Definition
A 75 MHz marker beacon used on some Category II and Category III ILS approaches, located between the middle marker and the runway threshold. When the aircraft passes over it, the marker beacon receiver activates a white indicator light and an audio tone of rapid dots, signaling the pilot is at the decision height for a Category II approach.
Plain English
A small radio beacon on the ground near the end of the runway that tells the pilot, during a low-visibility instrument approach, that they have reached the point where they must decide to land or go around.
Context Anchor
Seen on some instrument approach charts and heard or seen in the cockpit during certain Instrument Landing System approaches.
Derivation
Called 'inner' because it is the innermost of the three ILS marker beacons (outer, middle, and inner), sitting closest to the runway. 'Marker' because it marks a specific point along the approach path.
Why Pilots Care
It gives pilots a precise position fix near the threshold so they can confirm alignment and prepare for landing in very low visibility.
Intuition Check
“Inner” does not mean inside the airplane or inside the runway. Here it means the marker beacon closest to the runway in the approach marker system.
Example Sentence 1
Crossing the inner marker on the Category II approach, the captain confirmed the approach lights were in sight and continued to land.
Example Sentence 2
The approach plate showed the inner marker symbol between the middle marker and the threshold.