Definition
A marker beacon used with an ILS (Category II) precision approach, located between the middle marker and the runway threshold. It transmits a 3,000 Hz signal upward in a narrow beam, indicating to the pilot the point at which the aircraft is at decision height on the glide path during a Category II approach.
Plain English
A small radio transmitter on the ground near the runway. As the aircraft flies over it during a precision approach, the cockpit gives a signal showing the pilot has reached the point where they must decide to land or go around.
Context Anchor
Seen in instrument approach procedures and cockpit indications for certain instrument landing system approaches, especially older or low-visibility systems.
Derivation
Called 'inner' because it is the innermost of the ILS marker beacons — closest to the runway threshold, sitting inside the outer and middle markers along the approach path.
Why Pilots Care
It gives a precise final position reference before the runway, helping the pilot decide whether to land or go around in low visibility.
Intuition Check
Do not picture an Inner Marker as a painted sign or physical marker on the runway. In this context, it is an electronic signal received in the aircraft that marks a precise point on the approach path.
Example Sentence 1
As they crossed the inner marker, the captain confirmed the runway lights in sight and continued to land.
Example Sentence 2
Passing the inner marker confirmed the aircraft was aligned for the final landing segment.