Missile guidance
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Military missile systems use a variety of methods to guide the missile to its intended target. These can generally be classified into a number of categories, with the broadest categories being active vs. passive vs. preset.
Passive systems use signals generated by the target itself as a signal on which to "home in". A number of such systems have been developed, but by far the most common are sound in the case of torpedoes and infrared in the case of air-to-air missiles.
Active systems use some "input" sigal instead. One common sort of signal is a controller who watches the missile and sends corrections to its flight path. Another common system is to use radar signals or radio control. The semi-active radar homing is a crossover, homing passively on a reflected active radar signal generated by some other system.
Preset systems are used to attack targets at fixed locations, such as military bases and cities.
command guided, an active system in which signals are sent to the missile using radio control or some similar system. more specifically the term is typically used to describe anti-aircraft systems in which the tracking and guidance systems are all ground-based.
MCLOS, manually command to line of sight, the operator watches the missile flight and uses some sort of signaling system to command the missile back into the straight line between the operator and the target (the "line of sight"). Typically useful only for slower targets where significant "lead" is not required. MCLOS is a subtype of command guided systems.
SACLOS, semi-automatic command to line of sight, is similar to MCLOS but some automatic system positions the missile in the line of sight while the operator simply tracks the target. SACLOS has the advantage of allowing the missile to start in a position invisible to the user, as well as generally being considerable easier to operate.
beam riding, in which a "beam" of some sort, typically radio or laser, is pointed at the target and detectors on the rear of the missile keep it centered in the beam. Beam riding systems are often SACLOS, but don't have to be, in other systems the beam is part of an automated radar tracking system.
active radar homing uses a radar on the missile to provide a guidance signal. Typically electronics in the missile keep the radar pointed directly at the target, and the missile then looks at this "angle off" its own centerline to guide itself.
semi-active radar homing is an automated form of SACLOS in which the tracking radar provides a signal on which the missile homes. The signal does not nessessarily have to be pointed at the target, nor does the missile have to "ride" the beam. The missile itself is actually using passive guidance, homing on a signal reflected from the target. SALH is a similar system using a laser as a signal.
track-via-missile (TVM) is like a hybrid between command guidance, semi-active radar homing and active radar homing. The missile picks up radiation broadcast by the tracking radar which bounces off the target and relays it to the tracking station, which relays commands back to the missile.
Infrared homing, a passive system in which heat generated by the target is detected and homed on. Typically used in the anti-aircraft role to track the heat of jet engines, it has also been used in the anti-vehicle role with some success. This means of guidance is sometimes also referred to as "heatseeking".
inertial guidance uses sensitive measurement devices to calculate the location of the missile due to the acceleration put on it after leaving a known position. Early mechanical systems were not very accurate, and required some sort of external adjustment to allow them to hit targets even the size of a city. Modern systems use solid state ring gyros that are accurate to within metres over ranges of 10,000km, and no longer require additional inputs.
astro guidance, one form of additional information for early inertial guidance systems was to take a star fix while in flight.
TERCOM, for "terrain contour matching", uses altitude maps of the strip of land from the launch site to the target, and compares them with information from a radar altimeter onboard. More sophisticated TERCOM systems allow the missile to fly a complex route over a full 3D map, instead of flying directly to the target. TERCOM is the typical system for cruise missile guidance, but is being supplanted by GPS systems.