Laser-guided bomb

A laser-guided bomb (LGB) is a precision-guided munition (PGM) that uses semi-active laser homing to strike a designated target with greater accuracy than a free-fall bomb. LGBs are one of the most common and widespread PGMs, used by a large number of the world's air forces.

Overview

Laser-guided munitions use a laser designator to mark (illuminate) a target. The reflected laser light ("sparkle") from the target is then detected by the seeker head of the weapon, which sends signals to the weapon's control surfaces to guide it toward the designated point. Laser-guided bombs are generally unpowered, using small wings to glide towards their targets. Powered laser-guided missiles, such as some variants of the US AGM-65 Maverick and the French AS.30L, use the same guidance system, but have greater range and maneuverability because they are not limited to unpowered flight. Some LGBs have been fitted with strap-on rocket motors to increase their range; one such weapon is the USAF AGM-123 Skipper.

The earliest laser guidance seekers measured the intensity of the reflected laser light at four corners of the seeker window. The seeker then actuated the control fins to steer the weapon in the direction of the strongest signal return, thereby keeping the weapon centered on the laser sparkle. Later weapons have more sensitive seekers and more sophisticated control systems that waste less energy with course corrections, improving accuracy and range, but the principle remains essentially the same. The first such weapon to be developed was the Texas Instruments BOLT-117.

Most laser-guided bombs are produced in the form of strap-on kits: seeker heads, steering fins, and wings that can be attached to a standard general-purpose bomb or penetration bomb. Such kits are modular, allowing relatively easy upgrades, and are considerably cheaper than purpose-built weapons.

Development

Laser-guided weapons were first developed in the United States in the early 1960s. The USAF issued the first development contracts in 1964, leading to the development of the Paveway series, which was used operationally in Vietnam starting in 1968. Although there were a variety of technical and operational problems, the results were generally positive. LGBs proved to offer a much higher degree of accuracy than unguided weapons, but without the expense, complexity, and limitations of guided air-to-ground missiles like the AGM-12 Bullpup. The LGB proved particularly effective against difficult fixed targets like bridges, which previously had required huge loads of "dumb" ordnance, and large numbers of sorties, to destroy.

It was determined that 48 percent of Paveways dropped during 1972–73 around Hanoi and Haiphong achieved direct hits, compared with only 5.5 percent of unguided bombs dropped on the same area a few years earlier. [http://www.americanheritage.com/articles/magazine/ah/2006/6/2006_6_28.shtml From Saigon to Desert Storm] ] The average Paveway landed within 23 feet of its target, as opposed to 447 feet for gravity bombs. The leap in accuracy brought about primarily by laser guidance made it possible to take out heavily defended, point objectives that had eluded earlier air raids. The most dramatic example was the Thanh Hoa Bridge, 70 miles south of Hanoi, a critical crossing point over the Red River. Starting in 1965, U.S. pilots had flown 871 sorties against it, losing 11 planes without managing to put it out of commission. In 1972 the “Dragon’s Jaw” bridge was attacked with Paveway bombs, and 14 jets managed to do what the previous 871 had not: drop the span, and cut a critical North Vietnamese supply artery.

In the wake of this success, other nations, specifically the Soviet Union, France, and Great Britain, began developing similar weapons in the late 1960s and early 1970s, while US weapons were refined based on combat experience.

The USAF and other air forces are now seeking to upgrade their LGBs with GPS guidance as a back-up. These weapons, such as the USAF Enhanced Guided Bomb Unit (part of the Paveway family), use laser designation for precision attacks, but contain an inertial navigation system with GPS receiver for back-up, so that if the target illumination is lost or broken, the weapon will continue to home in on the GPS coordinates of the original target.

Problems and Limitations

While LGBs are highly accurate under ideal conditions, they present several challenges for successful use, making them somewhat less than the "silver bullet" sometimes suggested.

The first problem is designation. To ensure accurate guidance, the target must be illuminated by a laser designator and the pilot must deliver the weapon within the "weapon basket" (an imaginary zone in which the weapon seeker can observe the laser target marker and the weapon has sufficient energy to guide to the target). Laser guided bombs can be launched without the laser designator turned on, in which case it will follow a ballistic path, until such time as the seeker picks up the reflected laser signal. This allows the aircraft to deliver an LGB using techniques such as loft or toss bombing. However, if the designator is turned off, the laser spot moved, the laser signal is blocked (for example by cloud or smoke), the weapon's accuracy will be greatly reduced. A particular problem with laser designation can be path length under certain environmental conditions. Laser designators operate in the Infra-Red wavebands and attenuation of the laser signal by the moisture content of the atmosphere can reduce the signal strength below the threshold of the seeker. Laser designators modulate their output on a number of discrete codes and to guide correctly the bomb and designator must be matched.

For an accurate attack against a small target, uninterrupted designation is desirable. But, the simple guidance system of early LGBs (such as the American Paveway II) resulted in a rectilinear flight path, with a tendency to lag below the sightline. To compensate, crews will release their weapons on an unguided, ballistic flightpath, activating the designator only to refine the bomb's final impact point. This is more demanding of crew and aircraft, requiring a high standard of basic, unguided bombing accuracy and more attention to the bomb's flight.

In the 1970s and 1980s it was common for aircraft to rely on a separate designator, either carried by ground forces, operated by the forward air controller, or carried by another aircraft in the strike group. It was often deemed more practical for one aircraft to designate for its comrades. Modern conflicts and a growing emphasis on precision-guided weapons have pointed to the need for autonomous designation, and many fighter-bomber aircraft are now being fitted with designator pods to self-designate for laser-guided munitions.

Even if the launch aircraft is capable of autonomous designation, the problems described above remain. Laser illumination can be interrupted by smoke, fog, or clouds, limiting the usefulness of LGBs in poor weather or very dusty conditions. In desert warfare, such as the 1991 Gulf War, laser designation sometimes reflected off the sand, causing weapons to home on false targets. Furthermore, the need to provide designation may leave the aircraft dangerously exposed to ground fire or enemy air support.

An additional concern is the limited "weapon basket", if the weapon is released too low or too far from the target, or in a trajectory that puts the weapon outside the seeker's field of view, it will miss. Optimum altitude for an effective LGB attack is from medium altitude (20,000 - 30,000 ft), increasing the aircraft's vulnerability to surface-to-air missile (SAM) attacks.

For these reasons, while all modern air forces have put an increasing emphasis on LGBs and other precision-guided munitions, some tacticians still see an important role for the accurate delivery of unguided bombs. During their 1981 raid on the Iraqi nuclear reactor at Osirak, the Israeli Air Force chose to use unguided Mark 84 bombs rather than laser-guided weapons because they felt the need to designate the target would leave the attackers unacceptably vulnerable.

ee also

*JDAM
*KAB-500L
*Laser guidance
*Missile guidance

External links

* [http://www.fas.org/man/dod-101/sys/smart/lgb.htm Laser-guided bombs page – FAS]

References