Marker Beacons
Inner, Middle, and Outer
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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. |
| Marker Beacon | Pilot Alert | Distance to Threshold | Modulated frequency | Audio Keying | ||||
| Outer | Glide Path Intercept | 4 to 7nm | 400Hz | ------ | ||||
| Middle | Category 1 Decision Height | 3500 ft | 1300Hz | .-.-.- | ||||
| Inner | Category 2 Decision Height | 1000 ft | 3000Hz | ...... | ||||
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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. | |