This gives rise the concepts of instantaneous field of view (IFOV) which is the angle the detector sees, and the overall field of view, also known as the tacking angle or off-boresight capability, which includes the movement of the entire seeker assembly. Since the assembly cannot move instantly, a target moving rapidly across the missile's line of flight may be lost from the IFOV, which gives rise to the concept of a tracking rate, normally expressed in degrees per second.
