Precision-guided munition
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A precision-guided munition (PGM) is a weapon that adjusts its flight path to hit a specific target. It may be self-powered, unpowered with gravity and launcher momentum as its energy source but with aerodynamic control surfaces, or unpowered with its launch energy coming from an artillery propellant, or combinations of these families. Unfortunately, it should be noted that while accuracy and precision have different technical meanings in engineering and the sciences, a military convention has emerged in which an "accurate" weapon hits within 10 meters of its desired impact point, while a "precision" weapon hits within 3 meters.[1] This CEP also assumed a nonmoving target. The purpose of precision guidance is to make the weapon fly to the most desirable impact point on the target, which requires the munition to be able to change its flight path after release. Earlier approaches concentrated on making increasingly accurate bombsights, to have the weapon released at the point from which it should hit the ideal impact point. This reached its practical limit during the Vietnam War; as long as the bomb could not correct for wind and other effects after it was dropped, increasing the precision of the release point made no difference to the precision of the impact point. The effect of precision guidance is to redefine an axiom of warfare, the principle of mass. With conventional weapons, this principle required sending a large number of weapons, or using weapons of very great power, to be sure a target was destroyed. While a Second World War bombing raid might need to drop thousands of bombs to be sure a factory was destroyed, because individual bomb impact points could vary by kilometers, current technology may require only one bomb, if that bomb, of relatively low yield, can hit reliably between 1 and 3 meters from the center of the target. Indeed, some weapons, of this precision, do not even use an explosive. Filled with concrete or other dense material, they deliver enough kinetic energy to destroy the target — and cause minimal collateral damage near the target. Precision is a force multiplier. As part of a system of controlling close support to ground forces, it can change the mission of front-line soldiers and marines from overpowering solely with their own skills, to having potent resources under their control.
Effects-based targetingRecent weapons have made precision alone an inadequate criterion for judging the effects of weapons. When an unguided, or perhaps wind-corrected, cluster munition releases individually guided submunitions, is that system precision-guided? When the warhead has an area effect, such as running conductive carbon filaments across electrical power lines, shorting out a large area, is that a precision effect? Yet another problem is targeting, which takes place long before weapons release. A bomb precisely delivered to the wrong target is useless, or worse than useless if it hits a sensitive civilian installation. If, however, a bomb can hit the control room of an oil refinery, or the communications console of an air defense command post, fewer and smaller munitions would be needed to disrupt the target. Precision guidance is an incredibly useful feature, which can minimize unintended death and destruction. It should be remembered, however, that the munition is part of a larger system involving the intelligence (information gathering)|problem of finding targets, balancing the risks and benefits of striking them, selecting the munition most likely to cause the desired effect, and creating weapons when none exist with the desired effect. HistoryFor many years, weapons designers emphasized making the launch more accurate: better bombsights for airplanes and better gunsights for projectile weapons. One of the first attempts at precision was the Norden bombsight, a WWII analog device which could achieve a 1000 foot circular error probability (CEP) — if the aircraft could fly straight and level, regardless of the antiaircraft bursts and fighters around it, for 30 seconds.[1] As early as WWII, there were munitions whose flight could be adjusted by an operator, such as the German Fritz-X. Tactical delivery of such weapons, however, still required a human operator who could see both the weapon and target, and then mentally translate the relative positions into steering commands to the munition. The breakthrough came when the munition included a sensor such that the operator could designate the impact point and have the bomb fly to it. The earliest of such approaches used television cameras, but these necessarily had a narrow field of view, so the operator might have trouble acquiring the target, or reacquiring it if the bomb drifted. Using a laser designator to show the impact point to the weapon made for much greater precision for targets such as bridges. Improved television-guided bombs offered wide and narrow view, to improve the situational awareness of the operator.
By the Gulf War, accuracy from medium altitude improved somewhat, but the inherent inaccuracies of "dumb" munitions obviated the "smarter" delivery platform. Dumb bombs had reached their limit of utility against point targets.[3] In 1933, the former chief of the US Army Air Corps, Major General James E. Fechet, wrote:
Over sixty years later, COL Phillip Meininger, wrote:
World War Two precision guided weapons were "man-in-the-loop", sometimes literally so, as opposed to increasingly autonomous precision guidance made possible by computers, sensors, space-based navigation, and artificial intelligence. The first air-launched weapon[6] using machine-assisted intelligent guidance may well have been the Mark 24 torpedo, which used acoustic homing to force the German submarine, U-456, to the surface, where it was sunk by convoy escorts. [7] Anti-shipping missile technology first appeared on 9 September 1943, a German Fritz-X radio-guided anti-shipping missile (actually a rocket-boosted guided bomb) dropped from a Dornier Do 217 bomber sank the modern Italian battleship Roma as it steamed towards Gibraltar. [3] The first real example of the potential of employment of large numbers of precision-guided weapons are not often considered as such, but the many Japanese kamikaze and other weapons guided by a pilot who would die with the weapon was just that. Kamikaze attacks, especially at the Battle of Okinawa, were an example of the potential of anti-ship missiles.
GuidanceAbstract paradigmsPGMs, in general, follow one of two guidance paradigms:
Guidance mechanismMany PGMs combine more than one guidance mechanism
Some of the methods, such as television feedback to the operator of a guided bomb or missile, can be considered telepresence applications. The human stress of an operator experiencing a sense of "crashing and burning" should not be ignored as a psychological factor for operators. Classification by launcher and target locationThere are several ways to classify guided missiles. One of the most basic covers the launching platform and the target location, "location" here being agnostic to GOLIS or GOT. Each one of these types has further subdivisions, such as range, mobility, guidance, payload, etc.
Warhead typesThere have also been revolutions in the nature of warheads. With a unitary warhead, the entire payload triggers as a unit. Warheads with cluster submunition payloads can cover a large area with small warheads, possibly independently guided, which do not concentrate too much force than is needed for the target. There are significant humanitarian concerns with early cluster munitions, which had a high failure rate, unintentionally producing an antipersonnel minefield that could affect civilians. Newer submunitions are being designed either to function at once, or to become inert. Other changes include the nature of the casing and shape. Another is the fuze that triggers the payload, and a third is the payload inside the warhead. There were early demonstrations of radically new weapon payloads in World War Two, but, for various reasons, did not become a part of general air warfare until much later. UnitaryThe payload can be designed to maximize blast pressure, to produce a mixture of blast and fragmentation effect, or to produce a maximum amount of heat energy. With more powerful conventional explosives than were available in World War II, as well as precision guidance, a bomb in the 2,000 to 5,000 pound class can do more damage than a WWII bomb ten times its weight. If greater effects are needed, precision guidance can open a hole with one bomb, and then direct a second (and more) to the bottom of the first bomb's crater, digging even deeper. Given this effectiveness of explosive, smaller bombs become practical. The GBU-39 Small Diameter Bomb[8] is a 250-pound class munition effective against a majority of hardened targets previously vulnerable only to 2,000 or 5,000-pound class munitions like the GBU-28 Bunker Buster. [9] Containing only 50 pounds of explosive, basic assumption is that if a small bomb can detonate on a dictator's desk chair, not much force is necessary. The challenges are more penetration and guidance than yield. In the past, it was assumed that the force of a nuclear weapon would be needed to ensure destruction. Not only precision of delivery at a given set of coordinates, but at a depth or physical environment becomes part of the new approach. Traditional fuzes were limited to a few options. Simple impact was the most basic, which triggered the payload when the nose contacted the target. Some warheads had an extended nose probe, or possibly a radar, to cause them to detonate slightly above the surface. Other radar fuzes could produce an air blast. Other traditional variants, used with armor-piercing cases, would be base detonating, so that the warhead would trigger only after the tail hit the target. This could add a few milliseconds of penetration, which did allow slightly greater penetration before detonation. Mechanical fuzes, however, were much more limited in controlling penetration into a hard target than are radar proximity fuzes that do not have to stand the shock of impact. New fuze designs can count the number of floors they penetrate as they travel through a building, so they will detonate in precisely the right room, defined in three dimensions. [10] The Hard Target Smart Fuze (FMU-157/B HTSF) has three operating modes, and a backup programmable time delay, programmable in millisecond increments up to 250 msec.
While the initial deployments of this technology were on guided bombs, it is being implemented on the AGM-86 ALCM conventional-warhead air-launched cruise missile. There remain, however, targets where a different type of explosive effect is needed. Against a hardened but spread-out target, such as an underground factory, the type of shock wave conceived by the British engineer, Barnes Wallis, starting with the Tallboy bomb, were harbingers of the realization that hardening and burial may not be adequate defense against penetrating weapons. The larger Grand Slam bomb devastated extremely hardened targets, such as U-boat pens. Still, Grand Slam and Tall Boy were "dumb" bombs, delivered by the specialists of Royal Air Force 617 Squadron, the "Dam Busters". These bombs were not always aimed directly at the target. Against a bridge, for example, they were aimed beside it, to produce a cavity in the earth, which would then implode, reflecting earthquake-like shock waves against the distributed target. Large blast bombs, now using precision guidance, such as the GBU-43 Massive Ordnance Air Blast (MOAB), also reflect some of the capabilities of the earthquake bombs. These are useful for situations where a wide pressure wave is needed, as for clearing minefields, or destroying spread-out but "soft" structures. Image:MassiveOrdnanceBlastBomb.jpg| thumb | GBU-43/B Massive Ordnance Air Blast bomb While the MOAB uses a solid explosive, explosives#fuel-air explosive|fuel-air explosives and explosives#Thermobaric explosion|thermobaric explosives also are ways to get much greater power from physically smaller bombs. Perhaps the key missing ingredient in 1945 was the range of intelligence sensors (e.g., Electro-optical MASINT#Spectroscopic MASINT|spectroscopic MASINT, with Geophysical MASINT#Gravitimetric MASINT|gravitimetric MASINT on the horizon) that can locate hidden targets, followed closely by precision guidance to direct the penetrating weapon most effectively. Precision guidance, deep penetration, and advanced target acquisition are a revolutionary leap when joined. Guidance can be sufficiently precise that for some applications, the kinetic energy of a concrete-filled bomb, carrying no explosive, still can destroy the target. There are an assortment of experiments using ballistic missiles without nuclear or conventional explosives, delivering a solid mass to a point target, or bundles of metal rods for an area target. The speed of an incoming ballistic missile gives more kinetic energy than would be released by conventional explosives. ClusterAll of the types below describe the submunitions ejected by the main cluster munition payload of the weapon. Not all types are intended to be hazardous to people; the types marked with an asterisk do have the danger of creating antipersonnel minefields.
Recent experienceDuring the Gulf War, with less precise weapons than are fielded today, "Most Republican Guard divisions outside Baghdad were not reduced in number by 50% (as some reports at the time claimed) but they were reduced to only 20% of their original combat efficiency by the bombing. With a thousand Coalition planes in the sky, coupled with a number of Apache and Black Hawk helicopters, and thousands of munitions directed to precise locations by ground spotters, the U.S. infantry was able to obtain the auxiliary power of several traditional armoured divisions.">[11] References
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