localizer

An element in the instrument landing system (ILS) located approximately 1000 ft (300 m) beyond the upwind end of the runway and generally in line with the centerline. It indicates the extended centerline of the runway and the approach path onto the runway and directs the pilot to make his or her final approach along this path for landing. It operates in the frequency band of 108 to 111.95 MHz. A localizer transmits two signals, one modulated with a 90-hertz tone and the other with a 150-hertz tone. The two signals are transmitted from highly directional antennae in such a way that the two signals overlap with equal strength on the runway extended centerline. If the aircraft is not on the centerline, the ILS vertical needle swings to indicate lateral displacement. When the pilot is flying along the centerline, the needle stays in the center; the pilot makes an effort to maintain this throughout the approach. Identification is transmitted on the localizer frequency, using International Morse Code; it consists of a three-letter identifier preceded by the letter I (. . ). The localizer coverage is indicated in the illustration. On aeronautical charts, the availability of this facility is indicated by the symbol .