Ground-controlled interception

Specifically, the term was used to describe a new generation of radars that spun on their vertical axis in order to provide a complete 360 degree view of the sky around the station.

GCI radars began to replace CH starting in 1941/42, allowing a single station to control the entire battle from early detection to directing the fighters to intercept.

GCI systems grew in size and sophistication during the post-war era, in response to the overwhelming threat of nuclear attack.

Here the information from the radar was combined with reports from the Royal Observer Corps and radio direction finding systems and merged to produce a single set of "tracks", identified by number.

Because the Chain Home radar stations faced out to sea, once airborne intruders had crossed the British coast they could no longer be tracked by radar; and accordingly the interception direction centres relied on visual and aural sightings of the Observer Corps for continually updated information on the location and heading of enemy aircraft formations.

While this arrangement worked acceptably during the daylight raids of the Battle of Britain, subsequent bombing attacks of The Blitz demonstrated that such techniques were wholly inadequate for identifying and tracking aircraft at night.

SAGE used massive computers to combine reports sent in via teleprinter from the Pinetree Line and other radar networks to produce a picture of all of the air traffic in a particular sector's area.

When an interception mission started, the SAGE computers automatically flew the plane into range of the target, allowing the pilot to concentrate solely on operating the complex onboard radar.

This was upset by the introduction of the AMES Type 80 radar, which was originally intended as a very long-range early warning system for ROTOR but demonstrated its ability to control interceptions as well.

In the 1960s the Linesman/Mediator project looked to computerize the system in a fashion similar to SAGE, but was years late, significantly underpowered, and never operated properly.

In more recent years, GCI has been supplanted, or replaced outright, with the introduction of Airborne Early Warning and Control (AEW&C, often called AWACS) aircraft.

With GCI or AEW&C, the defending aircraft can be vectored to an interception course, perhaps sliding in on the intruder's tail position without being noticed, firing passive homing missiles and then turning away.