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Aviation History
1959
1959 - 0536.PDF
FLIGHT, 20 February 1959 FLYING AIDS ... 263 Automatic capture and following of a selected VOR radial and anI.L.S. localizer beam has made the autopilot-controlled approach down to heights of about 200ft a routine procedure. Distance measuring using an interrogator/responder techniquewas fir,st developed as a military aid and later applied to civil jet operations. Both Ultra and Murphy in Britain have producedD.M.E. under the military code-name Rebecca and the Comets which first flew with B.O.A.C. used the system in order to judgethe right point at which to begin the let-down from cruising altitude. Rebecca Mk 8 has been developed to the stage whereit is accurate enough for pilot-interpreted approaches in bad weather down to minima which compare favourably with thoseallowed under I.L.S. or G.C.A. guidance. Another type of D.M.E. was extensively installed in Australia for civil use in conjunctionwith VOR and proved both popular and useful in traffic control. But it is officially stated that the VOR/DME combination isnot considered suitable for traffic control purposes in high-density traffic in Australia. VOR and DME require separate receivers and aerials, but it ispossible to perform both the bearing and distance measurement in a single equipment. In order to effect this, and also to provide abeacon system which could be operated from ships and sub- marines, the U.S. Navy some years ago produced Tacan. TheU.S.A.F. also adopted Tacan and the system was then proposed as a new joint civil/military aid to repiace VOR. Vigorousopposition came from the civil agencies, but a compromise was adopted when the A.T.A., who proved to have the casting vote,was looking for a distance-measuring system to be available by 1959 for the big jet transports. The compromise was that VOR beacons should be co-locatedwith Tacan so that military aircraft could use Tacan while civil operators continued to derive bearing information from VORand used the distance-measuring element of Tacan. The DME portion of Tacan, known as DMET, is at this moment beingproposed as an international standard aid to complete the original VOR/DME requirement. Tacan has been standardized in NATOair forces and is being gradually introduced into service. Installa- tion of Vortac is also proceeding slowly in the U.S., with a totalof about 1,300 Vortacs planned for 1962. The sum invested in the system is variously reported as between $500 million and$800 million. Tacan beacons have a range of about 200 nautical miles in theline of sight and bearing accuracy considerably better than that of VOR. Beacon transmitter frequencies are in the U.H.F. bandbetween 962 and 1.024 Mc/s and between 1,151 and 1,214 Mc/s, with the responder frequencies for the DMET element lyingbetween the two. Any number of aircraft may derive bearing information simultaneously, and up to 120 can interrogate theDME responder at a time. Any one of 126 channels may be selected by rotary switches in the cockpit and both distance andbearing information are presented (in the Standard Telephones version) on one dial in the form of meter and veeder indications.Tacan was originally developed by International Telephone and Telegraph Corp and is manufactured in this country by StandardTelephones and Cables, Ltd. Complete test equipment is made in this country by G. and E. Bradley, Ltd. The American armed services now regard Tacan as an interimnavigational aid, intending apparently to introduce other systems as they become available. Such rapid replacement is not feasiblein civil operations and the declared intention is to make the utmost use of Vortac by adding further refinements in future,and to keep it in being as the basic American aid for many years to come. Whatever aid replaces VOR as the I.C.A.O. standard, Self-setting Decca Flight Log showing the digital tracks and photo- sensitive scanning unit beside the chart The navigator's position in the B-58 Hustler, showing the controls tor the Sperry stellar-inertial bombing and navigation system (see p. 266) Vortac is to be maintained in the U.S. The colossal investment isnot lightly to be thrown away. The Jet Operations Requirements Panel of I.C.A.O., theAmerican Curtis report in its Phase 2 recommendations, the International Federation of Airline Pilots Associations and theBritish Ministry of Transport and Civil Aviation have individu- ally stated that a point-source aid of the VOR or Vortac type willnot allow the necessary flexibility or accuracy required for trafnc- control purposes and that an area-coverage system is considerednecessary. Three of them also stated a requirement for pictorial indication in the cockpit. Each body made its own detailedinvestigation of the requirements. The reasoning which led the M.T.C.A. to favour an area-coverage system was recorded inFlight for January 23. There is a sharp division of opinion between America andmuch of the rest of the world on what a new navigational aid should be. The whole question is at this moment being thrashedout at a technical meeting of I.C.A.O. in Montreal. It would be rash to predict either a clear-cut solution or one based purelyon technical considerations. Much more is involved. In recording the general considerations from the technicalpoint of view it is well worth summarizing the United Kingdom views on the situation if only by way of establishing a smallmonument. Britain, of course, supports the Decca Navigator system which is still the only area-Coverage system to have beenextensively used in airline operations and for which a consider- able coverage of ground stations already exists. It should beremembered that it is the Decca Mk 10 system on which the Ministry bases its confidence.The M.T.C.A. notes that navigation requirements are today dictated by traffic control requirements and infers that theyshould not be dominated by military thinking which was origin- ally responsib'e for VOR and Tacan. Both the capital andmaintenance costs of these systems is in any case unnecessarily high and DMET is more complex than an equivalent designedspecially for civil use. Vertical separation alone is unsuitable for jet airliners becauseof the severe economic penalties imposed by operating at other than the ideal altitude. The ability to enforce lateral separationis therefore an important requirement for traffic control. The M.T.C.A. is convinced that the real requirement is foran aid which will:— (a) reduce lateral as well as longitudinal separation minima;(b) enable greater use to be made of lateral separation in order to provide the greatest possible freedom in the verticalplane; (c) be capable of serving any air-route configuration withoutthe costly, lengthy and difficult process of redeployment of ground facilities; (d) give the required quality of short-distance navigationalservice over sea areas; (e) permit flexibility in the siting of holding patterns andeconomy in their size; (f) enable pilots to make smooth and accurate approachesto both instrument and non-instrument runways; (g) provide accurate navigational information to helicoptersas well as to high-flying turbojet aircraft; (h) provide the pilot with a continuous and accurate presenta-tion of his position in a manner which will enable him to follow the desired track during all stages of the flight;
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