The essence of air defence is to detect hostile aircraft and destroy them. The critical issue is to hit a target moving in three-dimensional space; an attack must not only match these three coordinates, but must do so at the time the target is at that position. This means that projectiles either have to be guided to hit the target, or aimed at the predicted position of the target at the time the projectile reaches it, taking into account speed and direction of both the target and the projectile.
Throughout the 20th century air defence was one of the fastest-evolving areas of military technology, responding to the evolution of aircraft and exploiting various enabling technologies, particularly radar, guided missiles and computing (initially electromechanical analog computing from the 1930s on, as with equipment described below). Air defence evolution covered the areas of sensors and technical fire control, weapons, and command and control. At the start of the 20th century these were either very primitive or non-existent.
Initially sensors were optical and acoustic devices developed during the First World War and continued into the 1930s, but were quickly superseded by radar, which in turn was supplemented by optronics in the 1980s.
Command and control remained primitive until the late 1930s, when Britain created an integrated system for ADGB that linked the ground-based air defence of the army's AA Command, although field-deployed air defence relied on less sophisticated arrangements. NATO later called these arrangements an "air defence ground environment", defined as "the network of ground radar sites and command and control centres within a specific theatre of operations which are used for the tactical control of air defence operations".
Rules of Engagement are critical to prevent air defences engaging friendly or neutral aircraft. Their use is assisted but not governed by IFF (identification friend or foe) electronic devices originally introduced during the Second World War. While these rules originate at the highest authority, different rules can apply to different types of air defence covering the same area at the same time. AAAD usually operates under the tightest rules.
NATO calls these rules Weapon Control Orders (WCO), they are:
- weapons free: a weapon control order imposing a status whereby weapons systems may be fired at any target not positively recognised as friendly.
- weapons hold: a weapon control order imposing a status whereby weapons systems may only be fired in self-defence or in response to a formal order.
- weapons tight: a weapon control order imposing a status whereby weapons systems may be fired only at targets recognised as hostile.
Until the 1950s guns firing ballistic munitions were the standard weapon; guided missiles then became dominant, except at the very shortest ranges. However, the type of shell or warhead and its fuzing and, with missiles the guidance arrangement, were and are varied. Targets are not always easy to destroy totally, although damaged aircraft may be forced to abort their mission and, even if they manage to return and land in friendly territory, may be out of action for days or permanently. Ignoring small arms and smaller machine-guns, ground-based air defence guns have varied in calibre from 20 mm to at least 149 mm.
Ground-based air defence is deployed in several ways:
- Self-defence by ground forces using their organic weapons, AAAD.
- Accompanying defence, specialist aid defence elements accompanying armoured or infantry units.
- Point defence around a key target, such as a bridge, critical government building or ship.
- Area air defence, typically 'belts' of air defence to provide a barrier, but sometimes an umbrella covering an area. Areas can vary widely in size, belts along a nation's border, e.g. the Cold War MIM-23 Hawk and Nike belts that ran north–south across Germany, a military formation's manoeuvre area, or the area of a city or port. In ground operations air defence areas may be used offensively by rapid redeployment across current aircraft transit routes.
Air defence has included other elements, although after the Second World War most fell into disuse:
- Tethered barrage balloons to deter and threaten aircraft flying below the height of the balloons, where they are susceptible to damaging collisions with steel tethers.
- Searchlights to illuminate aircraft at night for both gun-layers and optical instrument operators. During World War II searchlights became radar controlled.
- Large smoke screens created by large smoke canisters on the ground to screen targets and prevent accurate weapon aiming by aircraft.
Passive air defence is defined by NATO as "Passive measures taken for the physical defence and protection of personnel, essential installations and equipment in order to minimize the effectiveness of air and/or missile attack". It remains a vital activity by ground forces and includes camouflage and concealment to avoid detection by reconnaissance and attacking aircraft. Measures such as camouflaging important buildings were common in the Second World War. During the Cold War some airfields painted their runways and taxiways green.
Read more about this topic: Flak
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