Ground-Controlled Approach---Its Development and Early Operational Use

Abstract

Ground-Controlled Approach (GCA) was developed to meet military requirements for a "universal" blind-landing system, needing no additional airborne equipment or pilot indoctrination beyond short-range voice radio and basic instrument training. The talk-down principle, previously discredited in favor of beam-approach systems, was proved feasible when accurate information was available. The requirements of rapid radar scanning, at low angles to the horizon, with a high degree of accuracy at short range, were met by linear dipole arrays at 3-cm wavelength, with mechanical variation of waveguide width varying the angle of radiated power to the array. Half-power beamwidth for the elevation antenna was 0.1°; for the azimuth antenna, 0.8°. To translate range/angle data into a linear relationship of range vs deviation in feet from a selected approach path, a new type of indication was developed, i.e., Expanded Partial PPI Indicator (EPI). This permitted direct interpretation and immediate transmission of the data to the pilot by the controller. GCA's wartime service record, beginning late in 1944, proved its effectiveness under all conditions and led to further modifications and simplification for postwar commercial as well as military use.