Ground-controlled interception (GCI) an air defense tactic whereby one or more radar stations are linked to a command communications centre which guides interceptor aircraft to an airborne target. This tactic was pioneered during World War II by the Royal Air Force with the Luftwaffe to follow closely. Today, GCI is still important for most nations, although Airborne Early Warning and Control, with or without support from GCI, tends to be more capable and flexible.
World War II
In the original RAF system, information from the various Chain Home coastal radar stations was relayed by phone to a number of operators on the "ground floor" of the interception direction center. They used the information to move markers on a large operational map, representing both the enemy intruder formations and own aircraft. Radio operators were located on a balcony overlooking the map, and relayed instructions directly to a particular squadron, or more typically, a remotely located station relaying information to a group of aircraft. Above them were status boards, consisting of a series of lights showing the current status of a particular squadron, on the ground, in battle, returning, etc. Overall direction of the battle was directed by commanders who thus had "instant access" to a picture of the battle as a whole.
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 Royal 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. Accordingly, the Royal Air Force established during 1941 a network of inland GCI radar stations, RAF Sopley, Sturminster Marshall, RAF Durrington, RAF Willesborough, RAF Waldringfield, RAF Orby, and RAF Avebury, whose task was to direct searchlights onto intruders, and also to guide interceptor nightfighters. This network did not become fully functional till after May 1941, when the main German night-time air assault against Britain was discontinued. During World War II, airborne radars were so primitive that the defending aircraft needed to close to within what would be easily visual distance during daytime. GCI was often used to vector the defending night fighters very close to the intruders and they then crept up on the often unsuspecting aircraft. GCI and night fighters eventually made night sorties over western Europe significantly more risky for bomber crews than they were at the beginning of the war (when they had relative impunity).
More recently, in both the Korean and Vietnam Wars the North Koreans and North Vietnamese had important GCI systems which helped them harass the opposing forces (although in both cases due to the superiority in the number of US planes the effect was eventually minimised). GCI was important to the US and allied forces during these conflicts also, although not so much as for their opponents.
The most advanced GCI system deployed to date was the US's Semi Automatic Ground Environment (SAGE) system. 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. The information was then displayed on terminals in the building, allowing operators to pick defensive assets (fighters and missiles) to be directed onto the target simply by selecting them on the terminal. Messages would then automatically be routed back out via teleprinter with instructions on them.
The system was later upgraded to relay directional information directly to the autopilots of the interceptor aircraft like the F-106 Delta Dart. The pilot was tasked primarily with getting the aircraft into the air (and back), and then flying in a parking orbit until called for. 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.
GCI is typically augmented with the presence of extremely large early warning radar arrays, which could alert GCI to inbound hostile aircraft hours before they arrive, giving enough time to prepare and launch aircraft and set them up for an intercept either using their own radars or with the assistance of regular radar stations once the bogeys approach their coverage. An example of this type of system is Australia's Jindalee over-the-horizon radar. Such radars typically operate by bouncing their signal off layers in the atmosphere.
Airborne Early Warning and Control
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. AEW&C tends to be superior in that, being airborne and being able to look down, it can see targets fairly far away at low level, as long as it can pick them out from the ground clutter. AEW&C aircraft are extremely expensive, however, and generally require aircraft to be dedicated to protecting them. A combination of both techniques is really ideal, but GCI is typically only available in the defence of one's homeland, rather than in expeditionary types of battles.
The strengths of GCI are that it can cover far more airspace than AEW&C without costing as much and areas that otherwise would be blind-spots for AEW&C can be covered by cleverly placed radar stations. AEW&C also relies on aircraft which may require defence and a few aircraft are more vulnerable than many ground-based radar stations. If a single AEW&C aircraft is shot down or otherwise taken out of the picture, there will be a serious gap in air defence until another can replace it, where in the case of GCI, many radar stations would have to be taken off the air before it became a serious problem. In both cases a strike on a command center could be very serious.
Either GCI or AEW&C can be used to give defending aircraft a major advantage during the actual interception by allowing them to sneak up on enemy aircraft without giving themselves away by using their own radar sets. Typically, to perform an interception by themselves beyond visual range, the aircraft would have to search the sky for intruders with their radars, the energy from which might be noticed by the intruder's radar warning receiver (RWR) electronics, thus alerting the intruders that they may be coming under attack. 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. Alternatively, they could turn their radars on at the final moment, so that they can get a radar lock and guide their missiles. This greatly increases the interceptor's chance of success and survival.
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