Missile guidance
Overview
 
Missile guidance refers to a variety of methods of guiding a missile
Missile
Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

 or a guided bomb
Guided bomb
A guided bomb is a precision-guided munition intended to precisely hit a specific target, and to minimize damage to things other than the target....

 to its intended target. The missile's target accuracy is a critical factor for its effectiveness. Guidance systems improve missile accuracy by improving its "Single Shot Kill Probability" (SSKP), which is part of combat survivability calculations associated with salvo combat model
Salvo combat model
The salvo combat model provides a mathematical representation of missile battles between modern warships. It was developed by Wayne Hughes at the U.S. Naval Postgraduate School in Monterey . The salvo model describes the basic elements of modern missile combat in a very simple way...

.

These guidance technologies can generally be divided up into a number of categories, with the broadest categories being "active," "passive" and "preset" guidance.
Encyclopedia
Missile guidance refers to a variety of methods of guiding a missile
Missile
Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

 or a guided bomb
Guided bomb
A guided bomb is a precision-guided munition intended to precisely hit a specific target, and to minimize damage to things other than the target....

 to its intended target. The missile's target accuracy is a critical factor for its effectiveness. Guidance systems improve missile accuracy by improving its "Single Shot Kill Probability" (SSKP), which is part of combat survivability calculations associated with salvo combat model
Salvo combat model
The salvo combat model provides a mathematical representation of missile battles between modern warships. It was developed by Wayne Hughes at the U.S. Naval Postgraduate School in Monterey . The salvo model describes the basic elements of modern missile combat in a very simple way...

.

These guidance technologies can generally be divided up into a number of categories, with the broadest categories being "active," "passive" and "preset" guidance. Missiles and guided bombs generally use similar types of guidance system, the difference between the two being that missiles are powered by an onboard engine, whereas guided bombs rely on the speed and height of the launch aircraft for propulsion.

History

The concept of missile guidance originated at least as early as World War I, with the idea of remotely guiding an airplane bomb onto a target. In World War II guided missiles were first developed, as part of the German V-weapons program.

Categories of guidance systems

Guidance systems are divided into different categories according to what type of target they are designed for - either fixed targets or moving targets. The weapons can be divided into two broad categories, Go-Onto-Target (GOT) and Go-Onto-Location-in-Space (GOLIS) guidance systems. A GOT missile can target either a moving or fixed target, whereas a GOLIS weapon is limited to a stationary or near-stationary target.
The trajectory that a missile takes while attacking a moving target is dependent upon the movement of the target. Also, a moving target can be an immediate threat to the sender of the missile. The target needs to be eliminated in a timely fashion in order to preserve the integrity of the sender. In GOLIS systems the problem is simpler because the target is not moving.

GOT systems

In every GOT system there are three subsystems:
  • Target tracker
  • Missile tracker
  • Guidance computer


The way these three subsystems are distributed between the missile and the launcher result in two different categories:
  • Remote Control Guidance: The guidance computer is on the launcher. The target tracker is also placed on the launching platform.
  • Homing Guidance: The guidance computers are in the missile and in the target tracker.

Remote control guidance

These guidance systems usually need the use of radars and a radio or wired link between the control point and the missile; in other words, the trajectory is controlled with the information transmitted via radio or wire.

System include
  • Command Guidance - The missile tracker is on the launching platform. These missiles are totally controlled by the launching platform that sends all control orders to the missile. The 2 variants are
  • Command to Line-Of-Sight (CLOS)
  • Command Off Line-Of-Sight (COLOS)
    • Line-Of-Sight Beam Riding Guidance (LOSBR) - The missile tracker is on board the missile. It has already some orientation capability, in order to fly inside the beam that the launching platform is using to illuminate the target. It can be manual or automatic.

Command to Line-Of-Sight (CLOS)

The CLOS system uses only the angular coordinates between the missile and the target to ensure the collision. The missile will have to be in the line of sight between the launcher and the target (LOS), correcting any deviation of the missile in relation to this line. Due to the amount of missiles that use this guidance system, they are usually are subdivided into four groups:
  • Manual Command to Line-Of-Sight (MCLOS), The target tracking and the missile tracking and control is performed manually. 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 or missiles against ships or the supersonic Wasserfall
    Wasserfall
    The Wasserfall Ferngelenkte Flakrakete , was a World War II guided surface-to-air missile developed at Peenemünde, Germany.-Technical characteristics:...

     against slow-moving B-17 Flying Fortress bombers this system worked, but as speeds increased MCLOS was quickly rendered useless for most roles.
  • Semi-Manual Command to Line-Of-Sight (SMCLOS), The target tracking is automatic and the missile tracking and control is manual
  • Semi-Automatic Command to Line-Of-Sight (SACLOS), The target tracking is manual and the missile tracking and control is automatic. 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 considerably easier to operate. SACLOS is the most common form of guidance against ground targets such as tanks and bunkers.
  • Automatic Command to Line-Of-Sight (ACLOS), The target tracking, missile tracking and control are automatic.

Command Off Line-Of-Sight (COLOS)

This guidance system was one of the first to be used and still is in service, mainly in anti-aircraft missiles. In this system, the missile tracker and the target tracker can be oriented in different directions. The guidance system ensures the interception of the target by the missile by locating both in space. This means that they will not rely on the angular coordinates like in CLOS systems. They will need another coordinate which is distance. To make it possible, both target and missile trackers have to be active. They are always automatic and the radar has been used as the only sensor in these systems. The SM-2MR Standard is inertially guided during its mid-course phase, but it is assisted by a COLOS system via radar link provided by the AN/SPY-1 radar installed in the launching platform.

Line-Of-Sight Beam Riding Guidance (LOSBR)

LOSBR uses a "beam" of some sort, typically radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

, radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 or laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

, 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
SACLOS
SACLOS is an acronym for Semi-Automatic Command to Line of Sight, a second-generation method of missile guidance. In SACLOS, the operator has to continually point a sighting device at the target while the missile is in flight...

, but do not have to be; in other systems the beam is part of an automated radar tracking system. A case in point is later versions of the RIM-8 Talos
RIM-8 Talos
The Bendix RIM-8 Talos was a long-range naval surface-to-air missile, and was among the earliest surface-to-air missiles to equip United States Navy ships. The Talos used radar beam riding for guidance to the vicinity of its target, and semiactive radar homing for terminal guidance...

 missile as used in Vietnam - the radar beam was used to take the missile on a high arcing flight and then gradually brought down in the vertical plane of the target aircraft, the more accurate SARH
Sarh
Sarh is the third largest city in Chad, after N'Djamena and Moundou. It is the capital of Moyen-Chari region and the department of Barh Köh. It lies 350 miles south-east of the capital Ndjamena on the Chari River...

 homing being used at the last moment for the actual strike. This gave the enemy pilot the least possible warning that his aircraft was being illuminated by missile guidance radar, as opposed to search radar. This is an important distinction, as the nature of the signal differs, and is used as a cue for evasive action.

LOSBR suffers from the inherent weakness of inaccuracy with increasing range as the beam spreads out. Laser beam riders are more accurate in this regards, but are all short-range, and even the laser can be degraded by bad weather. On the other hand, SARH becomes more accurate with decreasing distance to the target, so the two systems are complementary.

Active homing

Active homing uses a radar system 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" of its own centerline to guide itself. Radar resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...

 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
Fire-and-forget
Fire-and-forget is a third-generation method of missile guidance. The military uses the term for a type of missile which does not require further guidance after launch such as illumination of the target or wire guidance , and can hit its target without the launcher being in line-of-sight of the...

" air-to-air missile systems such as AMRAAM and R-77

Semi-active homing

Semi-active homing 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
Sarh
Sarh is the third largest city in Chad, after N'Djamena and Moundou. It is the capital of Moyen-Chari region and the department of Barh Köh. It lies 350 miles south-east of the capital Ndjamena on the Chari River...

 is by far the most common "all weather" guidance solution for anti-aircraft systems, both ground and air launched.

It has the disadvantage for air-launched systems that the launch aircraft must keep moving towards the target in order to maintain radar and guidance lock. This has the potential to bring it within range of shorter-ranged IR-guided missile systems, an important consideration now that "all aspect" IR missiles are capable of "kills" from head on, something which did not prevail in the early days of guided missiles. For ships and mobile or fixed ground-based systems, this is irrelevant as the speed (and often size) of the launch platform precludes "running away" from the target or opening the range so as to make the enemy attack fail.

SALH
Laser guidance
Laser guidance is a technique of guiding a missile or other projectile or vehicle to a target by means of a laser beam. Some laser guided systems utilise beam riding guidance, but most operate more similarly to semi-active radar homing . This technique is sometimes called SALH, for Semi-Active...

 is a similar system using a laser as a signal. However, most laser-guided weapons employ a turret-mounted laser designator which increases the launching aircraft's ability to maneuver after launch. How much maneuvering can be done by the guiding aircraft will depend on the turret field of view and the systems ability to maintain a lock-on while maneuvering. As most air-launched, laser-guided munitions are employed against surface targets the designator providing the guidance to the missile need not be the launching aircraft; designation can be provided by another aircraft or by a completely separate source (frequently troops on the ground equipped with the appropriate laser designator).

Passive homing

Infrared homing
Infrared homing
Infrared homing refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track and follow it. Missiles which use infrared seeking are often referred to as "heat-seekers", since infrared is just below the...

 is 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 "heat seeking".

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 AGM-65 Maverick
AGM-65 Maverick
The AGM-65 Maverick is an air-to-ground tactical missile designed for close-air support. It is effective against a wide range of tactical targets, including armor, air defenses, ships, ground transportation and fuel storage facilities....

, 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
Camouflage
Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...

 can render them unable to "lock on".

Retransmission homing

Retransmission homing, also called Track Via Missile(TVM), is a hybrid between command guidance
Command guided
Command guidance is a type of missile guidance in which a ground station or aircraft relay signals to a guided missile via radio and tell the missile where to steer in order to intercept its target. Additionally, it is possible to send a command to the missile to detonate, even if the missile...

, semi-active radar homing
Semi-active radar homing
Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...

 and active radar homing
Active radar homing
Active radar homing is a missile guidance method in which a guided missile contains a radar transceiver and the electronics necessary for it to find and track its target autonomously...

. 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.

GOLIS systems

Whatever the GOLIS guidance system, it must contain preset information about the target. These systems' main characteristic is the lack of target tracker. The guidance computer and the missile tracker are located in the missile. There is only one type of guidance system of this kind: Navigational Guidance.

Navigational guidance is any type of guidance executed by a system without target tracker. The other two units are on board the missile. These systems are also known as Self Contained Guidance Systems, however they're not always entirely autonomous due to the missile trackers used. They are subdivided function of their missile tracker's type:
  • Entirely autonomous - Systems where the missile tracker is not dependent of any navigation external source, and can be divided in:
  • Inertial Guidance
  • With gyro-stabilized platform
  • With strapdown platform
  • Preset Guidance

  • Dependent on natural sources - Navigational guidance systems where the missile tracker depends of some external source that is provided by nature:
  • Celestial Guidance
  • Terrestrial Guidance
  • Topographic Reconnaissance (Ex: TERCOM)
  • Photographic Reconnaissance (Ex: DSMAC)
  • Magnetic Guidance

  • Dependent on artificial sources - Navigational guidance systems where the missile tracker depends of some external source that is provided by any artificial means:
  • Satellite Navigation
  • Global Positioning System (GPS)
  • GLObal NAvigation Satellite System (GLONASS)
  • Hyperbolic Navigation
  • DECCA
  • LORAN C

Inertial guidance

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
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 ring laser gyros that are accurate to within metres over ranges of 10,000 km, and no longer require additional inputs. Gyroscope development has culminated in the AIRS
Advanced Inertial Reference Sphere
The Advanced Inertial Reference Sphere, or AIRS, is a highly accurate inertial guidance system designed for use in the LGM-118A Peacekeeper ICBM which was intended for precision nuclear strike against Soviet missile silos.-Details:...

 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 gyroscope
Ring laser gyroscope
A ring laser gyroscope consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the internal standing wave pattern in response to angular rotation...

, 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.

Inertial guidance is most favored for the initial guidance and reentry vehicles of strategic missiles
Intercontinental ballistic missile
An intercontinental ballistic missile is a ballistic missile with a long range typically designed for nuclear weapons delivery...

, because it has no external signal and cannot be jammed
Countermeasure
A countermeasure is a measure or action taken to counter or offset another one. As a general concept it implies precision, and is any technological or tactical solution or system designed to prevent an undesirable outcome in the process...

. Additionally, the relatively low precision of this guidance method is less of an issue for large nuclear warheads.

Preset guidance

Preset guidance is the simplest type of missile guidance. From the distance and direction of the target, the trajectory of the flight path is determined. Before firing, this information is programmed into the missile's guidance system, which, during flight, maneuvers the missile to follow that path. All the guidance components (including sensors such as accelerometers or gyroscopes) are contained within the missile, and no outside information (such as radio instructions) is used. An example of a missile using Preset Guidance is the V-2 rocket
V-2 rocket
The V-2 rocket , technical name Aggregat-4 , was a ballistic missile that was developed at the beginning of the Second World War in Germany, specifically targeted at London and later Antwerp. The liquid-propellant rocket was the world's first long-range combat-ballistic missile and first known...

.

Celestial guidance

Celestial guidance was first used in the American Snark missile (Nortronics stellar-inertial guidance) first flown on 06/08/1953. It uses star positioning to fine-tune the accuracy of the inertial guidance system after launch. As the accuracy of a missile is dependent upon the guidance system knowing the exact position of the missile at any given moment during its flight, 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 separate stars to achieve this), if it was not quite aligned to where it should be then this would indicate that the inertial system was not precisely on target and a correction would be made.

Terrestrial guidance

TERCOM
TERCOM
Terrain Contour Matching, or TERCOM, is a navigation system used primarily by cruise missiles. It uses a pre-recorded contour map of the terrain that is compared to measurements made during flight by an on-board radar altimeter. A TERCOM system considerably increases the accuracy of a missile...

, 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
Radar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...

 on board. 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
Cruise missile
A cruise missile is a guided missile that carries an explosive payload and is propelled, usually by a jet engine, towards a land-based or sea-based target. Cruise missiles are designed to deliver a large warhead over long distances with high accuracy...

 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 missile to its target.

DSMAC is reputed to be so un-robust that destruction of prominent buildings in the internal map of the system (by preceding cruise missiles, among other things) will spoil navigation.

See also

  • Missile
    Missile
    Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...

  • List of missiles
  • Countermeasure
    Countermeasure
    A countermeasure is a measure or action taken to counter or offset another one. As a general concept it implies precision, and is any technological or tactical solution or system designed to prevent an undesirable outcome in the process...

  • Semi-active radar homing
    Semi-active radar homing
    Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer-range air-to-air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external ...

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