A guidance system is a device or group of devices used to navigate a ship, aircraft, missile, rocket, satellite, or other craft. Typically this refers to a system that navigates without direct or continuous human control. Systems that are intended to have a high degree of human interaction are usually referred to as a navigational system .

One of the earliest examples of a true guidance system is that used in the German V-1 during World War II. This system consisted of a simple gyroscope to maintain heading, an airspeed sensor to estimate flight time, an altimeter to maintain altitude, and other redundant systems.

A guidance system has 3 major sub-sections: Inputs, Processing, and Outputs. The input section includes sensors, course data, radio and satellite links, and other information sources. The processing section, composed of one or more CPUs, integrates this data and determines what actions, if any, are necessary to maintain or achieve a proper heading. This is then fed to the outputs which can directly affect the system's course. The outputs may control speed by interacting with devices such as turbines, and fuel pumps, or they may more directly alter course by actuating ailerons, rudders, or other devices.

Major guidance systems

Long Range Navigation (LORAN) – This was the predecessor of GPS and was (and to an extent still is) used primarily in commercial sea transportation. The system works by triangulating the ship's position based on directional reference to known transmitters.

Global Positioning System (GPS) – This system of satellites provides extremely accurate position information. The receiver's position is triangulated using satellites in known orbits. Commercial receivers are limited in how accurately they may provide position data, as well as the maximum velocity at which they may operate. This is to prevent their use in manufacturing weapons.

Laser designation – This form of guidance is used exclusively for military munitions. A laser designator device highlights a spot on the target with an encoded laser beam. This spot provides reference information to an incoming munition that allows it to make in-flight corrections to its trajectory. The use of an encoded signal reduces the threat of jamming as well as reducing interference in high-noise combat environments. The primary limitation on this device is that it requires a line of sight to the target from both the munition and the designator. More advanced systems use the laser to designate a target, which is acquired by an orbiting satellite that then feeds GPS target data to a launch facility. This allows potential targets to be designated long before operations commence as well as eliminating the line-of-sight requirement for the munition.

Optical guidance – Another form of guidance used almost exclusively for military purposes, optically-guided missiles use stored images of the terrain they are to fly over and an external sensor to track their current position. This guidance system was extremely expensive and not suitable for use in small payload operations. These were used on cruise missiles before the advent of GPS, which is both cheaper and more accurate. Devices that implement optical guidance incur high costs because of the high on-board processing requirements needed to check the current location against the course data. At the time this type of guidance system was widely used by the military, processors capable of this were very expensive, although similar processing power is available in embedded architectures today. Although called optically-guided, most designs used infrared, ultraviolet, or radar imaging to scan the terrain, since the visible spectrum suffers from relatively poor clarity and high interference (other electromagnetic frequencies can see through dust and clouds, for instance).

See also: Missile, List of missiles