The middle marker transmission is modulated with a 1300 Hz tone that is a series of dots and dashes so as to not be confused with the all dash tone of the outer marker. It is located on approach approximately 3,500 feet from the runway.
Localizer antenna location on aircraft plus#
This, plus the oral tone in combination with the localizer and the glideslope indicator, positively locates the aircraft on an approach. A marker beacon receiver receives the signal and uses it to light a blue light on the instrument panel. The transmission is very narrow and directed straight up. It transmits a 75 MHz carrier wave modulated with a 400 Hz audio tone in a series of dashes.
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As mentioned, an outer marker beacon transmitter is located 4–7 miles from the threshold. They transmit signals that indicate the position of the aircraft along the glidepath to the runway. Marker BeaconsMarker beacons are the final radio transmitters used in the ILS. Once located, the pilot maneuvers the aircraft to fly down the glidepath to the runway. If a middle marker compass locator is in place, it is similar but is identified with the last two letters of the ILS identifier. The ADF receiver is used to intercept the locator so no additional equipment is required. It transmits omnidirectional LF radio waves (190 Hz to 535 Hz) keyed with the first two letters of the ILS identifier.
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The outer marker compass locator is a 25 watt NDB with a range of about 15 miles. Another may be located at the middle marker beacon about 3,500 feet from the threshold. There is typically one located at the outer marker beacon 4-7 miles from the runway threshold. A compass locator is a transmitter designed for this purpose. Compass LocatorsIt is imperative that a pilot be able to intercept the ILS to enable its use.