Definition
A 75 MHz radio beacon installed along the centerline of an ILS approach, located between the middle marker and the runway threshold, that transmits a narrow vertical signal indicating the point at which the aircraft is at decision height on a Category II ILS approach. When overflown, it activates a white marker light and a continuous series of dots in the cockpit at six dots per second.
Plain English
A small ground transmitter just before the runway on a precision approach. As the aircraft passes directly over it, the cockpit gets a quick light and audio signal telling the pilot they are at the point where, on a Category II landing, they must decide whether to land or go around.
Context Anchor
Seen in instrument landing system approach discussions and, on equipped aircraft, as a cockpit marker-beacon light and tone during final approach.
Derivation
Called 'inner' because it is the innermost of the three ILS marker beacons -- outer, middle, and inner -- with 'inner' meaning closest to the runway threshold.
Why Pilots Care
It supplies the last precise position reference before the runway, allowing the pilot to confirm they are at the correct decision height in very low visibility.
Grounding Statement
Picture crossing an invisible radio gate near the runway that triggers a cockpit alert.
Intuition Check
“Inner” does not mean inside the aircraft or inside the airport. Here it means the marker beacon closest to the runway on the final approach path.
Example Sentence 1
Crossing the inner marker, the captain confirmed the runway lights were in sight and continued the Category II approach to landing.
Example Sentence 2
On the approach chart the inner marker beacon is shown 0.2 miles from the runway threshold.