Outer Marker
The Outer Marker, which normally identifies the final approach fix (FAF), is situated on the same course/track as the localizer and the runway centerline, 4 to 7 nautical miles before the runway threshold. It is typically located about 1 NM (2 km) inside the point where the glideslope intercepts the intermediate altitude and transmits a 400 Hz tone signal on a low-powered (3 watts), 75 MHz carrier frequency. Its antenna is highly directional, and is pointed straight up. The valid signal area is a 2,400 ft (730 m) × 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-like 'dashes') indication. Some countries, such as Canada, have abandoned marker beacons completely, replacing the outer marker with a non-directional beacon (NDB), and more recently with GPS fixes. In the United States, the outer marker has often been combined with an NDB to make a Locator Outer Marker (LOM). Some ILS approaches have no navigation aid at all situated at the final approach fix, but use other means, such as VOR radial intersections, distance measuring equipment (DME), GPS, or radar fixes, to identify the position.
Read more about this topic: Marker Beacon
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