Localizer

The portion of an instrument landing system (ILS) that directs the pilot of an aircraft down the center line of the instrument runway for the final approach in an instrument landing. The localizer transmits two signals, one modulated with a 90-hertz tone, and the other with a 150-hertz tone. These two signals are transmitted from highly directional antennas so they overlap along the center line of the runway by two and a half degrees. The vertical needle of the cross-pointer indicator in the aircraft responds to these two signals. When the aircraft is flying along a path lined up with the center line of the runway, the needle is centered. When the aircraft moves to one side of the center line or the other, the signal for the side it is on is the stronger, and the needle swings over to show the side of the runway over which the airplane is flying.