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Marker Beacon Subsystem

Marker beacons are used to alert the pilot that an action (e.g., altitude check) is needed. This information is presented to the pilot by audio and visual cues. The ILS may contain three marker beacons: inner, middle and outer. The inner marker is used only for Category II operations. The marker beacons are located at specified intervals along the ILS approach and are identified by discrete audio and visual characteristics (see Table 1). All marker beacons operate on a frequency of 75 MHz.

Table 1: Marker Beacon Characteristics

Marker
Beacon

Pilot
Alert

Distance to
Threshold

Modulated
Frequency

Audio
Keying

Outer

Glide Path
Intercept

4 to 7 nm

400 Hz

- - - -

Middle

Category I
Decision Height

3500 feet

1300 Hz

. - . -

Inner

Category II
Decision Height

1000 feet

3000 Hz

. . . .

The marker beacon coverage provides adequate signal laterally throughout the localizer proportional guidance sector. Marker beacons produce cone or fan-shaped radiated patterns directed upward and, therefore, pose very few siting problems. The majority of problems in locating the marker beacon are the availability of real estate and access to utilities. If an acceptable site for the outer marker cannot be found, an alternative is to collocate a Distance Measurement Equipment (DME) transponder with the localizer. This DME then provides the range indication to the aircraft. The ILS models do not provide any information with regard to marker beacon performance.

 

Avionics Engineering Center
Russ College of Engineering and Technology
131 McFarland Avionics Building
Ohio University
Athens, OH 45701
Tel: (740) 593-1515
avionics@ohio.edu

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