Plain English Meaning
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.
Full 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.
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.