Marker beacon
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A marker beacon is a beacon used in aviation in conjunction with an instrument landing system (ILS), to give pilots a means to determine distance to the runway. There are three types of marker beacons on an ILS.
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[edit] Outer marker
The Outer Marker, which normally identifies the Final Approach Fix, is situated on the same line with the localizer and the runway centerline, four to seven nautical miles[citation needed] before the runway threshold. It is typically located about 1-nautical-mile (2 km) inside the point where the glideslope intercepts the intermediate altitude and transmits a low-powered (3 watt), 400 Hz tone signal on a 75 MHz carrier frequency. Its antenna is highly directional, and is pointed straight up. The valid signal area is 2,400 ft (730 m) by 4,200 ft (1,280 m) ellipse (as measured 1,000 ft (300 m) above the antenna.) When the aircraft passes over the outer marker antenna, its marker beacon receiver detects the signal. The system gives the pilot a visual (blinking blue outer marker light) and aural (continuous series of audio tone morse code 'dashes') indication. Some countries, such as Canada, have abandoned marker beacons completely, replacing the outer marker with a non-directional beacon (NDB). In the United States, the outer marker will often be combined with an NDB to make a Locator Outer Marker (LOM). Some ILS approaches have no navigation aid at all situated at the outer marker, but use other means, such as radial intersections, distance measuring equipment (DME), or radar fixes, to identify the position.
[edit] Middle marker
A middle marker works on the same principle as an outer marker. It is normally positioned 0.5 to 0.8 nautical miles (1 km) before the runway threshold. When the aircraft is above the middle marker, the receiver’s amber middle marker light starts blinking, and a repeating pattern of audio morse code dot-dashes at a frequency of 1,300 Hz in the headset. This is intended as an annunciator for the pilot, an alert that the missed approach point (typically 200 feet (60 m) above the ground level or AGL on the glideslope) has been passed and should have already initiated the missed approach if one of several visual cues has not been spotted. Middle Markers are typically associated with Category II or III approaches.
[edit] Inner marker
Similar to the outer and middle markers; located at the beginning (threshold) of the runway on some ILS approach systems (Category II and III) having decision heights of less than 200 feet (60 m) AGL. Triggers a flashing white light on the same marker beacon receiver used for the outer and middle markers; also a series of audio tone 'dots' at a frequency of 3,000 Hz in the headset.
[edit] Fan marker
A fan marker can be used for a final approach fix on an ILS back course. It is located in a location similar to the outer marker only on the back course or opposite end of the runway. A fan marker on a back course lights the white light and emits a series of audio tone 'dots' at a frequency of 3,000 Hz in the headset.
[edit] See also
- Instrument Landing System
- Distance Measuring Equipment (DME)
- Non-Directional Beacon (NDB)
- VHF Omni-directional Range (VOR)
- Global Positioning System (GPS)
- Transponder Landing System (TLS)
[edit] External links
- 2001 Federal Radionavigation Plan (FRP) - FRS publication has detailed description of ILS and other navigational systems
