Ground-controlled interception

Ground-controlled interception (GCI) is an air defence tactic whereby one or more radar stations or other observational stations are linked to a command communications centre which guides interceptor aircraft to an airborne target. This tactic was pioneered during World War I by the London Air Defence Area organization, which became the Royal Air Force's Dowding system in World War II, the first national-scale system. The Luftwaffe introduced similar systems during the war, but most other combatants did not suffer the same threat of air attack and did not develop complex systems like these until the Cold War era.

Today the term GCI refers to the style of battle direction, but during WWII it also referred to the radars themselves. 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. Previous systems, notably Chain Home (CH), could only be directed along angles in front of the antennas, and were unable to direct traffic once it passed behind their shore-side locations. 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. The US' SAGE system was perhaps the most complex attempted, using building-filling computers linked to dozens of radars and other sensors to automate the entire task of identifying an enemy aircraft's track and directing interceptor aircraft or surface-to-air missiles against it. In some cases, SAGE sent commands directly to the aircraft's autopilot, bringing the aircraft within attack range entirely under computer control.