Definition
A low-power radio transmitter on the ground that sends a narrow signal straight up along an instrument approach path. As an aircraft passes directly overhead, the receiver in the cockpit triggers a coded light and audio tone, telling the pilot they are at a known, fixed point along the approach.
Plain English
A small ground transmitter that beams a signal straight up. When you fly over it, a light flashes and a tone sounds in the cockpit so you know exactly where you are on the approach.
Context Anchor
Seen in instrument approach procedures and in aircraft equipped with marker beacon lights or audio, especially on some instrument landing system approaches.
Derivation
From Old English 'beacn,' meaning a signal or sign — originally a fire lit on a hilltop to signal others. The word has always meant 'something that marks a position.' A marker beacon does exactly that: it marks a specific point along an approach path.
Why Pilots Care
Marker beacons give a positive, unmistakable confirmation that the aircraft has crossed a known point on the approach. This is useful for cross-checking altitude and timing during an instrument approach in low visibility.
Intuition Check
A marker beacon is not a runway light or a flashing tower. In this context, it is a radio signal that marks a specific point by triggering a cockpit light and sound as you pass over it.
Example Sentence 1
As they crossed the outer marker, the blue beacon light flashed and the slow Morse tone confirmed their position on the approach.
Example Sentence 2
The middle marker beacon signals the point where the aircraft should be at decision altitude on the glide slope.