Ground-controlled Interception - World War II

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).