Precision-guided munition
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Precision-guided munitions (smart munitions or smart bombs) are self-guiding weapons intended to maximize damage to the target while minimizing "collateral damage". Because the damage effects of an explosive weapon scale as a power law with distance, quite modest improvements in accuracy (and hence reduction in miss distance) enables a target to be effectively attacked with fewer and/or smaller bombs. Thus, even if some bombs miss, fewer aircrews are put at risk and collateral damage is greatly reduced. The creation of precision-guided munitions resulted in the renaming of older bombs as "gravity bombs", "dumb bombs" or "iron bombs".
Ironically, despite the greater precision of precision-guided weapons, the failure of their guidance systems can cause greater damage than a miss from an ordinary bomb. Misses from older, unguided munitions are generally normally distributed around the aim point. Thus it can be assumed that the further you are from the target, the safer you are. On the other hand, most smart bomb misses are caused by system failures—a jammed steering fin, computer failure, loss of homing signal, etc. In this case, the weapon is actually more likely to miss the target by a very large distance, than by a small distance.
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Types of precision-guided munitions
Radio-controlled weapons
The United States Army began experimenting with radio-controlled remotely guided planes in the First World War, but the program had few successes. The first successful experiments with guided bombs were conducted during World War II when television-guided bombs, flare sighted bombs and other steerable munitions were developed. The Germans developed several types of steerable munitions, such as the Fritz X. There was even an attempt to produce a glider bomb that was released from a larger plane over the target, but the program stopped with the nuclear attacks in Japan.
Visually-guided weapons
The programs started again in the Korean War, where the political ramifications of nuclear war would have been unthinkable. In the 1960s, the electro-optical bomb or "fire and forget" camera bomb was introduced. They were equipped with television cameras and steerable flare sights, in 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 collateral damage.
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.
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 conditions 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 that the bomb cannot be confused by an ordinary laser.
Laser-guided weapons did not become commonplace until the advent of the microchip. 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 of the "dumb" kind. 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.
Satellite-guided weapons
The problem of poor visibility does not affect satellite-guided weapons such as JDAM (Joint Direct Attack Munition) and JSOW (Joint Stand-Off Weapon), which uses satellite navigation systems, specifically the United States' GPS system. This offers improved accuracy compared to laser systems, and can operate in all weather conditions, without any need for ground support. Because it is possible to jam GPS, the bomb reverts to inertial navigation in the event of losing the GPS signal. Inertial navigation is significantly less accurate; JDAM achieves a CEP of 13 m under GPS guidance, but typically only 30 m under inertial guidance. Further, the inertial guidance CEP increases as the dropping altitude increases, while the GPS CEP does not.
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. 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.
Other uses
In video games (particularly arcade-style shooters such as Defender), the concept of smart bombs has often, somewhat ironically, been assigned to a type of weapon that obliterates any and all targets in sight.