Military

BGM-71 / M-220 Tube-launched, Optically tracked, Wire-guided missile (TOW)

The TOW anti-tank missile of Iran-Contra fame was introduced for service in the US Army in 1970. Current versions are capable of penetrating more than 30 inches of armor, or "any 1990s tank," at a maximum range of more than 3,000 meters. It can be fired by infantrymen using a tripod, as well from vehicles and helicopters, and can launch 3 missiles in 90 seconds. It is primarily used in antitank warfare, and is a command to line of sight, wire-guided weapon. TOW is used to engage and destroy enemy armored vehicles, primarily tanks. Secondary mission is to destroy other point targets such as non-armored vehicles, crew-served weapons and launchers. This system is designed to attack and defeat tanks and other armored vehicles. The system will operate in all weather conditions and on the "dirty" battlefield.

The TOW system is used on the HMMWV, the M151 jeep, the armored personnel carrier, the Bradley Fighting Vehicle (BFV) COBRA helicopters, the ITV, and the US Marine Corps light armored vehicle.

Considerable improvements have been made to the missile since 1970. There are six missiles available for the TOW. Three of the five TOW missile versions -- Basic TOW, Improved TOW and TOW 2 -- are no longer being produced for US forces. However, these versions are still used by 40 allied countries.

The basic TOW Weapon System was fielded in 1970. Manufactured by Hughes Aircraft Company, the TOW is the most widely distributed anti-tank guided missile in the world with over 500,000 built and in service in the U.S. and 36 other countries. The TOW has extensive combat experience in Vietnam and the Middle East. Iran may have obtained 1,750 or more TOWs and used TOWs against Iraqi tanks in the 1980s. The TOW 2 launcher is the most recent launcher upgrade. It is compatible with all TOW missiles. The TOW 2 Weapon System is composed of a reusable launcher, a missile guidance set, and sight system. The system can be tripod mounted. However because it is heavy, it is generally employed from the HMMWV. The missile has a 20-year maintenance-free storage life. All versions of the TOW missile can be fired from the current launcher.

The TOW is a crew portable, vehicle-mounted, heavy anitarmor weapon system consisting of a launcher and one of five versions of the TOW missile. It is designed to defeat armored vehicles and other targets such as field fortifications from ranges up to 3,750 meters. After firing the missile, the gunner must keep the cross hairs of the sight centered on the target to ensure a hit. The system will operate in all weather conditions in which the gunner can see a target throughout the missile flight by using either a day or night sight. Guided missiles of this type fly to the point indicated in the crosshairs of a launcher's sight within a design range of the missile, e.g. 3,750 meters in case of a TOW missile. The tracking system optically tracks a flare originating from the back of the missile, e.g. in the infra-red range, and sends guidance signals through two fine steel wires attached to and dispensed from the missile during its flight. The tracking system may be mounted on the missile launcher tube and its major components are a trackable optical sight, a missile tracker and a guidance set. A gunner tracks the target by keeping the crosshairs of the optical sight on the target. The missile tracker senses the optical, e.g. infra-red flare on the missile and generates signals representing the missile's deviation from the gunner's line-of-sight in the horizontal and vertical plane. This deviation appears in the form of error signals which are processed by a feedback control loop in the guidance set. The guidance set produces a correction guidance signal which is sent to the missile by the wire link and the missile responds to these signals by servomechanism deflection of control fin surfaces located at the rear of the missile.

The TOW missile system uses a command to line-of-sight guidance concept. The function of the guidance set is to minimize missile trajectory deviations due to error sources such as gunner jitter, tracker and sensor noise, gusts and cross-winds, flight motor thrust axis misalignments, system imbalances, variations in launch conditions, etc.

Since its development, the TOW missile system has seen combat experience which has provided important information as to its overall utility. While the missile itself is capable of a range of 3,750 meters, the original guidance set was not designed to effectively utilize the maximum range. The need to utilize the missile at its maximum range is apparent, especially in regard to developments which have increased the firing range of new tanks. Investigation of the hit probabilities for the existing TOW missile system have revealed that the chance of a target hit at a range of 3,500 meters is 73.5% for a good gunner. For a poor gunner, the chance of a target hit for a range of 3,500 meters is only 7.5%. Thus, it would be desirable to improve the performance of the original TOW missile system guidance set so as to exploit the longer range of the basic TOW missile.

An analysis into the performance of the existing guidance set was performed and the following was revealed. The inherent TOW missile construction provides low aerodynamic damping which makes it highly susceptible to the effects of aerodynamic forces, resulting in fluctuations during its flight. Over the longer range of flight, the gunner must maintain his sight on the target for a longer period of time. The feedback control system depends on the steadiness and the aiming ability of the gunner. During flight internal inertial forces are relied upon to guide the missile to its target, and initially introduced errors in its trajectory grow over time. The control loop is thus required for reconciling the need to correct larger errors as time goes on, with the fact that with longer flight time, i.e. with an increase of the error gain, the control loop may become unstable. Large oscillations of the missile around trim position create a decrease in missile speed through increased wind resistance.