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. 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.
|“||Admiral James O. Ellis, former commander, U.S. Strategic Command, said "We've migrated the number of sorties it will take to hit a target to the number of targets that one sortie can strike. ||”|
Recent 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 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.
For 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.
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.
|War||Number of bombs||Number of Aircraft||CEP (in feet)|
|World War II||9,070||3,024||3,300|
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. In 1933, the former chief of the US Army Air Corps, Major General James E. Fechet, wrote:
In the past, wars’ slaughter has been largely confined to armed combatants. Soldier has slain soldier. Unfortunately, in the next, despite all peace time decrees and agreements, the principal effort will be directed at trade and manufacturing centers [sic]. Obviously the airman, riding so high above the earth that cities look like ant hills, cannot aim his deadly cargo at armed males. All below will be his impartial target.
Over sixty years later, COL Phillip Meininger, wrote:
Precision air weapons have redefined the meaning of mass ... The result of the trend towards ‘airshaft accuracy’ in air war is a denigration in the importance of mass. PGMs provide density, mass per unit volume, which is a more efficient measurement of force. In short, targets are no longer massive, and neither are the aerial weapons used to neutralise them. One could argue that all targets are precision targets—even individual tanks, artillery pieces, or infantrymen. There is no logical reason why bullets or bombs should be wasted on empty air or dirt. Ideally, every shot fired should find its mark.
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 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.  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. 
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.
The Kamikaze was the deadliest aerial anti-shipping threat faced by allied surface warfare forces in the war... Despite radar detection and cuing, airborne interception and attrition, and massive anti-aircraft barrages, a distressing 14 per cent of Kamikazes survived to score a hit on a ship; nearly 8.5 per cent of all ships hit by Kamikazes sank. As soon as they appeared, then, Kamikazes revealed their power to force significant changes in allied naval planning and operations, despite relatively small numbers. Clearly, like the anti-shipping cruise missile of a later era, the Kamikaze had the potential to influence events all out of proportion to its actual strength.
PGMs, in general, follow one of two guidance paradigms:
- Go onto location in space (GOLIS), where guidance directs the weapon to a specific set of geographical coordinates and whatever is located at those coordinates;
- Go onto target (GOT), in which the weapon recognizes a specific target, which may be moving, and adjusts its course based on sensor input or human commands, in order to hit that target.
Many PGMs combine more than one guidance mechanism
- Human command
- Terrain contour matching
- Radar guided
- Target characteristics
- against a land background
- against a water background
- against an air background with land or water below
- Radar source
- Target characteristics
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 location
There 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.
|Launching platform||Target location||Energy source||Missile type||Example|
|Surface||Surface||Rocket||surface-to-surface missile (SSM)anti-shipping missile, anti-radiation missile (ARM), cruise missile [Note 1]||U.S. LGM-30 Minuteman ICBM, German V-2 MRBM, Soviet SS-1 SCUD, French Exocet, U.S. BGM-109 Tomahawk|
|Surface||Air||Rocket||surface-to-air missile (SAM)||Soviet/Russian S-75 Dvina (NATO designation SA-2 and reporting name GUIDELINE), MIM-104 Patriot|
|Surface||Underwater||Rocket||surface-to-underwater missile (SUM)||Russian RPK-2 Viyuga (NATO designation SS-N-15 and reporting name STARFISH, U.S. RUM-139 Vertical Launch ASROC|
|Surface||Space [Note 2]||Rocket||anti-ballistic missile (ABM), anti-satellite missile||U.S. RIM-161 Standard SM-3, Israeli Arrow|
|Air||Air||Rocket, jet or both||air-to-air missile (AAM)||U.S. AIM-9 Sidewinder, Russian Vympel R-27, (NATO designation AA-10 ALAMO)|
|Air||Space||Rocket||anti-satellite missile||U.S. ASAT|
|Air||Surface||Rocket||air-to-surface missile (ASM), anti-shipping missile[Note 2], anti-radiation missile (ARM), cruise missile [Note 1]||U.K. Brimstone, U.S. AGM-88 HARM, Russian Raduga KSR-5 (NATO designation AS-6 KINGFISH)|
|Air||Surface||Gravity||guided bomb||U.S. Joint Direct Attack Munition (JDAM), AGM-154 Joint Standoff Weapon (JSOW)|
|Air||Surface||Cannon launch||guided shell||U.S. M712 Copperhead, M982 Excalibur|
|Air||Underwater||Gravity, then underwater method (propeller, pump jet)||air-dropped homing torpedo||U.S. Mark 50 torpedo|
|Underwater||Surface||Ballistic missile||submarine-launched ballistic missile||U.S. and U.K. UGM-133 Trident D5, French M45|
|Underwater||Surface||Rocket or jet||anti-shipping missile, cruise missile:||AGM-84 Harpoon; Exocet; BGM-109 Tomahawk|
|Underwater||Underwater||Rocket in air; torpedo pumpjet or propeller||underwater-to-underwater missile (UUM)||U.S. UUM-44 SUBROC, Russian RPK-2 Viyuga (NATO designation SS-N-15 and reporting name STARFISH|
- Note 1: There are additional subcategories based on subcategories of the GOT paradigm. Anti-shipping missiles must be able to identify a target against the background of a moving ocean surface, anti-radiation missile must track a source of electromagnetic radiation such as a radar transmitter, and some types of cruise missiles using terrain contour mapping (TCM) must identify the path to a target by matching the land surface to a precise topographic map.
- Note 2: anti-ballistic missiles may actually intercept in space, in the upper atmosphere, or lower atmosphere. For this chart, the types are all considered "surface-to-space", but the specific articles will be for anti-ballistic missile or anti-satellite missile. Some missiles, such as the SM-3, have both capabilities. A given ABM may be able to be able to intercept the slower, shorter-ranged ballistic missiles, but not ICBMs.
There 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.
The 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 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.  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.  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.
- hard-layer detection: triggers a programmed time after contacting the surface
- void detection: counts the number of air spaces (e.g., an office on a floor)
- depth of burial: combines information on time after impact and number of voids with programmed initial delay, so that a timer might start only after the main hard surface was penetrated. These techniques, along with an even more reinforced case to allow greater penetration, was used in Operation Allied Force in Kosovo in 1999.
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.
Perhaps the key missing ingredient in 1945 was the range of intelligence sensors (e.g., spectroscopic MASINT, with 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.
All 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.
- Electronic warfare
During 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.">
- Sine, Jack (Spring 2006), "Defining the “Precision Weapon” in Effects-Based Terms", Air & Space Power Journal
- "Precision Engagement Redefining Warfare: Admiral Jim Ellis", Defender: Spotlight on National Defense Technologies: 2-3, 8, 2004
- Hallion, Richard P. (1995), Precision Guided Munitions and the New Era of Warfare, Air Power Studies Centre (Australia), APSC Working Paper No. 53
- Fechet, James E. (1933), Flying, Williams & Wilkins,in cooperation with The Century of Progress Exposition, at 135
- Meininger, Phillip (1995), 10 Propositions Regarding Air Power, U.S. Air Force History and Museums Project
- The first operational homing torpedo was the submarine-launched German Falke
- "U-456 the First Victim of US Smart Weapon", Defender: Spotlight on National Defense Technologies: 1, 2004
- U.S. Air Force, GBU-39B Small Diameter Bomb Weapon System
- Lane, Gary W. (April 2001), New Conventional Weapons, reducing reliance on a nuclear response toward aggressors, Air University, United States Air Force
- "FMU-157/B Hard Target Smart Fuze [HTSF]", Globalsecurity
- Coker, Christopher (2005), "The Second Gulf War And The Debate On Military Transformation", Pointer: Journal of the Singapore Armed Forces