Missile guidance

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A guided bomb strikes an underground facility

Missile guidance technologies of missile systems use a variety of methods to guide a 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" signal 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. In the case of glide bombs missiles against ships or the supersonic Wasserfall against slow-moving B-17 bombers this system worked fine, but as speeds increased MCLOS was quickly rendered useless for most roles.

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. SACLOS is the most common form of guidance against ground targets such as tanks and bunkers.

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. Radar resolution is based on the size of the antenna, so in a smaller missile these systems are useful for attacking only large targets, ships or large bombers for instance. Active radar systems remain in widespread use in anti-shipping missiles, and in "fire and forget" air-to-air missile systems such as AMRAAM and R-77

semi-active radar homing (SARH) systems combine a radar receiver on the missile with a radar broadcaster located "elsewhere". Since the missile is typically being launched after the target was detected using a powerful radar system, it makes sense to use that same radar system to track the target, thereby avoiding problems with resolution or power. SARH is by far the most common "all weather" guidance solution for anti-aircraft systems, both ground and air launched. 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. Gyroscope development has culminated in the highly accurate 'baryllium baby' floating gyro found on the MX missile, allowing for an accuracy of less than 100m at intercontinental ranges. Many civilian aircraft use inertial guidance using the ring laser gyro, which is less accurate than the mechanical systems found in ICBMs, but which provide an inexpensive means of attaining a fairly accurate fix on location (when most airliners such as Boeing's 707 and 747 were designed, GPS was not the widely commercially available means of tracking that it is today). Today guided weapons can use a combination of INS, GPS and radar terrain mapping to achieve extremely high levels of accuracy such as that found in modern cruise missiles.

Stellar-inertial guidance was first used in the American Poseidon missile and uses star positioning to fine-tune the accuracy of the inertial guidance system after launch. As the accuracy of a ballistic missile is dependent upon the guidance system knowing the exact position of the rocket at any given moment during its boost phase, the fact that stars are a fixed reference point from which to calculate that position makes this a potentially very effective means of improving accuracy. In the Polaris system this was achieved by a single camera that was trained to spot just one star in its expected position (it is believed that the missiles from Soviet submarines would track two seperate stars to achieve this), if it was not quite alligned to where it should be then this would indicate that the inertial system was precisely on target and a corection would be made. Aparently this system is sufficiently sensative to detect stars in daylight.

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 and by DSMAC, Digital Scene-Matching Area Correlator, which employs a camera to view an area of land, digitizes the view, and compares it to stored scenes in an onboard computer to guide the missle to its target.

Contrast seekers use a television camera, typically black and white, to image a field of view in front of the missile, which is presented to the operator. When launched, the electronics in the missile look for the spot on the image where the contrast changes the fastest, both vertically and horizontally, and then attempts to keep that spot at a constant location in its view. Contrast seekers have been used for air-to-ground missiles, including the famous AGM-65 Maverick, because most ground targets can be distinguished only by visual means. However they rely on there being strong contrast changes to track, and even traditional camouflage can render them unable to "lock on".

Missile guidance

See also