Precision-guided munition

A precision-guided munition (PGM, smart weapon, smart munition) is a guided munition intended to precisely hit a specific target, and to minimize collateral damage.

Because the damage effects of explosive weapons decrease with distance due to an inverse cube law, even modest improvements in accuracy (hence reduction in miss distance) enable a target to be attacked with fewer or smaller bombs. Thus, even if some bombs miss, fewer air crews are put at risk and the harm to civilians and the amount of collateral damage may be reduced.

The advent of precision-guided munitions resulted in the renaming of older bombs as "gravity bombs", "dumb bombs", or "iron bombs".

Types


Recognizing the difficulty of hitting moving ships during the Spanish Civil War, the Germans were first to develop steerable munitions, using radio control or wire guidance. The U.S. tested TV-guided (GB-4), semi-active radar-guided (Bat), and infrared-guided (Felix) weapons.

Radio-controlled weapons
The Germans were first to introduce PGMs in combat, with the 1,400-kg (3,100-lb) Fritz X, to successfully attack the Italian battleship Roma in 1943 and the Henschel Hs 293 missile (also in use since 1943, but only against lightly armored or unarmored ship targets). The closest Allied equivalents were the 1000-lb (454-kg) AZON (AZimuth ONly), used in both Europe and the CBI theater, and the US Navy's Bat, primarily used in the Pacific Theater of World War II. In addition, the U.S. tested the rocket-propelled Gargoyle, which never entered service. Japanese PGMs - with the exception of the air-launched, rocket-powered, human-piloted Ohka suicide flying bomb - did not see combat in World War II.

Prior to the war, the British experimented with radio-controlled remotely guided planes laden with explosive, such as Larynx. The United States Army Air Forces used similar techniques with Operation Aphrodite, but had few successes; the German Mistel (Mistletoe) "parasite aircraft" was no more effective, guided by the human pilot flying the single-engined fighter mounted above the unmanned, explosive-laden twin engined "flying bomb" below it, released in the Mistel's attack dive from the fighter.

The U.S. programs restarted in the Korean War. In the 1960s, the electro-optical bomb (or camera bomb) was reintroduced. They were equipped with television cameras and flare sights, by which the bomb would be steered until the flare superimposed the target. The camera bombs transmitted a "bomb's eye view" of the target back to a controlling aircraft. An operator in this aircraft then transmitted control signals to steerable fins fitted to the bomb. Such weapons were used increasingly by the USAF in the last few years of the Vietnam War because the political climate was increasingly intolerant of civilian casualties, and because it was possible to strike difficult targets (such as bridges) effectively with a single mission; the Thanh Hoa Bridge, for instance, was attacked repeatedly with iron bombs, to no effect, only to be dropped in one mission with PGMs.

Although not as popular as the newer JDAM and JSOW weapons, or even the older laser-guided bomb systems, weapons like the AGM-62 Walleye TV-guided bomb are still being used, in conjunction with the AAW-144 Data Link Pod, on US Navy F/A-18 Hornets.

Infrared-guided weapons
In World War II, the U.S. National Defense Research Committee developed the VB-6 Felix, which used infrared to home on ships. While it entered production in 1945, it was never employed operationally.

Laser-guided weapons
In 1962, the US Army began research into laser guidance systems and by 1967 the USAF had conducted a competitive evaluation leading to full development of the world's first laser-guided bomb, the BOLT-117, in 1968. All such bombs work in much the same way, relying on the target being illuminated, or "painted," by a laser target designator on the ground or on an aircraft. They have the significant disadvantage of not being usable in poor weather where the target illumination cannot be seen, or where it is not possible to get a target designator near the target. The laser designator sends its beam in a coded series of pulses so the bomb cannot be confused by an ordinary laser, and also so multiple designators can operate in reasonable proximity.

Laser-guided weapons did not become commonplace until the advent of the microchip. They made their practical debut in Vietnam, where on 13 May 1972 they were used in the second successful attack on the Thanh Hoa Bridge ("Dragon's Jaw"). This structure had previously been the target of 800 American sorties (using unguided weapons) and was partially destroyed in each of two successful attacks, the other being on 27 April 1972 using Walleyes. That first mission also had laser-guided weapons, but bad weather prevented their use. They were used, though not on a large scale, by the British forces during the 1982 Falklands War. The first large-scale use of smart weapons came in 1991 during Operation Desert Storm when they were used by coalition forces against Iraq. Even so, most of the air-dropped ordnance used in that war was "dumb," although the percentages are biased by the large use of various (unguided) cluster bombs. Laser-guided weapons were used in large numbers during the 1999 Kosovo War, but their effectiveness was often reduced by the poor weather conditions prevalent in the southern Balkans.

There are two basic families of laser-guided bombs in American (and American-sphere) service: the Paveway II and the Paveway III. The Paveway III guidance system is more aerodynamically efficient and so has a longer range, however it is more expensive. Paveway II 500-pound LGBs (such as GBU-12) are a cheaper lightweight PGM suitable for use against vehicles and other small targets, while a Paveway III 2000-pound penetrator (such as GBU-24) is a more expensive weapon suitable for use against high-value targets. GBU-12s were used to great effect in the first Gulf War, dropped from F-111F aircraft to destroy Iraqi armored vehicles in a process referred to as "tank plinking."

Radar, infrared, IR imaging and electro-optical guided weapons
Precision guidance has been applied to weapons other than conventional bomb warheads. The Raytheon Maverick heavy anti-tank missile has among its various marks guidance systems such as electro-optical (AGM-65A), imaging infra-red (AGM-65D), and laser homing (AGM-65E). The first two, by guiding themselves based on the visual or IR scene of the target, are fire-and-forget in that the pilot can release the weapon and it will guide itself to the target without further input, which allows the delivery aircraft to manoeuvre to escape return fire. The Israeli Elbit Opher is also an I.R imaging "drop and forget" guided bomb that has been reported to be considerably cheaper than laser-homing bombs and can be used by any aircraft, not requiring specialized wiring for a laser designator or for another aircraft to illuminate the target. During NATO's air campaign in 1999 in Kosovo the new Italian AF AMX employed the Opher.

Millimeter-wave radar
The Lockheed-Martin Hellfire II light-weight anti-tank weapon in one mark uses the radar on the Boeing AH-64D Apache Longbow to provide fire-and-forget guidance for that weapon.

Satellite-guided weapons


Lessons learned during the first Gulf War showed the value of precision munitions, yet they also highlighted the difficulties in employing them — specifically when visibility of the ground or target from the air was degraded. The problem of poor visibility does not affect satellite-guided weapons such as Joint Direct Attack Munition (JDAM) and Joint Stand-Off Weapon (JSOW), which make use of the United States' GPS system for guidance. This weapon can be employed in all weather conditions, without any need for ground support. Because it is possible to jam GPS, the guidance package reverts to inertial navigation in the event of GPS signal loss. Inertial navigation is significantly less accurate; the JDAM achieves a published Circular Error Probable (CEP) of 13 m under GPS guidance, but typically only 30m under inertial guidance (with free fall times of 100 seconds or less).

The precision of these weapons is dependent both on the precision of the measurement system used for location determination and the precision in setting the coordinates of the target. The latter critically depends on intelligence information, not all of which is accurate. According to a CIA report, the accidental bombing of the Chinese embassy in Belgrade during Operation Allied Force by NATO aircraft was attributed to faulty target information. However, if the targeting information is accurate, satellite-guided weapons are significantly more likely to achieve a successful strike in any given weather conditions than any other type of precision-guided munition.

Advanced guidance concepts
Responding to after-action reports from pilots who employed laser or satellite guided weapons, Boeing developed a Laser JDAM (LJDAM) to provide both types of guidance in a single kit. Based on the existing JDAM configurations, a laser guidance package is added to a GPS/INS-guided weapon to increase its overall accuracy. Raytheon has developed the Enhanced Paveway family, which adds GPS/INS guidance to their Paveway family of laser-guidance packages. These "hybrid" laser and GPS guided weapons permit the carriage of fewer weapons types, while retaining mission flexibility, because these weapons can be employed equally against moving and fixed targets, or targets of opportunity. For instance, a typical weapons load on an F-16 flying in the Iraq War included a single 2,000-lb JDAM and two 1000-lb LGBs. With LJDAM, and the new Small Diameter Bomb, these same aircraft can carry more bombs if necessary, and have the option of satellite or laser guidance for each weapon release.

Cannon Launched Guided Projectiles
A Cannon-Launched Guided Projectile (CLGP), a precursor to modern PGMs, is fired from artillery, ship's cannon, or armored vehicles. Several agencies and organizations sponsored the CLGP programs. The United States Navy sponsored the Deadeye program, a laser-guided shell for its 5" guns and a program to mate a Paveway guidance system to an 8" shell for the 8"/55 caliber Mark 71 gun in the 1970s (Photo). Other Navy efforts include the BTERM, ERGM, and LRLAP shells.

The U.S. Army's MGM-51 Shillelagh missile can be considered a type of CLGP. Intended for use on the M551 Sheridan light tank, the Shillelagh missile was fired out of the Sheridan's cannon to provide robust anti-tank capability. The Army's M712 Copperhead laser guided artillery round was used in Desert Storm. Army CLGPs include the M982 Excalibur 155mm artillery shell, the XM395 Precision Guided Mortar Munition, and the XM1156 Precision Guidance Kit to refit existing 155mm shells with precision guidance, as the Air Force's JDAM program converts dumb bombs into precision munitions.

Guided small arms
Precision-guided small arms prototypes have been developed which use a laser designator to guide an electronically actuated bullet to a target. Another system in development uses a laser range finder to trigger an explosive small arms shell in proximity to a target. The U.S. Army plans to use such devices in the future.