In naval warfare, anti-air warfare (AAW) is the intelligent coordination of detecting, avoiding if appropriate, neutralizing, and protecting against hostile air units. AAW is generally assumed to defend against fixed-wing aircraft, helicopters, air-to-surface missiles and cruise missiles, but not ballistic missiles.
The heart of detection is radar, but it is possible to sense the existence of a radar before it can identify a target. A practical implication is that turning off one's radars may reduce the change of being found, but it also reduces the chance your side's radar will detect the enemy.
Many technologies and compromises apply to this problem. The defending force may send out radar aircraft to a considerable distance from the surface force, so when those radars are detected, all they indicate is that enemy forces are within several hundred miles (i.e., the range of the communications from the early warning platform). The early warning sensors can also be on unmanned aerial vehicles or satellites; one important U.S. Navy system of the Cold War did not try to find Soviet forces by their radar reflections, WHITE CLOUD satellite, but by triangulating on the Soviet ships' radio and radar emission.
If it is impossible to avoid radiating signals, they may be radiated in a manner that deceives the enemy side. For example, there are techniques where pairs of helicopters simulate an important target such as a carrier, and, when a missile commits to that apparent target, the helicopters rapidly separate, releasing decoys such as chaff.
Again assuming a modern navy, there are multiple concentric rings of defenses through which attackers must pass. During the Cold War, the U.S. Navy believed in starting with the outer air battle, with long-range F-14 Tomcat interceptors armed with the very long range AIM-54 Phoenix air-to-air missile.
There may be a ring of shorter-ranged fighters between the longest-ranged surface-to-air missiles, or simply closer in than specialized aircraft such as the F-14 could operate. Putting multiple friendly systems in the same airspace is an invitation to fratricide, and the group anti-air commander must deconflict the many systems. Complex anti-air warfare is the key strength of the AEGIS battle management system, and even more so when it is enhanced by the Cooperative Engagement Capability; AAW control is normally aboard a Ticonderoga-class cruiser. Burke-class destroyers also have AEGIS, although they have less group command capability than a Ticonderoga.
In the (U.K.) Royal Navy, the strongest AAW ship will be a Type 45-class destroyer, which are replacing the Type 42-class. NATO vessels can exchange detailed tracking and status information using the Joint Tactical Information Distribution System. Type 45 destroyers, rather than AEGIS, have the SAMPSON air warfare system.
In a U.S. AAW system, the RIM-156 Standard SM-2 is the long-range weapon, with one per VLS cell, which can engage targets at least 130 nm/240 km away. ESSM, with a range of 27+ nm/50+ km, forms the next band. For final defense, there is the 9 km (5 nm) RIM-116 Rolling Airframe Missile or the shorter-ranged Phalanx close-in weapons system autocannon.
Ideally, it is best to shoot down aircraft before they release missiles. Aircraft often carry multiple missiles, and the missiles themselves are smaller and more difficult targets. Anti-air specialists try to "shoot the archer, not the arrows."