Missile guidance

Missile guidance is a specialized field of engineering. It belongs to the study of guidance, navigation, and control. These systems are designed to maximize the reliability and accuracy of missiles and guided bombs. By improving the likelihood that a weapon reaches its target, these systems ensure precision.

Engineers generally divide these systems into two broad categories. The first is Go-onto-target (GOT) systems, which are used against moving targets. The second is Go-onto-location-in-space (GOLIS) systems, which attack fixed geographical positions. While missiles use onboard engines for propulsion, guided bombs rely on gravity and the speed of the launch aircraft.

Homing guidance is a primary method used in GOT systems. In this process, the missile tracks a target using its own onboard sensors. These sensors can detect radar, infrared light, or other forms of light. Homing missiles are often called "fire-and-forget" weapons because they do not need constant communication with a ground station. There are three distinct types of homing. Active homing occurs when the missile illuminates the target with its own radiation, such as radar. Semi-active homing relies on an external source of radiation to find the target. Passive homing is different because it only tracks the emissions or the contrast produced by the target itself. Many modern missiles use proportional navigation to steer during the final terminal phase of flight.

Another category is remote control guidance, which requires a link between the missile and a control point. This connection can be a radio signal, a beam, or a physical wire. Command guidance is a specific type where instructions originate from outside the missile. This system requires two separate links: an information link to find the missile's position and a command link to send steering instructions. One disadvantage is that the target must be illuminated by an external energy source, which might allow the target to perform evasive maneuvers. Beam riding is a related method where a missile follows an electromagnetic beam, such as a laser or radar. Sensors on the rear of the missile calculate steering commands to keep the projectile within the beam.

While beam riding allows multiple missiles to use one beam, it can become less accurate as the beam spreads out over long distances.

GOLIS systems operate without a target tracker. Instead, they use navigational guidance to reach a preset location. The guidance computer and the missile tracker are both located on the missile itself. This is often called a self-contained guidance system. The simplest form is preset guidance, where the flight path is programmed into the system before the missile is even fired. The missile then uses internal components like accelerometers or gyroscopes to follow that path. The V-2 rocket is a historical example of a missile that used this type of preset guidance.

Inertial guidance is a sophisticated form of navigational guidance. It uses gyroscopes and accelerometers to estimate a missile's position and velocity through a process called dead reckoning. Early mechanical versions were not very precise and often needed external adjustments to hit targets. Modern technology has improved this significantly. Today, solid state ring laser gyros can be accurate to within meters even over distances of 10,000 km.

Some advanced systems, like the AIRS on the MX missile, allow for accuracy of less than 100 meters at intercontinental ranges. Because inertial guidance does not rely on external signals, it is very difficult to jam.

For even greater precision, some systems use astro-inertial guidance. This is a form of sensor fusion that combines inertial guidance with celestial navigation. This method is particularly useful for submarine-launched ballistic missiles (SLBMs). Because submarines are moving, calculating a starting point is much harder than it is for a fixed silo. Astro-inertial guidance uses star positioning to fine-tune the accuracy of the inertial system after launch. By observing the fixed positions of stars, the missile can correct small errors in its calculated position. For example, the Trident missile system uses a camera to spot a single star to ensure the missile remains on its intended path.

The history of these technologies spans over a century. In the late 1880s, the writer Jules Verne imagined a rocket-powered missile with a target seeker in his fiction. During World War I, various nations began experimenting with guided systems. Archibald Low is known as the father of radio guidance for his work on early drones. During World War II, Germany developed guided missiles as part of its V-weapons program to bypass treaty limits. After the war, the United States faced significant budget cuts in 1946, known as the "black Christmas." This led to the cancellation of 10 guided missile projects. However, the start of the Korean War caused development to accelerate rapidly once again. The PGM-11 Redstone became the first U.S. ballistic missile to use a highly accurate inertial guidance system.