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
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
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"
- 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
- 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.