Ground-controlled interception
Ground-controlled interception is a technique whereby one or more radar stations are linked to a command center with communications equipment in order to launch and/or guide aircraft to intercept incoming airborne threats. This technique was pioneered during World War 2 by the British. 'Fighter Command' played a pivotal role in the defence of Britain during the war, especially in the Battle of Britain. It is still important today, although nations that can afford it find AWACS, with or without support from GCI, to be more powerful and flexible.
The strengths of GCI are that it can cover far more airspace than AWACS without costing as much and areas that otherwise would be blind-spots for AWACS can be covered by cleverly placed radar stations. AWACS also relies on aircraft which may require deference and a few aircraft are more vulnerable than many ground-based radar stations. If a single AWACS 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 (although a strike on the command center(s) could be very serious indeed).
AWACS 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. A combination of both techniques is really ideal, but GCI is typically only available in the defence of one's homeland, rather than in expiditionary types of battles.
GCI can also be 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.
Either GCI or AWACS 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, this alerting the intruders that they may be coming under attack. With GCI or AWACS, 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. This greatly increases the defender's chance of success and survival. Alternatively, they could be guided to an interception course and then GCI or AWACS can notify then of the best time to turn their radars on and giving them a vector to the intruders, so that they can get a radar lock and fire their missiles without giving any warning.
During World War 2, 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).