Missile guidance

Missile guidance technologies of missile systems use a variety of methods to guide a missile to its intended target. The missile accuracy on its target was always the critical factor for its effectiveness. Guidance systems improve missile accuracy by improving its SSKP (Single Shot Kill Probability).

These can generally be classified into a number of categories, with the broadest categories being active vs. passive vs. preset.

Categories of guidance systems

Guidance systems are divided into different categories depending on what type of target they were designed for. These targets can be divided in two groups; fixed targets and moving targets. Thus, the categories for each target group are:

For moving targets: Go-Onto-Target (GOT) guidance systems
For fixed targets: Go-Onto-Location-in-Space (GOLIS) guidance systems

GOT systems are used against targets that present immediate danger, which demands a higher speed of response, in which its trajectory is dependent on the target. In GOLIS systems those constraints don't exist.

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 or the launcher result in two different categories:

Remote Control Guidance: The guidance computer is not in the missile but on the launcher. The target tracker is also placed on the launching platform.
Homing Guidance: The guidance computer is in the missile and also the target tracker.

Remote Control Guidance

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

This category can be divided in:

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. Depending on the way these orders are calculated, it is possible to divide in:

Command to Line-Of-Sight (CLOS)
Command Off Line-Of-Sight (COLOS)

Line-Of-Sight Beam Riding Guidance (LOSBR) - The missile tracker is onboard the missile. It has already some orientation capability, in order to fly inside the beam that the launching platform is illuminating the target. It can be manual or automatic.

Command Guidance

Command to Line-Of-Sight (CLOS)

This 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 that the missile does in relation to this line. Due to the amount of missiles that use this guidance system, they usually are subdivided in four groups:

Manual Command to Line-Of-Sight (MCLOS), in which 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 missiles against ships or the supersonic Wasserfall against slow-moving B-17 Flying Fortress bombers this system worked fine, but as speeds increased MCLOS was quickly rendered useless for most roles.

Semi-Manual Command to Line-Of-Sight (SMCLOS), in which the target tracking is automatic and the missile tracking and control is manual

Semi-Automatic Command to Line-Of-Sight (SACLOS), in which 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), in which the target tracking and the 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-air missiles. In this system, the missile tracker and the target tracker can be oriented in different directions. The guidance system ensures the interception missile-target by locate 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 midcourse phase, but it is assisted by a COLOS system via radar link provided by the SPY-1 radar installed in the launching platform.

Line-Of-Sight Beam Riding Guidance (LOSBR)

in which a "beam" of some sort, typically radio, radar 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. A case in point is later versions of the Talos 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 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 a missile guidance, as opposed to search, radar - 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.

Homing Guidance

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 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 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 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. 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.

Passive Homing

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".

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".

Retransmission Homing or Track Via Missile (TVM)

This is 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.

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 onboard 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 they're 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 ring laser gyros that are accurate to within metres over ranges of 10,000km, and no longer require additional inputs. Gyroscope development has culminated in the AIRS 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, 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.

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. [1]

Celestial Guidance

Celestial 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 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. Apparently this system is sufficiently sensitive to detect stars in daylight.

Terrestrial Guidance

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 missile to its target.

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