Editing Ground-controlled interception (section)

==World War II==

{{Main article|Dowding system}}
[[Image:Battle of Britain operations room.JPG|thumb|right|The restored Operations Room in the underground bunker at [[RAF Uxbridge]] showing the map and plotters and with the [[Royal Air Force station|RAF Station]] names and readiness status boards on the wall behind. Also shown is the [[Sector clock]]]]

In the original [[Dowding system]] of fighter control, information from the [[Chain Home]] coastal [[radar]] stations was relayed by phone to a number of operators on the ground floor of the "filter room" at Fighter Command's headquarters at [[RAF Bentley Priory]]. 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. These tracks were then telephoned to the Group headquarters that would be responsible for dealing with that target. Group would assign fighter squadrons to the tracks, and phone the information to Section headquarters, who were in direct contact with the fighters. These fighter aircraft could then be "[[Scrambling (military)|scrambled]]" to intercept the aircraft.

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.

Experiments in addressing this problem started with manually directed radars being used as a sort of radio-searchlight, but this proved too difficult to use in practice. Another attempt was made by using a [[Height finder|height finding radar]] turned on its side in order to scan an arc in front of the station. This proved very workable, and was soon extended to covering a full 360 degrees by making minor changes to the support and bearing systems. Making a display system, the "[[Plan Position Indicator]]" (PPI), that displayed a 360 degree pattern proved surprisingly easy, and test systems were available by late 1940.

Starting in 1941 the RAF began deploying production models of the GCI radar, first with expedient solutions known as the [[AMES Type 8]], and then permanent stations based on the much larger [[AMES Type 7]]. Unlike the earlier system where radar data was forwarded by telephone and plotted on a map, GCI radars combined all of these functions into a single station. The PPI was in the form of a 2D top-down display showing both the targets and the intercepting [[night fighter]]s. Interceptions could be arranged directly from the display, without any need to forward the information over telephone links or similar. This not only greatly eased the task of arranging the interception, but greatly reduced the required manpower as well.

As the system became operational the success of the RAF night fighter force began to shoot up. This was further aided by the introduction of the [[Bristol Beaufighter]] and its [[AI Mk. IV radar]] which became available in numbers at the same time. These two systems proved to be a potent combination, and interception rates doubled every month from January 1941 until the ''Luftwaffe'' campaign ended in May.

The Germans were quite slow to follow in terms of PPI and did not order operational versions of their [[Jagdschloss radar]] until late in 1943, with deliveries being relatively slow after that. Many were still under construction when the war ended in 1945.