Integrated air defense system
An integrated air defense system puts all antiaircraft sensors (e.g., radar, visual observers, and other technical means) as well as antiaircraft weapons (e.g., anti-aircraft artillery, surface-to-air missiles, air superiority fighters and interceptors, etc., under a common system of command, control, communications and intelligence (C3I). Depending on the national doctrine involved, the control is more or less decentralized. NATO doctrine is concerned with deconfliction, but allowing a fighter pilot the discretion to pursue the final attack. Soviet doctrine, including that of Iraq, was more centralized and less flexible for the pilot.
Today's battlefield, in many respects, starts as a duel between the IADS and the suppression of enemy air defense campaign against it.
The first operational IADS, with no computer assistance other than in the brains of the defenders, was in the Battle of Britain. The term IADS had not yet been invented, but more important was that the Germans did not see the British system as a system. They saw airfields, radars, etc., but did not grasp that the most critical and vulnerable part were the control centers. Indeed, some of the German radars of the time were more advanced, but were not as well integrated.
Requirements of an IADS
The basic rules of an IADS are:
- Find the attackers
- Direct your defensive platforms (e.g., surface-to-air missiles (SAM), anti-aircraft artillery (AAA), etc.) against them
- Let your defensive platforms attack the enemy
- Don't let your defensive platforms attack your own side
Radar is the backbone of modern IADS, and radar of many types: early warning, perhaps over-the-horizon early warning, "look down" airborne radar that can detect low-flying aircraft and cruise missiles, ballistic missile acquisition radar, missile fire control radar, ground- and air-based radar that direct fighters to their targets, missile guidance radar, to say nothing of weather radar to pick out routes that the enemy may try and that your forces can use. Still, it is possible to become overdependent on radar, or make too many assumptions that you are superior to the enemy's radar.
Some Soviet/Russian SAMs, such as the 2K12 (NATO reporting name SA-6 GAINFUL), have, in addition to radar, electro-optical assistance to visual guidance.  These were a rude surprise to NATO forces that believed they had destroyed all Serbian radars, or forced them to shut down. Anthony Cordesman observed that a lesson learned from the Kosovo campaign was "the continuing survivability of land-based air defenses, and the threat posed by “non-cooperative” air defenses that do not emit or deploy in ways that can be easily targeted. According to NATO figures, some 90% of Serbia’s SA-6 assets survived the war, and could fire using pop-up radar and/or electro-optical techniques at the end of the war." Further, he observed that modern air and missile power can achieve very high levels of suppression, [but\ cannot kill mobile systems or prevent land-based systems from riding out an extensive..."suppression of enemy air defense (SEAD) campaign. In contrast to the Iraqi KARI  IADS was largely made up of fixed assets.
A modern IADS has multiple layers of sensors and defensive systems. This is most obvious in the protection of a U.S. naval carrier group, in which the outermost ring consists of fighters and well-escorted airborne radar aircraft, with the next ring made up of long-range surface-to-air missiles (SAM) such as the RIM-156 Standard SM-2, another ring of medium-range SAMs such as the # RIM-162 ESSM, and point defenses such as the Phalanx close-in weapons system (i.e., autocannon) and RIM-116 Rolling Airframe Missiles.
When there was a still a major threat from Soviet Naval Aviation bombers launching long-range anti-shipping missiles (ASM), there were two rings of fighters, the Outer Air Battle aircraft being F-14 Tomcat interceptors armed with the extremely long range, but relatively unmaneuverable, AIM-54 Phoenix air-to-air missile. On the theory that "it is better to stop the archer than the arrows", the Tomcats would try to intercept the bombers outside their maximum missile range. Another ring of fighters would engage any threat aircraft not carrying long-range weapons, and probably engage the fast but straight-running ASMs.
Ballistic missile defense can have many layers. Israel is exploring acquiring the very long range RIM-161 Standard SM-3 missile intended to engage theater ballistic missiles, and adapting them to a land-based role. The next ring would be the still long-range, very-high-altitude Arrow-2 missiles co-developed with the U.S. For critical areas of 15 mile or so radius, the remaining incoming warheads would face MIM-104 Patriot missiles. Final point defense of key installations might use autocannon such as the Oerlikon AHEAD shotgun-like pellets, or the developmental Nautilus, formerly the Mobile Tactical High Energy Laser (MTHEL).
Command and control
Today, if the enemy knows where your IADS command is located, and it is not mobile and well-defended, he will make it a high priority to kill. It becomes a center of gravity of one's defense. This was not true in the Battle of Britain; the Germans did not fully understand the network of sector stations linked to one another and to Fighter Command, and they did not have the precision-guided munitions to achieve sure kill on the command posts if they knew their significance.
Iraqi IADS command posts, however, were some of the first targets of the 1991 Operation Desert Storm attack, although an early warning radar was the very first target. The latter was on the Saudi-Iraq border, and was unconventionally attacked by Army AH-64 Apache attack helicopters, led to the target by Air Force Special Operations MH-53 PAVE LOW helicopters with advanced low-level navigation.
While Airborne Warning and Control System (AWACS) such as the E-3 Sentry or Russian Beriev A-50 seem safe in the air, that depends on the sophistication of the opponent. It is unlikely that a fighter can claw through the escorts surrounding an AWACS, but long-range air-to-air missiles that also have an anti-radiation missile capability may get through. The Soviet/Russian Vympel R-33/AA-9 AMOS has been considered an "AWACS killer". Longer-ranged versions of the U.S. AIM-120 AMRAAM may have the same capability toward the other side.
Air defense platforms
Active DCA uses air, sea, and ground based assets that either physically destroy the enemy, using antiaircraft artillery (AAA), surface-to-air missiles (SAM), and fighter aircraft armed with autocannon or air-to-air missiles (AAM). Fighters have the greatest range and flexibility, but cannot provide continuous coverage of a protected area.
In the past, certain fighter aircraft were specialized for the aircraft interception mission. Multirole combat aircraft have replaced them, but some interceptors, such as the Russian MiG-25. Interceptors tend to have very high speed, but little maneuverability if they were intended to go against bombers. Their high speed and altitude capability can make them useful as reconnaissance platforms, such as the MiG-25R.
Such systems can be divided by their area of coverage. Fighters and longer-range SAMs can cover areas containing numerous surface targets, while point defense of short-range SAMs and AAA can protect specific targets. A special case is the protection of high-value assets such as AWACS and tankers, which usually involves fighters, and possibly electronic warfare aircraft, escorting them.
Deconfliction is one of the key aspects of an IADS. In general, there are a series of concentric circles around a target: the outermost might be assigned to long-range fighters, the next to long-range SAMs, the next to shorter-range fighters, and the innermost to AAA and short-range SAMs. These circles may be three-dimensional; there may be a rule that while aircraft at high altitude over troop concentrations are not to be engaged by the ground missiles, if they descend below a given altitude, they become targets.
Both air force and air defense force [Egyptian] commanders confirmed that, while it was an operational goal to use the MiG-21 as the first force to engage enemy aircraft at maximum range, it also was tactical doctrine for the interceptors to fight within the missile belt and continue harrying attacking forces all the way to their targets. They agreed that losses from friendly missiles were so relatively small that the tactics of using both interceptors and missiles in the same airspace was operationally sound and militarily effective against the offensive formations.
IADS over time
Battle of Britain
Named after its architect and commander, Generalleutnant Josef Kammhuber of the Luftwaffe, this IADS, originally extending to the French coast, defended Germany from World War II strategic bombing. Obviously, its western installations were lost as the Allies moved inland, but the German defenses were still very credible. The radars were technically excellent, and one of the major problems was Adolf Hitler's unwillingness to invest in defensive systems.
While the North Vietnamese had relatively little air defense when first bombed after the Gulf of Tonkin Incident, they steadily improved. As a very significant difference from the Korean War, North Vietnamese air defenses moved quickly to integrate radar, command post, ground observers, anti-aircraft artillery, and, when the Soviets supplied them, surface-to-air missiles.
By 1960, the U.S. was starting to direct strategic SIGINT at North Vietnam, to understand their air defense as well as other structures.
After a regiment of PRC MiG-17 fighters arrived at Mengtzu in 1963, SIGINT predicted jet fighters would enter the DRV air defense network. This was reinforced with learning that high-level DRV and PRC personnel would have a meeting at Mengtzu in May 1964.
Early DRV Air Defense Buildup
In the weeks immediately following the Gulf of Tonkin incident, the most important SIGINT role was providing defensive information to US air strikes. This was done at three levels of generality. First, overall monitoring of the DRV air defense network, SIGINT could maintain situational awareness of North Vietnamese tracking via radar and visual observers. Second, SIGINT detected the activation of specific weapons systems in the air defense network, such as S-75 Dvina surface-to-air missiles (SAM), Anti-aircraft artillery (AAA), and fighter interceptors. Finally, it could detect immediate threats, such as missile launches or impending attacks by fighters.
Reports from the roughly 40 visual observation stations were sent to sector headquarters, which controlled AAA. These reports were sent by high-frequency (HF) radiotelegraphy, in standardized message formats where only the specific details needed to be transmitted. It could take up to 30 minutes for a report to work its way through the system, so that more specific tracking or interception orders could be given. According to the NSA history, air defense communications did not change significantly during the war, so COMINT analysts were able to become very familiar with its patterns and usage.
Command and control applied to four system components: air warning from radar and observer stations, limited radar tracking, AAA and SAMs, and fighters. Rapid upgrades started to go into place after the Gulf of Tonkin incident, with the arrival, within two days, of 36 MiG-15 and MiG-17 fighters. These arrived from China and were probably flown, at first, by Chinese pilots, but Vietnamese pilots were soon in familiarization flights.
Two main communications links between the DRV and PRC were established, from Hanoi to Kuangchow and K'unming. These liaison networks allowed access to Chinese radar covering the Gulf of Tonkin, Laos, and Hainan Island, as well as the DRV itself. Air Defense headquarters was at Bac Mai.
The DRV system matures, 1964
Air Defense headquarters was at Bac Mai. North Vietnam's air defense system, as of 1965, had three main subsystems:
In 1965, the DRV had full radar coverage, with Chinese input, out to 150 miles from its borders. Detection and processing times dropped to five minutes. In contrast, the US did not have full radar coverage over the DRV, and SIGINT was seen as a way of filling the gaps in US knowledge of their air defense operations. 
By January 1966, all major air defense installations, including those in the PRC, were linked by a common HF radio network with standardized procedures. There was an Air Situation Center and an Air Weapons Control Staff. The latter assigned targets to the various defense weapons. By 1967-1968, there were approximately 110,000 persons in the DRV air defense system, supporting 150 radars, 150 SAM sites (rarely all active at the same time), and 8,000 AAA pieces. There were 105 fighters, including the MiG-21. At any given time, one-third to one-half of the fighters were based at PRC airfields.
A wider range of communications systems emanated from Air Defense Headquarters, including [[ITU frequency bands|VHF voice}}, landlines, and HF/MF. Due to the need to move information quickly, without any automation, most communications were either in low-grade ciphers or were unencrypted.
1973 Arab-Israeli war
Egyptian tactical IADS took the unusual risk of operating MiG-21 (fighter) interceptors in the same airspace as SA-2 GUIDELINE SAMs, possibly with the specific intent of bringing Israeli aircraft to low level. The Egyptian low-altitude system was the Soviet SA-6 GAINFUL, which the Israelis knew had a radar mode they believed they could counter; they did not know that these missiles also had an electro-optical tracking system that could not be jammed.
In 1999, Kosovo kept up an IADS for a surprising amount of time  While there has not been official confirmation, the SA-6 may have been involved in shooting down a U.S. F-117 Nighthawk "stealth" fighter over Kosovo in 1999. Actual detection may have used an older long-wave radar, with the SA-6 fired using electro-optical guidance. When the missile threatened the F-117, the latter may have maneuvered away, and into the path of fighters or AAA. 
- Royal Air Force, Battle of Britain
- Clark, Gregory C. (March 1997), Deflating British Radar Myths of World War II, Air Command and Staff College, ADA397960
- Anthony Cordesman (August, 2000), The Effectiveness of the NATO Tactical Air and Missile Campaign Against Serbian Air and Ground Forces in Kosovo: A Working Paper, Center for Strategic and International Studies
- a French-built system; KARI is the French word "Irak" spelled backwards
- Press, Michael C. (09 June 1978), Tactical Integrated Air Defense System, U.S. Army Command and General Staff College, ADA057830
- Robert Hotz, Offense, Defense Tested in 1973 War, Both Sides of the Suez: Airpower In the Mideast, Aviation Week and Space Technology
- Hanyok, Robert J. (2002), Chapter 6 - Xerxes' Arrows: SIGINT Support to the Air War, 1964-1972, Spartans in Darkness: American SIGINT and the Indochina War, 1945-1975, Center for Cryptologic History, National Security Agency
- National Security Agency (11/30/2005 and 05/30/2006). Gulf of Tonkin. declassified materials, 2005 and 2006. Retrieved on 2007-10-02.
- A Lost Illusion