Missile Guidance

The Guidance system is that part of a missile which decides when, and by how much, the control system must change the trajectory of the missile. There are many types of Guidance used, some of which are briefly discussed below:

Radar Command

This system uses two radars, one to track the target and the other to track the missile. The bearing and elevation of the radar trackers is monitored by a computer which directs the control system to alter the missile trajectory until it reaches the target.

Radio Command

This system uses a human operator to observe the trajectory of the missile, and direct its control system by remote control radio link.

Wire Guidance

This system is broadly similar to radio command, but is less susceptible to electronic counter measures. The command signals are passed along a wire (or wires) dispensed from the missile after launch.

Inertial Guidance

This system is totally contained within the missile and is programmed prior to launch. Three accelerometers, mounted on a platform space-stabilised by gyros, measure accelerations along three mutually perpendicular axes; these accelerations are then integrated twice, the first integration giving velocity and the second giving position. The system then directs the control system to preserve the pre-programmed trajectory.

Astro Guidance

This system constantly measures star angles and compares them with the pre-programmed angles expected on the missile's intended trajectory. The guidance system directs the control system whenever an alteration to trajectory is required.

Infra-Red Guidance

Infra-Red homing is used mainly by missiles designed to destroy aircraft, the target aircraft's exhaust shining like a beacon to the infra-red sensor in the nose of the missile. The missile guidance system uses the data from the infra-red sensor to decide when an alteration in missile trajectory is required.

Semi-Active Laser Homing

This system uses a laser designator to 'illuminate' the target for the sensor in the nose of the missile. The designator can be operated from an aircraft or by an observer on the ground.

Terrain Comparison

Terrain Comparison, (Tercom) is used invariably by cruise missiles. The system uses sensitive altimeters to measure the profile of the ground directly below and checks the result against stored information. Each set of readings is unique to one strip of land. If the missile is unable to find a specific target due to ground interference, (damage to buildings causing unexpected height readings) it can fly around in a search pattern until it finds the intended target, perhaps from a different approach route.

Active Radar Homing

In this system the missile carries its own target-seeking radar. The integral radar searches for the size of target it was programmed to reach. However, this system can become confused by other targets in the close vicinity of the intended target. The system is also very susceptible to electronic countermeasures.

Semi-Active Radar Homing

This system uses a radar to illuminate the target for the sensor in the nose of the missile. The missile will only home on the reflected radar signals which originated from its parent radar.