The development of precision-guided munitions (PGMs) is perhaps the most significant factor in air warfare since aircraft first began to carry bombs. While bombing has proved of considerable military value, it has always been gravely hampered by one key factor, namely a lack of accuracy. Precision-guided weapons, while not providing the ‘one target, one bomb’ equation supposed by popular opinion, greatly enhance the tactical effectiveness of attack aircraft, and have become particularly important in modern warfare with the increased concern over the avoidance of civilian casualties. With the increase in the number of weapons with guidance systems, it is important to define what PGMs are. They may be regarded as weapons which are air launched and guided to their target through the use of laser, electro-optical sensors, global positioning systems, or inertial navigation systems. Some anti-armour missiles, such as the American TOW system, are highly accurate and guided, but they do not quite fall into the category of PGMs. The term does not normally encompass weapons which are fired from the ground or sea, and generally, but not exclusively, applies to unpowered bombs with guidance systems attached.
The first precision weapons were employed in WW II. Most of the development occurred in Germany and the USA. The two key German weapon types were the Ruhrstahl/Kramer X1, or Fritz X, and the Henschel Hs 293, both used for anti-shipping purposes. The Fritz X was a guided glide bomb, while the Hs 293 was powered by a small rocket motor for the initial stage of its journey, gliding the rest of the way. Both were guided to their targets by radio signals. The Hs 293 was the first into action, sinking the sloop HMS Egret on 27 August 1943. The Fritz X was first employed the following month. Both systems had the disadvantage that the launch aircraft were required to fly at slow speed, loitering in the target area, which made them vulnerable to attack by enemy fighters or anti-aircraft fire.
The development of guided weapons on the Allied side has been curiously ignored by historians. The first guided bomb, the GB-1, was perversely not guided, simply being a bomb with wings to enable it to glide to the target. The GB-series of bombs were little used, but began the principle of precision guidance being made available to aircrews. The GB-4 was guided through the use of a television camera system; GB-6A possessed an infra-red seeker; GB-8 was a visual-controlled glide bomb and GB-12 was an over-water light-contrast weapon. Derivations of these methods of control are to be found in modern PGMs. As well as the winged GB-series, the US employed the VB (Vertical Bomb) system, which did not possess flying surfaces, only tail assemblies with guidance systems to adjust the tail fins. The only one to be used in WW II was the VB-1, also known as the Azon, a contraction of Azimuth Only. This meant that while the bomb aimer had control of the bomb through wireless, he could not adjust its trajectory to prevent it from falling short of the target or overshooting it; he could only control its horizontal position over the target. Although trials demonstrated that the Azon was 29 times more accurate than unguided ordnance, under combat conditions, results were patchy. A development of Azon, the enormous 12, 000 lb (5, 443 kg) Tarzon, proved equally disappointing in Korea, even though it did destroy a number of bridges. Precision-guided weapons were not to be seen in large numbers until the Vietnam war.
The first types of PGM employed in Vietnam were the laser-guided Paveway system and the electro-optically guided bomb. These types have been greatly refined, and the laser-guided bomb is the most well-known and currently the most important type. The Paveway system, on which all western laser-guided bombs are based, was a simple kit to be fitted to ordinary bombs. The kit involved a guidance system to be mounted on the nose of the bomb, and control surfaces. A target is designated by laser, either from a ground observer or from a podded laser system carried by the aircraft or a companion (‘buddy lasing’ in the latter instance). The laser-guided bomb is then tossed or dropped into the inverted cone of laser light reflected from the target. The seeker system detects the laser light and its guidance computer corrects the angle of the control surfaces to ensure that the bomb arrives on or near to the point of the cone of laser radiation. The system has been highly effective, and has been used to equip several thousand bombs weighing between 500 lb and 2, 000 lb (between 226.8 and 907 kg) in weight. Although much more accurate than unguided weapons, the laser-guided bomb does not guarantee complete accuracy. The figures are greatly disputed, particularly since the qualifications for accuracy change from assessment to assessment, but it is estimated that a laser-guided system, if used under adequate conditions, can achieve in the region of 80 per cent of bombs dropped hitting within 10 to 30 feet (3 to 9 metres) of the designated spot.
The electro-optically guided bomb has been less successful. The first to see widespread use were the US navy's Walleye and the USAF's GBU-8 HOBOS in Vietnam, followed into US service by the GBU-15 glide bomb and the AGM-130 rocket-assisted weapon. Soviet forces employed a number of similar weapons, and development of the type continues in Russia. The principle behind such weapons is simple: the pilot or weapons system operator acquires the target through the weapon's electro-optical seeker, locks the seeker onto the target, and then launches the weapon. The newer weapons can have corrections sent to them through a data-link pod to ensure accuracy, but many of the older types broke lock through the failure of the seeker to maintain the contrast between the target and the background. This has led to the use of imaging infra-red seekers as alternatives to the optical systems on the GBU-15 and the AGM-130. The major difficulty with acquiring electro-optical PGMs is their cost; the sophisticated seeker equipment is considerably more expensive than laser-guided types.
Laser-guided and electro-optical weapons have a major obstacle to their effective utilization, and this is the weather. Both rely upon visual acquisition of the target so that it can be designated. Increasingly sophisticated designation pods such as the British TIALD are not proof against this problem, and when conditions of visibility are poor, it is not unknown for missions to be aborted. This leaves dependency upon intertially guided systems which are highly expensive, and are to be found only in cruise missiles. These did not originate as conventional weapons, but were designed to deliver nuclear warheads to a target over long ranges. The two key types in use are the American Tomahawk Land Attack Missile and Conventional Air Launched Cruise Missile. Both rely upon terrain profile matching, so that they navigate by matching the contours of the ground beneath the missile with pre-installed navigational instructions. This gives the weapons their famed ability to turn at key road junctions as if navigating along the highway beneath; they do not rely upon the presence of the road, but on the contours along which it runs. The addition of Global Positioning System (GPS) equipment makes the weapons even more accurate. Essentially, GPS enables the missile to communicate with satellites so that it can work out exactly where over the earth's surface it is. When the satellite systems tell the weapon that it is approaching or over the target's co-ordinates, the weapon adjusts its control surfaces so that the PGM hits the target. The order of accuracy may be measured in a few feet.
The use of GPS would appear to be the way forward for PGMs. It means that they can be employed in any weather, and conventional bombs can be modified relatively inexpensively with the fitting of a fixed aerodynamic jacket around the bomb body to keep it falling at a constant angle, and a tail unit with GPS equipment and movable fins. The GPS-guided munitions currently in service are the USA's GAM-84 and GAM-113, deployed on the B-2 bomber.
PGMs are essential tools for modern war. The political implications which now attach to civilian casualties mean that their use will increase. The search for the PGM which hits its target every time will be eternal; however, the purpose of the PGM is not to ensure 100 per cent accuracy, but to ensure that targets are much more likely to be destroyed or damaged, increasing the effectiveness of the aircraft employing them and reducing the risk of civilian casualties on the ground. In spite of their expense when compared to unguided weapons, they are a worthwhile investment, and desirable acquisitions for all military services. They will not replace unguided ordnance entirely, but PGMs will be the air weapon of choice in the future.
Bibliography
- Cordesman, Anthony H., and Wagner, Abraham R., The Lessons of Modern War, vol.
iv . The Gulf War (Boulder, Colo., 1996). - Gunston, Bill, The Illustrated Encyclopedia of Aircraft Armament (Salamander, 1987)
— David Jordan
