In order to provide a pilot with an indication of his distance from the runway, marker-beacon transmitters are installed with the outer-marker transmitter at approximately 5 miles from the runway and the mid-marker approximately 5 mile from the end of the runway.

The marker-beacon transmitter operates at a frequency of 75 MHz and produces both aural and visual signals. The outer-marker transmitter produces a 400-Hz intermittent signal which causes a blue indicator light on the instrument panel to glow intermittently.

The midmarker transmitter produces a signal modulated at 1,300 Hz which causes the amber marker-beacon light on the instrument panel to glow. Thus, when the airplane is approaching the runway and is approximately 5 miles from its end, the blue light will flash.

A short time later, when the airplane is within 2/3 mile of the runway, the amber light will flash. This system provides an excellent indication to the pilot of his distance from the runway.

The marker-beacon receiver for a typical large aircraft navigation system is a crystal-controlled fixed frequency superheterodyne designed to operate only on a frequency of 75 MHz. The receiver is equipped with output circuits which enable it to deliver both aural and visual signals to the flight crew.

A portion of the output signal is fed through a transformer to audio filters tuned to 400, 1,300, and 3,000 Hz. The 75 MHz - signal from the marker-beacon transmitters is modulated with the three different audio tones, depending upon whether the transmitter is a midmarker, an outer marker, or an airways Z, or fan, marker.

Each of the audio filters is designed to select one of the frequencies and with this signal activate a switching circuit which causes the appropriate signal light on the instrument panel to flash. The indicator lights are white, amber, and blue, thus making it possible for the pilot to know what type of marker he is passing over.

For example, if the blue light flashes, the pilot will know that he is passing over the outer marker.

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