Missile guidance methods are used to guide amissile or aguided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness. Guidance systems improve missile accuracy by improving its Probability of Guidance (Pg).[1]
These guidance technologies can generally be divided up into a number of categories, with the broadest categories being "command", "homing", and "non-homing" guidance.[2] 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 of the launch aircraft and gravity for propulsion.
In the late 1880s,Jules Verne featured in his fiction books a rocket-powered missile with a target seeker, proximity fuze, and a warhead.[3]
DuringWorld War I, various nations experimented with guided missiles. Systems were developed for thefirst powered drones byArchibald Low (the father of radio guidance).[4] In France, Pierre Lorin developed a radio-powered missile with the intention of using it to strikeBerlin, but the French military was not interested in the project.[3]
DuringWorld War II, guided missiles were developed as part of the GermanV-weapons program.[2] At the time, Germany was limited by theTreaty of Versailles from developing conventional weapons, so they focused their efforts on new weapons outside the provisions of the treaty. Guided missiles were one such avenue of development. The AmericanArmy Air Forces had dozens of various programs experimenting with "flying bombs, glide bombs, and vertical bombs".[3]
Following World War II, in the winter of 1946,President Harry S. Truman ordered funding cuts from programs across the American armed forces, particularly targeted atresearch and development. In response to these cuts, which became known as "the black Christmas of 1946", theAir Staff reduced the guided missile budget by 55%. By the end of March of the following year, 10 guided missile projects had been cancelled and 19 remained.[3]
Upon the opening of theKorean War, American development of guided missiles was rapidly accelerated.
The first U.S. ballistic missile with a highly accurate inertial guidance system was the short-rangePGM-11 Redstone.[5]
Guidance systems are divided into different categories according to whether they are designed to attack fixed or moving targets. The weapons can be divided into two broad categories:Go-onto-target (GOT) andgo-onto-location-in-space (GOLIS) guidance systems.[6][7] 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 GOT missile takes while attacking a moving target is dependent upon the movement of the target.
| Category | Method of navigation | Sensors | Examples |
|---|---|---|---|
| Active homing |
|
| AIM-120 AMRAAM,R-77 |
| Semi-active homing |
|
| AIM-7 Sparrow,R-27R |
| Passive homing |
|
| FIM-92 Stinger,9K38 Igla |
| Command |
|
| MIM-104 Patriot,S-300P/PT |
| Beam rider (CLOS) |
|
| Seaslug,9K121 Vikhr |
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Homing guidance systems use sensors within the missile to sense the target and then use that information to generate control commands. Possible sensors include radar, infrared sensors, or light sensors. Homing missiles usually do not need to communicate with a ground station or other launch platform.[9] Homing guidance is useful in situations where afire-and-forget missile is needed.[2]
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, beam, or wire (seeWire-guided missile). Some missiles will use both command guidance and homing guidance at different phases of flight. Commonly missiles will use command guidance during the boost and middle phases of flight, then switch to homing guidance in the terminal phase.[2]
Command guidance is a system in which the guidance commands originate outside the missile.[2] Command guidance requires two links between the missile and the transmitter: the information link and the command link. The information link allows the controller to determine the position of the missile, and the command link allows commands to be transmitted from the controller to the missile. In some systems, both links are accomplished using the same tracking unit (i.e. radar, optical, laser, or infrared), but others have a distinct tracking unit for each system.[9][2] A disadvantage of command guidance is it requires the target to be illuminated by an external energy source, from the launcher or elsewhere. This can alert the target, which could then conductevasive maneuvers orSEAD.[2]
Beam riding missiles use an electromagnetic beam of some sort, typicallyradar orlaser, which is pointed at the target. Sensors on the rear of the missile receive the beam and the control systems of the missile use this information to calculate steering commands, attempting to keep the missile in the beam.[2] These are sometimes considered distinct from command guidance.[9]
Beam riding systems are oftenSACLOS, but do not have to be; in other systems the beam is part of an automated radar tracking system. A case in point is the later versions of theRIM-8 Talos missile as used by theUnited States largely during theVietnam War – 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 accurateSARH 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.
An advantage of beam riding is multiple missiles may be launched at once using the same beam, due to the reduced tracking load on the launcher.[2] Beam riding suffers from the inherent weakness of inaccuracy with increasing range as the beam spreads out. Laser beam riders are more accurate in this regard, but they tend to be shorter range, and the laser beam can be degraded by bad weather. SARH becomes more accurate with decreasing distance to the target, so the two systems are complementary.[10]
Whatever the mechanism used in a GOLIS guidance system is, it must contain preset information about the target. These systems' main characteristic is the lack of a target tracker. The guidance computer and the missile tracker are located in the missile. The lack of target tracking in GOLIS necessarily implies navigational guidance.[11]
Navigational guidance is any type of guidance executed by a system without a target tracker. The other two units are on board the missile. These systems are also known as self-contained guidance systems; however, they are not always entirely autonomous due to the missile trackers used.
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 of the guidance components (including sensors such asaccelerometers orgyroscopes) are contained within the missile, and no outside information (such as radio instructions) is used. An example of a missile using preset guidance is theV-2 rocket.[11][9]
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 usesolid statering 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 theAIRS found on the MX missile, allowing for an accuracy of less than 100 m at intercontinental ranges. Many civilian aircraft use inertial guidance using a 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.[5]
Inertial guidance is most favored for the initial guidance and reentry vehicles ofstrategic missiles, because it has no external signal and cannot bejammed.[2] Additionally, the relatively low precision of this guidance method is less of an issue for large nuclear warheads.
A variant of inertial guidance for engaging slow-moving targets ispredicted line of sight (PLOS), which flies the missile along a pre-calculated curved path to remain on the line of sight between launcher and target. Since PLOS missiles do not rely on onboard seekers or post-launch command links, they are immune to many countermeasures. This method is employed in anti-tank weapons such as theNLAW andFGM-172 SRAW.[12]
Astro-inertial guidance, or stellar-inertial guidance, is asensor fusion-information fusion ofinertial guidance andcelestial navigation. It is usually employed onsubmarine-launched ballistic missiles. Unlike silo-basedintercontinental ballistic missiles, whose launch point does not move and thus can serve as areference, SLBMs are launched from moving submarines, which complicates the necessary navigational calculations and increasescircular error probable. Stellar-inertial guidance is used to correct small position and velocity errors that result from launch condition uncertainties due to errors in the submarine navigation system and errors that may have accumulated in the guidance system during the flight due to imperfectinstrument calibration.
The USAF sought a precision navigation system for maintaining route accuracy and target tracking at very high speeds.[citation needed]Nortronics,Northrop's electronics development division, had developed an astro-inertial navigation system (ANS), which could correctinertial navigation errors withcelestial observations, for theSM-62 Snark missile, and a separate system for the ill-fatedAGM-48 Skybolt missile, the latter of which was adapted for theSR-71.[13][verification needed]
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 fixedreference point from which to calculate that position makes this a potentially very effective means of improving accuracy.
In theTrident missile system this was achieved by a single camera that was trained to spot just one star in its expected position (it is believed[who?] 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.[14]
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 aradar altimeter 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 forcruise missile guidance, but is being supplanted byGPS systems and byDSMAC, 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 lacking in robustness that destruction of prominent buildings marked in the system's internal map (such as by a preceding cruise missile) upsets its navigation.[5]
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