A military missile guided during flight to a target by internal preset, or self-reacting, devices and external electronic signals.

See: ballistic missile

Guided Missiles for Dummies

In general, a projectile which moves under its own power and can adjust its own path to reach a target is considered a guided missile.

While guided missiles exist in a huge variety of types, and are used in different roles, most of them have a few things in common: they have a propulsion system, a guidance system, and a warhead. The exact forms these components take vary with the intended use of the missile.

The most basic classification of a guided missile specifies where it's launched from and where it's going: a surface to air missile (SAM), air to surface missile (ASM), surface to surface missile (SSM), or air to air missile (AAM). Since modern warships frequently carry missiles, and travel on the water's surface, you can think of the S as meaning "ship" as well as "surface".

In general, a missile intended for aerial targets needs to be fast and maneuverable, but doesn't need a very large warhead. Surface targets don't tend to dodge as much as aircraft (though this doesn't eliminate a missile's need for speed and maneuverability), but frequently can take more punishment, so warhead size is a larger factor. These are general trends, of course; the US used to field an air-to-air missile, the AIR-2 Genie, with a nuclear warhead!

It's not unusual for the same basic missile design to appear in both SAM and AAM forms (AIM-7 Sparrow and RIM-7 Sea Sparrow), or in both SSM and ASM forms (AGM-84 Harpoon), though there's sometimes a booster rocket associated with the surface- launched version. A few missiles (such as the US RIM-67 Standard series) are capable of engaging both air and surface targets.

A missile's propulsion system is usually either a jet engine, solid- or liquid-fueled rocket engine, or some combination thereof. Rocket engines tend to be faster, while jet engines, since they don't need to carry their own oxidizer, tend to be capable of longer range. A missile which includes wings to provide lift is called a cruise missile; these are almost always long-ranged, jet-propelled, anti-surface weapons.

Some missiles have multiple stages, such as a rocket booster stage to quickly gain altitude and airspeed, with a jet cruise stage to go a long way. A more recent development puts a rocket- powered "intercept stage" on top of a jet cruise missile; the jet stage provides long range while the rocket stage covers the last part of the distance at high speed, reducing the time the target has to shoot it down. The Russian SS-N-27 Alfa antiship missile uses this design.

A huge variety of guidance systems have been attached to missiles over the years. Here are general descriptions of the more important types.

  • Inertial or positional systems are used against ground targets; the missile "knows" where it is via dead reckoning, GPS, TERCOM, or some other method, and knows where its target is. Such systems are useless against any moving target, but are difficult or impossible to "spoof" or jam.
  • Infrared or heat-seeking systems (IR), as the name suggests, guide themselves towards heat sources. While early infrared seekers could only "lock on" to the heat of a jet exhaust from behind, newer, more sensitive ones can home on airframe heat from any angle ("all-aspect"), or the heat of a ground vehicle engine. The useful range of IR seekers is relatively short, usually under 10km. Early IR systems were easily fooled by launching flares; newer ones are smart enough to see the difference between a big fuzzy heat source and a small point heat-source, and thus promote the development of more sophisticated flares.
  • Wire-guided missiles are steered by electronic signals on physical control wires running from the launcher to the missile. For obvious reasons, these are restricted to very short ranges, and wire guided missiles are typically either man-carried or mounted on ground vehicles or attack helicopters, for use against tanks and other armored vehicles.
  • Command-guided missiles rely on radio commands from the launcher or another platform to steer them to their target. This obviously requires a good view of the action on the part of the launcher, and are subsceptible to radio jamming. Some missiles use command guidance to get them near their target, then switch over to another method such as IR homing to hit it. The command station may use active radar or some other method to track the target while steering the missile, or it may rely on a camera on board the missile itself; this is known as man-in-the-loop TV guidance.
  • Semi-active radar homing (SARH) seekers put a radar transmitter on the launching platform and a radar receiver on the missile. The missile homes in on the radar signal reflected from a target. The major disadvantage to such missiles is that the launching platform (or another platform) has to keep its radar on the target (in the parlance, this is called "illuminating") for the entire flight time of the missile. In an air-to-air fight, this means flying straight towards your target, who may in turn be shooting missiles at you in a high-speed game of chicken. If you turn away to avoid the incoming missile, your missile will almost certainly miss. SARH missiles are commonly medium-range ones, some with ranges over 40km.
  • Active radar homing puts both the radar transmitter and receiver on the missile. This means the launcher doesn't have to keep illuminating the target, but adds weight to the missile. Both SARH and active radar missiles can be spoofed by chaff (radar- reflective metal strips, which reflect radar signals and catch the attention of a radar-guided missile) or by active electronic countermeasures (ECM), which essentially amounts to transmitting false radar returns or radar noise towards the missile to confuse it. See also Electronic Counter Counter Measures (ECCM), which includes such straightforward techniques as increasing the power of your radar so that the real radar return drowns out the jamming.
  • Anti-radiation missiles (ARMs) have nothing to do with nuclear weapons, or for that matter, nuclear anything. The "radiation" refers to radar. Anti-radiation missiles seek radar transmitters. These are frequently used in SEAD missions, reasonably enough, to target enemy surface-to-air launchers by way of their radars. Clever ones remember where they detected radars, so if the radars turn off they can still try hit their targets. Insidious ones like the AGM-136 Tacit Rainbow slow down and fly in circles when the radars turn off, and wait for them to turn back on. Since radar antennae can't be well-armored, ARMs tend to trade off large warheads for high speeds, so the SAM launchers and anti-aircraft guns they're trying to hit have trouble shooting them down on the way in. Not all ARMs have small warheads, though -- some Russian air-launched cruise missiles are capable of carrying anti-radar seekers and nuclear warheads (so I guess anti-radiation missiles can have something to do with nuclear weapons after all). Passive measures like chaff can't defend against ARMs, but advanced active ECM presumably can.
  • In a laser guided missile system, a platform "designates" or "paints" the target with a (visible or infrared) laser beam, which the missile homes on. Frequently, in such systems, the designating platform is not the launching platform; unlike a radar, it's tricky to determine the source of a laser, so a relatively stealthy platform (like a plane that isn't emitting radar, or even an infantryman) can paint the target and call on a faraway launcher to send the missile in. This is also a popular guidance system for unpowered guided weapons.
The design of the missile warhead is usually determined by the target it's intended to attack. Air targets are hard to hit, but relatively fragile, thus anti-aircraft missile warheads are relatively small, but are usually equipped with a proximity fuse in order to detonate when they get close to the target rather than needing a direct hit. Anti-surface missiles generally carry larger conventional warheads which detonate on impact or nuclear warheads.
Information obtained from multiple sources including the Federation of American Scientists Military Analysis Network (www.fas.org) and the PBeM wargame Salmon Alley (http://www.estarcion.com/kaleja/sa.html). Information on military hardware is usually compiled from a variety of once-classified, unclassified, and just-plain-lying sources, and as such is fragmentary and inconsistent; please do not use this information to plan a war.

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