Radar control of aircraft from the ground
Ground Controlled Interception - GCI

Night Fighting aircraft, unassisted by radar (Catseye Aircraft), achieve but little success against night raiders, except, perhaps over the target in bright moonlight. A successful technique of night interception (employing radar aids) has however been evolved.
In the GCI-AI system, radar assistance to the night fighter is two-fold:

1. The night fighter carries a portable radar equipment which allows it to detect and “home" on to other from aircraft ranges of 4 to 5 miles and less. This airborne radar set is known as Al equipment. (Al = Aircraft Interception.)

2. Because the Al equipment has a small maximum range (4 to 5 miles) it is necessary first to vector the fighter into the vicinity of the raider and on to a favourable course so that it may detect the bomber on its Al equipment and complete the interception unassisted. The fighter is therefore controlled in the preliminary stages of an interception from a specialised radar ground station known as a GCI (Ground Controlled Interception) station.  At this station the relative positions in plan and the courses of the fighter and bomber are judged from a PPI  display tube; the heights of the two aircraft are also measured. On the  basis of this information the Controller issues appropriate vectors to the pilot until an Al contact is announced by the pilot or the interception is seen to have failed.