Definition
A 75 MHz radio beacon associated with an Instrument Landing System (ILS), located along the final approach course typically 4 to 7 nautical miles from the runway threshold. When the aircraft passes directly over it, the beacon transmits a narrow vertical signal that triggers a blue light and a series of low-pitched audio dashes in the cockpit, marking the point at which the aircraft should intercept the glideslope at the published altitude.
Plain English
A small ground transmitter on the approach path to the runway. As the aircraft flies over it, a blue light and a steady tone come on in the cockpit, telling the pilot they have reached a known fixed point on the approach.
Context Anchor
Seen on some instrument approach charts and heard or seen in the cockpit through the marker beacon receiver during an ILS approach.
Derivation
Called 'outer' because it is the outermost of the marker beacons along the approach (compared with the middle and inner markers, which sit closer to the runway). 'Marker' because its job is simply to mark a known position along the approach path.
Why Pilots Care
It confirms the aircraft is at the proper point to intercept and descend on the glideslope, which is essential for a stabilized approach in low visibility.
Intuition Check
“Outer” does not mean outside the airport. Here it means the marker farther from the runway in the marker-beacon system.
Example Sentence 1
Crossing the outer marker, the pilot confirmed the aircraft was on the glideslope at 1,800 feet as published.
Example Sentence 2
Approach clearance included crossing the OM at or above 2,000 feet to remain on the glideslope.