radio-marker beacon

The beacons that serve to identify a particular location in space along an airway or approach to an instrument runway. This is done by means of a 75 MHz transmitter, which transmits a directional signal to be received by aircraft flying overhead. These markers generally are used in conjunction with en route navigational aids and an ILS (instrument landing system) as point designators. The three types of en route markers are the fan marker (FM), low-powered fan markers (LFM), and Z-markers. They transmit the letter R (.-.) as identification, or if additional markers are in the same area, the letters K, P, X, or Z. The class FMs provide positive identification of positions at definite points along the airway and have a power output of 100 watts. The class LFMs have a rated output of 5 watts, and their antenna array produces a circular pattern that appears elongated at right angles to the runway because of the directional characteristics of an aircraft’s receiving antenna. The station location, or Z-marker gives a positive position for aircraft operating under instrument flight conditions to show the pilot when he or she was passing directly over a low-frequency navigational aid. There are three such marker beacons in an ILS: the outer marker, which is sited 4 to 5 NM from the touchdown and transmits two low-pitched dashes per second; the middle marker, which is located 1/4 to 1/2 NM from the touchdown and transmits high-pitched dots and dashes alternately; and the inner marker, which is located at the runway threshold and transmits six high-pitched dots per second. Normally, the inner marker is not installed. The signals from the marker are beamed vertically upward in a fan shape and can be heard in the aircraft only when it is directly above the beacons. See marker beacon.