Proportional navigation (PN) (Pro-Nav) 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

Collision course

A collision course, also known as a kamikaze run, is the deliberate maneuver by the operator of a moving object to collide with another object...

 when their direct Line-of-Sight

Line-of-sight

Line-of-sight may refer to:* Line-of-sight propagation, electro-magnetic waves travelling in a straight line* Line of sight * Direct fire or Line-of-sight fire* Sightline, an unobstructed line-of-sight between a subject and object...

 does not change direction. PN dictates that the missile velocity vector 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.

Where is the acceleration perpendicular to missile velocity vector, is the proportionality constant (dimensionless), is the line of sight rate, and V is the velocity.

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.