Proportional navigation

From Wikipedia, the free encyclopedia

Proportional Navigation (PN) is a guidance law used in some form or another by most homing air target missiles. It is based on the fact that two vehicles are on a collision course when their direct line of sight does not change direction. PN dictates that the missile should rotate at a rate proportional to the rotation rate of the line of sight (Line-Of-Sight rate or LOS-rate), and in the same direction. For example, if the line of sight rotates slowly from north to east, the missile should turn to the right, but a certain factor faster than the LOS-rate. This factor is called the Navigation Constant (Knav).

A rather simple hardware implementation of this guidance law can be found in early sidewinder missiles. These missiles use a rapidly rotating parabolic mirror as a seeker. Simple electronics detect the directional error the seeker has with its target (an IR source), and apply a moment to this gimballed mirror to keep it pointed at the target. Since the mirror is in fact a gyroscope it will keep pointing at the same direction if no external force or moment is applied, regardless of the movements of the missile. The voltage applied to the mirror while keeping it locked on the target is then also used (although amplified) to deflect the control surfaces that steer the missile, thereby making missile rotation proportional to line of sight rotation. Although this does not result in a rotation rate that is always exactly proportional to the LOS-rate (which would require a constant airspeed), this implementation is equally effective. The basis of proportional navigation was first discovered at sea, and was used by navigators on ships to avoid colisions.