Close air support
Close air support (CAS) is defined, by the U.S. Department of Defense, as "air action by fixed- and rotary-wing aircraft against hostile targets that are in close proximity to friendly forces and that require detailed integration of each air mission with the fire and movement of those forces."
It is distinguished from battlefield air interdiction (BAI), which attacks forces that are not in close proximity to friendly units, but are a potential threat to those units, or support the attacking forces.
Originally, CAS used bombs, machine guns, and autocannon, aimed purely by the pilot's vision and awareness of friendly and hostile units. Especially in the early parts of the Second World War, when the supported ground troops and the aircraft had no radio communications, there was a very significant danger of fratricide unless the aircraft stayed a significant distance from friendly units.
Precision improved with the advent of forward air controllers (FAC), who were pilots or otherwise trained to direct, by a shared radio channel, the aircraft's path against the target. While FACs first used only descriptions of landmarks to orient the attack pilot, smoke grenades, high-visibility colored panels spread on the ground, or other visual references were soon deployed by supported troops. Forward observers have a similar function for controlling artillery and shore bombardment.
Another advancement was having FACs in aircraft, typically small and slow, from which they could observe the position of their own troops and the enemy, and then fire marking rockets at the target. The marking rockets, usually with a filler of white phosphorus, produced a highly visible cloud of glowing smoke.
It is fair to say that the advent of precision guided munitions was a revolution for CAS. With some weapons, the pilots had a television link that let them see where the weapon was going. In the Vietnam War, lasers were used to designate a precise location; the weapon homed on the laser reflection. The laser designator was first in other aircraft, but designators suitable for use from the ground came into use.
While it emphasizes CAS directed by special operations troops, see a description of the state of the art in ground control of CAS.
Aircraft for CAS
Ground forces typically prefer CAS aircraft to be slow enough to see targets clearly, to have a long loiter time, and very accurate weapons. Air forces tend to prefer faster aircraft that are more survivable against air defenses, and also more flexible to deploy. Armed helicopters, attack helicopters and short takeoff and vertical landing (S/VTOL) aircraft are the realization of the ground orientation, while air forces tend to assign multirole fighter-bomber.
One controversial approach, pioneered by the Soviet Union with the World War II Il-2 Shturmovik, is to build a dedicated, extremely survivable fixed-wing aircraft that does use a conventional runway rather than S/VTOL. The U.S. A-10 and Russian Su-25 (NATO designation FROGFOOT) aircraft are heavily armored jet aircraft with many survivability features. They do not, however, have the same survivability against sophisticated air defense as does a high-performance fighter-bomber.
One hybrid approach, used in the Gulf War, was to conduct extensive suppression of enemy air defense (SEAD) operations before ground troops, and close air support, were committed. Not all campaigns will have the luxury of time to conduct SEAD, but wise commanders carefully balance SEAD and CAS priorities.
Another guideline is to keep the true CAS aircraft for CAS, and perhaps limited BAI, but not to try to use such manned resources for deep strike (e.g., Battle of Karbala. When quick-reaction deep strike is needed, ground commanders in advanced military forces often control surface-to-surface missiles that can hit the deep threats.
The use of unmanned aerial vehicles and of clandestine special reconnaissance teams, with "bomb truck" high-altitude, large-capacity aircraft carrying large numbers of precision-guided munitions is yet another way to deal with the challenge of moderate to deep strike.