From: mckinney@athena.mit.edu (Ethan L Mckinney)
Subject: Re: Missile types (was Re: TR-3a)
Organization: Massachusetts Institute of Technology
Date: Tue, 17 Nov 1992 19:09:26 GMT

Andrew, what do you mean by "initial guidance"?  Do you mean to say "inertial," 
ie., they fly on a pre-determined course with internal reference to acceleration
and velocity?

For the uninitiated, a guide to missile guidance procedures:

Beam-riding:  This was one of the earliest forms of guidance for anti-aircraft
missiles.  You point a radar beam directly at the target.  The missile has
receivers which "look" back at your radar and keep the missile within the beam. 
It is very resistant to jamming, but the width of the radar beam, especially at
long range, seriously degrades accuracy.  Thus, a big warhead is necessary to get
any chance of a hit.  Also, the missile tends to fly an inefficient "pure pursit"
path (ie., right at the target rather then "cutting it off at the path" by
flying to where the target will be.)  Some later BR's allowed the guidance beam
to be slewed independently, so the missile could be flown on a ballistic "up and
over" path to long-range targets or to lead a crossing target.  Early BR missiles
included the AA-1 Alkali and the Talos SAM (early versions).

Passive-homing:  The missile seeks on emissions from the target.  Against
aircraft, this means infra-red radiation (heat).  ARMs seek radar waves from
operating radars (usually on the ground).  The main limitation here is that you
have to fly directly at the target, because passive-homing gives no indication of
range.  Also, the target can sometimes change its emissions and cause the missile
to lose its lock.  Classic example -- AIM-9 Sidewinder.  Also, HARM.

Semi-active homing:  The missile homes on emissions *reflected* from the target
and originally emitted by a friendly designator.  This means radar-homing in
anti-aircraft missiles, but this general type would include laser-designation
against ground targets.  Examples:  most radar-homing missiles, Hellfire
laser-guided anti-tank missile, Laser-guided bombs.

Active-homing:  The missile homes on emissions reflected from the target and
originally emitted by a designator mounted on the missile itself.  Thus, the
missile ca nbe fired and then ignored, once the seeker head goes active.  There
is no need for the firer to keep designating the target, so it can run away, fire
at another target, take evasive action, or do whatever it needs to.  Example: 
AIM-54 Phoenix, AIM-120 AMRAAM, many modern anti-ship missiles.

Command-guidance:  The launcher sends radio signals to the missile telling it
where to fly and when to turn, etc.  The target and the missile are usually both
tracked by radar.  Accuracy is not very high with this system.  Classic example: 
SA-2.  TOW anti-tank missile is actually rather similar in operation, although it
sends the signals down an unraveling wire.

Track-via-missile:  The best of semi-active and command-guidance combined.  The
launch platform illuminates the target with radar, a seeker in the head of the
missile sees the reflection, sends messages to the launcher about what it sees,
the launcher sends messages back about where to fly.  You can put a lot more
guidance electronics on the launcher than you can on a missile!  At the same
time, the seeker in the head of the missile is a lot closer to the target, so
angular errors are smaller.  This is the system Patriot uses.

Laser-guidance:  Like beam-riding, eacept the missile flies up a laser beam, and
thus angular errors are many times smaller, since laser beams can be really tight.

Optical guidance:  Someone looks at the taget through a sight, which is connected
to electronics which determine what direction and elevation the sight is looking.
 The missile is then controlled much like a command-guided missile.

Questions?  Did I leave anything out?
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