Suppression of enemy air defense
Suppression of enemy air defense (SEAD), according to the U.S. Department of Defense, is an activity that neutralizes, destroys, or temporarily degrades surface-based enemy air defenses by destructive and/or disruptive means." It is a subset of offensive counter-air (OCA) operations. Electronic warfare is often a major component of SEAD, but a given SEAD mission may or may not -- more likely may -- use any elements of electronic warfare. SEAD may draw significantly from electronic intelligence at national or theater level.
The ability to conduct SEAD has become a major part of the politicomilitary doctrine of technologically advanced nations. As the loss of aircraft crews becomes more unacceptable, most campaigns start with SEAD from a distance, opening routes for attack aircraft. The use of stealthy manned aircraft and of armed unmanned aerial vehicles can either complement or replace conventional attack aircraft.
Germany lost the Battle of Britain when the Luftwaffe failed to recognize it faced an integrated air defense system and its first mission, if any other goals were to succeed, was to continue the initial SEAD attacks against British defense systems until those systems collapsed. In the Vietnam War, while the U.S. bombing of North Vietnam might not have resolved an inherently political problem, political restrictions against SEAD significantly limited the effectiveness of attacks against strategic targets. Israel suffered initial severe losses in 1973 when it encountered unexpected air defense system elements: SA-6 GAINFUL surface-to-air missiles guided by passive, visual, electro-optical means that could neither be detected nor attacked by electronic warfare.
In practice, SEAD involves multiple levels of deception with drones or electronic warfare, electronic countermeasures against the air defense electronics, long-range anti-radiation missiles (ARM) that home on radars, and bombing or other physical attacks on land- and sea-based air defense facilities. Deception may have the purpose of tricking the enemy into turning on radars so they can be targeted.
Before any of these actions can take place, however, the enemy integrated air defense system needs to be understood, with signals intelligence being a key discipline; the SEAD planner cannot simply rely on electronic intelligence to find the radars, but use communications intelligence to understand how the system is operated. Imagery intelligence is needed to locate physical defenses, such as airfields, missile launchers, and guns; there have been nasty surprises for attackers who encounter guns and missiles that are guided not by detectable radar signals, but by passive electro-optical means such as forward-looking infrared.
SEAD is usually considered restricted to air defense command and control, radars, surface-to-air missiles (SAM) and anti-aircraft artillery (AAA), but not enemy fighters. Complementary OCA activities include fighter sweeps intended to lure enemy fighters into air-to-air combat on unfavorable terms, as well as attacks on fighters and associated resources on the ground.
Prior to the need to operate against an integrated air defense system (IADS) of radars, surface-to-air missiles, antiaircraft artillery, and fighters, there were incidental attacks, usually accompanying strikes against AAA.
One notable example of the need for SEAD came during the Korean War, in an incident where AAA interfered with both ground troops and close air support, for which MAJ Charles J. Loring Jr. was recognized with a posthumous Medal of Honor
LORING, CHARLES J., JR.
Rank and organization: Major, U.S. Air Force, 80th Fighter-Bomber Squadron, 8th Fighter-Bomber Wing. Place and date: Near Sniper Ridge, North Korea, 22 November 1952. Entered service at: Portland, Maine. Born: 2 October 1918, Portland, Maine. Citation: Maj. Loring distinguished himself by conspicuous gallantry and intrepidity at the risk of his life above and beyond the call of duty. While leading a night of 4 F-80 type aircraft on a close support mission, Maj. Loring was briefed by a controller to dive-bomb enemy gun positions which were harassing friendly ground troops. After verifying the location of the target, Maj. Loring rolled into his dive bomb run. Throughout the run, extremely accurate ground fire was directed on his aircraft. Disregarding the accuracy and intensity of the ground fire, Maj. Loring aggressively continued to press the attack until his aircraft was hit. At approximately 4,000 feet, he deliberately altered his course and aimed his diving aircraft at active gun emplacements concentrated on a ridge northwest of the briefed target, turned his aircraft 45 degrees to the left, pulled up in a deliberate, controlled maneuver, and elected to sacrifice his life by diving his aircraft directly into the midst of the enemy emplacements. His selfless and heroic action completely destroyed the enemy gun emplacement and eliminated a dangerous threat to United Nations ground forces. Maj. Loring's noble spirit, superlative courage, and conspicuous self-sacrifice in inflicting maximum damage on the enemy exemplified valor of the highest degree and his actions were in keeping with the finest traditions of the U.S. Air Force.
Had the CAS mission been conducted under modern doctrine, and it was known the AAA was present, it would have been engaged by SEAD aircraft, using anti-radiation missiles if it was radar-guided, or cluster bombs or precision guided munitions if it was not. Alternatively, the gun positions might have been attacked by surface-to-surface missiles, long-range multiple rocket launchers, or howitzers if in range. As long as a CAS mission would be in direct support of troops, the AAA would have received priority for the long-range strikes.
Early modern SEAD
SEAD, as a distinct set of tactics against an IADS, emerged in the Vietnam War, in the bombing campaign against North Vietnam. "In World War II, there was the element of surprise, which materially assisted in degrading the quality of the AAA. This was not the case in North Vietnam. There was no element of surprise because of the small geographical area involved, limited approach routes, and the few targets available for attack. As a consequence of the geography, strike forces had to meet the defenses almost head on. That is, there was limited opportunity to strike from multiple directions and to feint the enemy defenses out of position. It was necessary to designate specific elements of the strike force to counter AAA fire in the target area. For fighter operations, this isn't a unique technique. It was used in World War II and Korea." 
Within a fighter-bomber strike, a certain number of aircraft were armed with cluster bomb unit (CBU) munitions, which spread out over a large area. CBU were more effective than unitary bombs, because AAA often had multiple guns, control positions, radars, etc., which all needed to be suppressed, but could be suppressed with a small amount of blast and fragmentation. Using the larger bombs appropriate for structures, oil tanks, etc., put too much blast into too small a space.
The aircraft carrying the CBU had to time their attacks precisely. They could not expect to destroy all the AAA, but to reduce its effectiveness. If they attacked the AAA too soon, the gunners could recover and concentrate their remaining firepower on the strike aircraft. If they attacked too late, fratricide could result from strike aircraft flying into CBU fragments and possible secondary explosions from AAA ammunition detonating on the ground.
Counter-SAM (Wild Weasel)
Most commonly called Wild Weasels, or sometimes Iron Hand Flights, these were generally agreed to be the elite of strike pilots. By whatever name, these aircraft, often two-seat F-105s so there was a dedicated electronic warfare officer, would enter the target area before the strike aircraft, and expose themselves, or generate electronic noise, to encourage the SAM crews to turn on their radar and engage these tempting targets. Preferably before the SAMs were launched, the Wild Weasels would fire anti-radiation missiles at the SAM radars.
The best situation was when the Wild Weasel could destroy the early warning radar before the separate fire control radar could lock on the aircraft. If the SAM launched but the ARM destroyed the fire control radar in time, the SAM would go out of control, and possibly self-destruct.
Increasingly bad scenarios were that the fire control radar was not destroyed until even a self-destructing SAM explosion could hit the Wild Weasel. Worst, of course, is that the radar and SAM were not damaged and the SAM remained on course. At that point, the pilots had last-second evasive maneuvers that might force the SAM to break lock and let the Weasel escape. These maneuvers did not always work.
Sometimes, the SAM sites would realize that Wild Weasels were coming in, and shut down their radars. With sufficiently short intervals between the Weasels and the strike aircraft, this might let the strike take place. The Wild Weasels also might be accompanied by CBU-carrying aircraft that would attack the SAM sites, which often also had AAA, while the radar was shut down.
The Wild Weasel mission is still part of SEAD, although the aircraft, when Air Force, has evolved from F-105 to F-4G to F-16 CJ Block 50/52. If electronic warfare can do a "soft kill" on the radars, other aircraft, especially when the IADS component locations are known from prestrike reconnaissance, can attack them with PGMs and go for a hard kill. Standard Navy F-18s also can act as Weasels; the EF-18 Growler the replacement for the EA-6B Prowler can do both "soft" and "hard" kill.
Versions of the Panavia Tornado fly SEAD missions for Italy, Germany, Saudi Arabia, and the United Kingdom. Italy and Germany use the dedicated electronic combat/reconnaissance (ECR) version, which can fire the AGM-88 HARM. The Saudi and British ground attack versions fire BaE Systems ALARM.
Before the main strike, if it is at night so they cannot be acquired visually, stealth bombers can slip in for a hard kill on IADS components. Cruise missiles may be able to take out soft targets such as antennas and SAM launchers.
Also, the IADS may be tricked into turning on their radars, and firing on drones, perhaps drones with radar amplifiers to appear to be a good target. Against Syria in 1973, Israel used Harpy drones to get the enemy radar to present ARM targets.
In a mission, called "Poobah's Party" against Baghdad's KARI IADS in January 1991, a large number of Air Force ground-launched BQM-34 and Navy tactical air-launched decoys (TALD) drones followed the stealth and cruise missile attacks, and the Iraqi missile crews seemed relieved to have targets they could actually track. A large number of Wild Weasels were behind the drones, expecting just such a response, and showered the defenses with AGM-88 HARM anti-radiation missiles (ARM).
Also in Desert Storm, air defense facilities were attacked with the MGM-140 ATACMS tactical ballistic missile, using cluster submunitions. These Army assets were most commonly used to eliminate threat to Army helicopters.
An armed version of Harpy is operated by is operated by Turkey, Israel, Korea, China and India.  A second-generation development of the Harpy UAV, Cutlass (combat uninhabited target locate and strike system) is a hybrid of UAV and cruise missile. It can loiter in the target area like a UAV, drawing radar attention, but it has a warhead and can attack. The U.S. was concerned about such an approach violating arms limitation treaties affecting land-based cruise missiles, but those restrictions do not apply to sea-launched missiles, and a team of Raytheon and Israel Aerospace Industries proposed Cutlass to the U.S. Navy.  Cutlass was also a finalist in the UK Loitering Munition Capability Demonstration (LMCD) program. 
- US Department of Defense (12 July 2007), Joint Publication 1-02 Department of Defense Dictionary of Military and Associated Terms
- Momyer, William W. (July 1983), "The Evolution of Fighter Tactics in SEA", Air Force Magazine 56 (7)
- Erwin, Sandra I. (June 2001), "Loitering, Smart Cruise Missile Marketed to U.S. Navy", National Defense Magazine
- "Harpy Air Defense Suppression System", International Online Defense Magazine, 22 December 2004
- "Hunter Killer drone target Ballistic Missile Launchers", International Online Defense Magazine, 3 April 2006