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Aviation History
1947
1947 - 0395.PDF
MARCH 2OTH, 1947 aircraft equipment beyond efficient tele-phone facilities. Airlines had hesitated to accept the principle that in normalconditions substantial responsibility for the safety of an aircraft during instru-ment approach should be transferred to a human agency on the ground. It couldbe argued, of course, that G.C.A. reduced the load on the pilot. This was per-tinent, but the present trend of develop- ment aimed at reducing the pilot'sburden, and pointed towards automatic flight control. G.C.A. appeared likely tobe junior partner to the SCS-51 type of instrument-approach system which hadbeen named as a primary international standard. Short-distance Aids •Ljn densely flown areas the short-ttisiance aids were integral components of any scheme of traffic control. Duringinstrument weather their co-ordinated use with final approach aids was essen-tial for orderly traffic handling. Principal aims of traffic control wereto help aircraft navigate in safety and to feed them smoothly through the '' gate ''to the landing runway at regular intervals governed by the airfield capacity.Ideally, the smoothest traffic flow would result by. fitting all take-offs and landingsin a given area into a cohesive plan ol movement. That was the only way olminimizing time spent in the air. Other- wise arrivals would always bunch at cer-tain times and might build-up to the point where the capacity of an airfield toabsorb them was temporarily exceeded. In these circumstances a regular landingrate could be preserved only by causing aircraft to lose time in holding patterns.These "air sidings" were essential, also, to cater for such possibilities as missedapproaches, a blocked runway or emer- gency landing. Short-distance aids must be air inter-preted for adequate traffic-handling capacity, accurate for proper checks ofposition and ground speed, and, above all, simple. Radar aids in the short-distance group were D.M.E., Gee and A.C.R. D.M.E. (Distance-measuring Equipment) The object of D.M.E. was to give adirect indication of the distance from a ground responder beacon cf known posi-tion, usually at airfields. It would give a pilot an accurate check on E.T.A. overthe last stage of his flight, thus helping to ensure that his movement fitted pro-perly into the traffic flow. Range would be governed by line-of-sight perform-ance—i.e., up to 100 miles depending on height. The airborne equipment consisted of a«-power transmitter and a receiver d either with separate stub aerials or FLIGHT 243 Interior of G.C.A. mobile operations room. Numbered positions are : (1) TrafficDirector ; (2) Plane Selector ; (3) Azimuth Tracker ; (4) Controller ; (5 Elevation Tracker. with a common T/R arrangement. Thedistance was indicated on a circular scale meter fitted to the instrument panel, andeach beacon was identified by a special code communicated to the pilot by ablinking light or by oral signals. D.M.E. could be made light and com-pact. The overall system had been selected by P.I.C.A.O. topartner the V.H.F. con- tinuous - wave omni - direc-tional radio range as the standard combination ofshort-distance aids for future use on main trunk routes.Before D.M.E. could start its career one difficulty hadto be overcome—the oper- ating wavelengths must bestandardized, but Great Britain and the U.S.A., whowere likely to be the two largest users, strongly dis-agreed over what would be the most suitable allocationof wavelengths. Gee (Position-fixing System) The broad principles and purpose ofGee will be familiar to most readers, and Mr. Willis' remarks on its qualities wereas follows: For the proper operation of Gee, careful synchronization between master and slave transmissions -vas essen-tial. This called normally f^r skilled human monitoring. The range of Geevaried with height, but was about 450 statute miles at 15,000ft. The accuracyof fix varied with the position of the air- craft, but could be as high as one-third ofa square mile. AIRCRAFT RESPONSE MAIN PULSE SECTOR TO \ AIRCRAFT ABOUT BE EXPANDED \ TO LAND 3° GLIDE PATH STROBE BRILLIANCE HILLECHO FROM MOUNTAIN G.C.A. displays. (.Left) Search display. (Right) Composite precision display- 3MI 1OOFT ABOVE GROUND LEVEL 5MI CENTRE TRACK (Centre) Azimuth precision display, a recently evolved improvement. Association of distance and localizer glide pathindicators for D.M.E. Improvements to Gee have aimed at re-ducing operating costs and simplifying operation. Ground equipment havingfully automatic monitoring was being de- veloped and the original airborne Mark IIset would be superseded eventually by a smaller and lighter model designed toallow cockpit installation. This Mark lit version embodied many refinements, in-cluding a daylight viewing cathode-ray tube, semi-automatic counting of Geeunits and a left/right meter for flying down selected lattice lines. The Gee system, owing to its highaccuracy and the tracking flexibility given by the lattice pattern, had beenproposed as a basis for a scheme of traffic control in Europe where there was a widediversity of routes and traffic. The use of Gee in Europe was an example of ap-plication to special regional problems, and there were no immediate proposalsfor using it in other parts of the world.
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