At gates with automatic systems, there are metal wires embedded in the ramp at strategic points that sense the nose gear as it rolls up the yellow line. It is similar to the triggering method used for traffic lights at intersections throughout the U.S.
Position data is then sent to a box positioned directly in front of the aircraft in good view of the pilots. There are lights which indicate the aircraft is off center and others which illuminate in sequence for final positioning of the aircraft. Also, before the aircraft arrives, personnel must enter the aircraft type, using a selector-type switch. The A/C type is then displayed for the pilots so they can be assured that the system is tuned for the correct aircraft type.
America West's original T4 concourses at Phoenix have this system installed, although they are no longer used. They proved to be less reliable than human beings.
They can be handy for 747s and other aircraft with high cockpits because as planes get larger, it becomes increasingly difficult for visual contact between ground personnel and the flight deck.