Ground Controlled Approach
Ground Controlled Approach (GCA) is a relatively obsolete aviation term that still has occasional specialized applications, especially with damaged aircraft. The classic example is when an aircraft needs to land in bad visibility.
For the particular aircraft and conditions, there is an optimal landing approach to a specific runway. Approach controllers will watch the aircraft on radar, and give commands to the pilot to bring him onto a course that, projected into the air, will align with the center of the runway.
Once the pilot is aligned on that course, control passes to a final controller (or sometimes a team of controllers), who give precise directions, in three dimensions, on how to land. Corrections may be on the level of "you are 5 degrees right of center" or you are "you are 20 feet below the glideslope." In some cases, the controller may order an approach be abandoned and retried, but, in any approach, there is a point at which the aircraft is committed to land. The controller will get the pilot to the point where the runway is visible, and say "take over and land manually".
For a variety of reasons, GCA is uncommon, just as human landing signal officers on aircraft carriers have been replaced for most operations. Automated instrument landing systems on aircraft have greatly improved. Few airport radars have the needed precision. The controller needs additional skills; sometimes a final controller is a pilot with additional radar training. GCA, however, sometimes is improvised, especially in military situations with a damaged aircraft or wounded pilot.