Surface-to-air missile (SAM) defines a wide range of guided missiles, usually rocket-powered, fired from land or sea against aircraft. The lightest SAMs can be fired from a soldier's shoulder; the class is called a man-portable air defense system (MANPADS). Larger SAMs vary from short-range systems intended to protect a small area such as an individual ship or ground headquarters, which is called "point defense". "Area defense" SAMs cover much larger areas, with a radius as large as 250 mi/400 km. Some SAMs either have an intrinsic capability for ballistic missile defense, or they are the basis for more extensive variants with BMD or anti-satellite ASAT capability.
SAMs have largely replaced anti-aircraft artillery in modern militaries, although vehicles often still mount heavy machine guns for defense against helicopters and slow aircraft, as well as use against ground targets. Some naval SAMs have an anti-shipping missile capability.
While the German military experimented with SAMs in the Second World War, the first generation of SAMs became operational in the early 1950s. They were first deployed in fixed sites, with a radius, depending on target altitude, on the order of 30mi/45km. They could not engage targets at low altitude.
The first operational U.S. SAM was the MIM-3 Nike Ajax. While a different missile was deployed first, only in the Moscow area, the first widely used operational Soviet SAM was the S-75 Dvina (NATO designation SA-2, reporting name GUIDELINE).
In this essentially obsolete technique, the surface control radar holds a continuous radar lock on the target, and the missile is just intelligent enough to stay in the center of the beam. It does not have a conventional radar receiver, so that it cannot detect radar reflections from the target.
Some beam rider systems may be able to send commands to the missile, most commonly "detonate" or "turn on proximity fuze". Such techniques were used by North Vietnam against B-52 bombers in the first phase of Operation LINEBACKER II. The S-75 Dvina missiles were not even sent by beam to the target; the bombers did have electronic warfare equipment to interfere with the beam. Instead, the missiles were mechanically aimed, in groups, at the flight path of the bombers. Once near the bomber, they might detonate by ground command, or have their very limited intelligence, a proximity fuze, enabled when they neared the target. The aircraft did not have a means of jamming the proximity fuze.
Search plus target illuminator with seeker on missile
Much more common, and, for surface-to-air rather than air-to-air applications, is a combination of a search radar to get the missile close to the target, and then using a second ground radar to illuminate the target for the missile's final attack. This latter technique, semi-active radar homing, requires only a brief illumination time. The AEGIS battle management system can track hundreds of targets with its AN/SPY-1 search radar, but AEGIS ships time-share their 3 or 4 AN/SPG-62 illuminator radars among many RIM-156 Standard SM-2 missiles being directed at targets.
A track-by-missile system, such as on the MIM-104 Patriot, uses a ground radar that tracks both the target and missile, but also has an active radar in the missile itself. The ground control station compares the two separate radar perspectives, which gives extreme accuracy. Since the Patriot is now primarily used for hit-to-kill applications in ballistic missile defense, the accuracy is critical since the missile and target must actually collide. The ground radar in this system, as in AEGIS, can track multiple targets yet handle multiple missiles.
SAMs range from shoulder-fired man-portable air defense systems (MANPADS), which must be light enough to be held by one soldier, to large, long-range missiles. RIM-156 Standard SM-2 ship-fired missiles can engage air targets at least 240 km/130 nmi distant. The SM-2 also has a secondary surface-to-surface capability, which, with a launch weight of 3200 pounds and a speed in the Mach 3.5 range, will hit a ship with very substantial kinetic energy, to which is added fires caused by unused fuel and the explosion of the warhead.
Some British naval SAMs, such as the Sea Cat, became obsolete for anti-aircraft, but were retained and given concrete warheads for anti-shipping use. Solid concrete warheads, impacting at high speed, can actually do more damage than a relatively small explosive. The blast-fragmentation warheads on SAMs are intended to damage aircraft targets with near-misses that still bring the target into the explosion. That explosion, however, is intended more to knock the aircraft out of control, possibly explode weapons or the engine, but is not sufficiently powerful, on its own, to shred the aircraft.