Precision-guided munitions (
PGMs,
smart munitions,
smart bombs,
guided bomb units or
GBUs) are guided weapons intended to maximize damage to the target while minimizing civilian damage. Because the damage effects of an
explosive weapon fall off with distance according to a
power law, even modest improvements in accuracy (and hence reduction in miss distance) enable a target to be effectively attacked with fewer or smaller bombs. Thus, even if some bombs miss, fewer air crews are put at risk and the harm to civilians is somewhat reduced. The creation of precision-guided munitions resulted in the renaming of older bombs as "
gravity bombs", "dumb bombs" or "iron bombs". A guided bomb differs from a
guided missile in that a bomb relies on the speed and height of the launch aircraft for propulsion, whilst a missile has an onboard engine.
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, using 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 Pacific, and the
US Navy's
Bat, primarily used in the Pacific Theater of WW II. In addition, the U.S. tested the rocket-propelled
Gargoyle; it never entered service.Japanese PGMs did not see combat in WW2.
The
United States Army Air Force experimented with radio-controlled remotely guided planes in
Operation Aphrodite, but had few successes; the German
Mistel (Mistletoe) "parasite aircraft" was no more effective.
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 weaponsIn World War Two, 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.
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Laser-guided weaponsIn 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 series of
encrypted 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 when 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/Electro-Optical Guided WeaponsPrecision 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 escape return fire.
Millimeter-wave radarThe 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.
