the latest Decca Mark 7 airborne receiver is far less critical in "purple," and to some extent, therefore, inherently reduces the seriousness of atmospheric noise effects. The development of the new Mark 7 receiver has also obviated the speed limitation to which the Decca system was susceptible. Whereas in an aircraft flying along the "purple" base-line, where the lanes are but 385 yd wide, the earlier receivers could lose "purple" integration at speeds greater than 250 m.p.h., the latest receiver is reliable at speeds up to three times those at present contemplated for passenger aircraft. Another argument frequently levelled against Decca is its susceptibility to jamming in the event of war. This argument is, to some extent, specious, in that (a) the con- sideration is of civil aids, and (b) Decca is less easily jammable than its rival aids. To jam a long-wave (3,000- metre) system such as Decca requires a very large aerial installation with high-power transmitters generating con- tinuous waves exactly on frequency; in fact, it entails a
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