Pursuit guidance, or a pursuit course, is a form of guidance widely used in older

guided missile In military terminology, a missile is a guided airborne ranged weapon capable of self-propelled flight usually by a jet engine or rocket motor. Missiles are thus also called guided missiles or guided rockets (when a previously unguided rocket ...

s. In pursuit guidance, the missile is steered so that the

velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity is a ...

vector of the missile always points at the target, i.e. it has always the direction of the

line of sight The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/observer/spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken ...

. This was the natural outcome of many guidance systems, notably

beam riding Beam-riding, also known as Line-Of-Sight Beam Riding (LOSBR) or beam guidance, is a technique of directing a missile to its target by means of radar or a laser beam. The name refers to the way the missile flies down the guidance beam, which is ai ...

systems where the missile followed the radar signal that was illuminating the target. As the radar typically used

conical scanning Conical scanning is a system used in early radar units to improve their accuracy, as well as making it easier to steer the antenna properly to point at a target. Conical scanning is similar in concept to the earlier lobe switching concept used ...

in order to keep itself pointed at the target (

radar lock-on Lock-on is a feature of many radar systems that allow it to automatically follow a selected target. Lock-on was first designed for the AI Mk. IX radar in the UK, where it was known as lock-follow or auto-follow. Its first operational use was in t ...

), and the missile guided itself by centering itself in this beam, this caused it to always fly directly at the target. It was also found in early

laser-guided bomb A laser-guided bomb (LGB) is a guided bomb that uses semi-active laser guidance to strike a designated target with greater accuracy than an unguided bomb. First developed by the United States during the Vietnam War, laser-guided bombs quickly p ...

s with the aerodynamically stabilized seeker/guidance ("birdy") on the front of the bomb. The birdies sit on a

universal joint A universal joint (also called a universal coupling or U-joint) is a joint or coupling connecting rigid shafts whose axes are inclined to each other. It is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges lo ...

and thus will point in the direction of the relative wind. If the laser receiver is placed on this wind stabilized reference, the angle that the laser is off the wind vector is the same as is measured relative to the centerline of the seeker. Thus, the angle is constructed easily in analog hardware. The disadvantage of this guidance law is that in most scenarios, it will bring the missile into a tail-chase towards the end of the missile's flight. This usually requires the missile to fly for longer, increasing the likelihood that the missile will run out of fuel or otherwise not reach the target. On the other hand, tail-chases can be better for heat-seeking air-to-air missiles, since the hottest part of the target is generally the jet exhausts. Another disadvantage is that this guidance law tends to require more maneuvering from the missile than laws such as

proportional guidance For example, if the line of sight rotates slowly from north to east, the missile should turn to the right by a certain factor faster than the LOS-rate. This factor is ''N''. Proportional navigation (also known as PN or Pro-Nav) is a guidance law ...

, and that maneuvering may exceed the missile's performance envelope. Since modern computers allow the implementation of more complex guidance laws with relatively little cost in weight or other design factors, pursuit guidance is rarely used in modern missiles.

References

* {{Missile types Guided missiles Missile guidance