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A marker beacon at or near the glideslope intercept altitude of an ILS approach. It is keyed to transmit two dashes per second on a 400 Hz tone, which is received aurally and visually by compatible airborne equipment. The OM is normally located four to seven miles from the runway threshold on the extended centerline of the runway.
Listed in the Pilot/Controller Glossary as a cross-reference to OUTER MARKER, whose text is shown above.
Source FAA Pilot/Controller Glossary · checked 2026-08-25