G.C.A.

The high, barbed-wire fence was small protection last week against the drift of fine snow across Indianapolis' icy airport. The fence was no protection against questions, either. But it served notice that CAA's Ground Controlled Approach (G.C.A.) equipment, housed in a chunky Army truck and trailer, was still a military secret.

Essential details of G.C.A. and its operation are available in the current issue of the Bendix Radio Engineer, a quarterly published by one of the equipment manufacturers.

G.C.A. is a fairly complicated radar system for guiding aircraft in for blind landings. It was a wartime development of the famed M.I.T. Radiation Laboratories. Army & Navy airforces accepted it in 1943.and used it to a limited extent in the last months of the war. G.C.A.'s big advantage over other blind landing systems: all the aircraft needs in the way of equipment is a standard two-way radio. The plane's position is figured out from the ground by radar; the pilot is "talked" down by a ground operator.

G.C.A. is made up of two microwave radar systems. The first is a rotating search beam: it is a modification of the two-dimensional radar used during the war to warn against the approach of enemy planes. Function of the search system is to find the approaching aircraft, and guide it through air traffic into the ten-mile range of the precision system.

The precision system is the heart of G.C.A. It consists of two radars which "scan" the approach area with fixed antennae. The elevation beam shows the plane's altitude and its rate of descent. The azimuth beam shows the direction of the plane's approach and its distance from the landing field. In both indicators, the plane's air track appears on calibrated error meters--with a hairline representing the ideal approach. On the basis of these meter readings, the pilot gets such verbal instructions as "Change course to 064 degree heading" or "Increase rate of descent to 150 feet per minute," etc., etc.

It takes five trained operators to run G.C.A. Two work on the rotating search beam--one as general policeman for all air traffic in the area and one to guide individual planes into precision range. The three precision system operators follow the plane's altitude, check its distance from the runway and keep up constant radio contact with the pilot.

CAA officials predict that G.C.A. will be a valuable aid to commercial and private flying--as soon as the new system is coordinated with other blind flying aids such as radio beams and airborne radar.

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