From AMS Glossary
(Commonly abbreviated GCA.) A system for the precise control of the approach and landing of aircraft by an operator on the ground.
Precision approach radar is employed to view the aircraft, and the radar operator "talks" the pilot down by giving him his position and suggested corrections for maintaining the "perfect approach." Currently, GCA is a required facility at an all-weather airport. If perfectly operated, this system would ensure safe landing under zero ceiling and visibility conditions; however, to provide a safety factor, GCA minimums are prescribed at nearly all installations.