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Aviation History
1946
1946 - 1357.PDF
JULY IITH, 194c FLIGHT Approach by Ground Control The Q.C.A. System Described : "Search" and "Precision" Indi- cators : Talking the Pilot Down THE G.C.A. (Ground-controlled Approach) system ofblind approach is a radar navigational aid which wasoriginally developed by the Massachusetts Institute of Technology early in 1940 and is in extensive use in many parts of the world. At the moment nine R.A.F. airfields are equipped with it: they include those at Prest- wick, Manston, Lyneham, St. Eval and Waddington. It differs from all other approach systems in requiring no special equipment in the aircraft (apart from normal H.F. or V.H.F. R/T), and landings have been made in conditions of zero visibility by pilots who have had no pre- vious training and, in- deed, have never even previously heard of the system. Obviously, if a Controller on the ground can be given exact and continuous information concerning an aircraft's position, he should be able to "talk the pilot down" to an airfield by giving him the correct courses and rates of descent. This is not particularly difficult once the correct technique has been worked out—a sample of the R/T "patter" is given later—and any reluctance on the part of a pilot to obey a disembodied voice is easily overcome once he has made a few practice approaches in conditions of good visibility. G.C.A. Controllers are chosen for what might be called their "bedside manner," and any pilot who has used the system operationally will testify to the beneficial effect of a calm, confident voice telling him not only where he is and what he should do next, but even reminding him of his cockpit drill at the correct moments. The G.C.A. equipment is fully mobile and consists of a large trailer towed by a six-wheeled truck. The truck contains the generating plant and the prime-mover, while the trailer houses the radar gear and its operators. It is possible to change runways in under thirty minutes/and \- The G.C.A. trailer which housesthe radar gear and its operators. This trailer, with its generatingplant, is placed some 375ft. from the centreline of the runway, andat an angle of 85 degrees to it. the equipment is normally sited about a mile from the touchdown point and three to four hundred feet to the left of the runway from the point of view of the pilot. The Two Stages An actual G.C.A. landing is made in two stages. Since an aircraft may arrive at an airfield from any direction, it is first necessary to put it into the correct position rela- tive to the runway before the approach can be started. The G.C.A. equipment is therefore fitted with a "search " radar system, using two Plan Position Indicator tubes which give a 360-degree picture of everything within thirty miles of the airfield. When an aircraft has asked for assistance, its echo is identified on the Plan Position In- dicator tube (by D/F or other means) and it is directed by R/T into a circuit which will bring it to the correct position for the approach. This part of the operation is not unlike wartime G.C.I., and was, indeed, evolved from it. Two P.P.I.s are provided so that large numbers of air- 3 CUDE PATH Diagrammatic representations of the sections of the radar beams used in the G.C.A. system. Above is the search "lobe," forwhich two plan position indicator tubes are used to give a 360 degree picture of the area within thirty miles of the airfield. On the right is a detail diagram of the "precision " part of the system, which is used to bring the aircraft in on to the runway.
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