Definition
A low-power ground-based transmitter that radiates a narrow, fan-shaped signal straight up at 75 MHz to mark a specific point along an instrument approach or airway. As the aircraft passes directly overhead, a receiver in the cockpit triggers a colored light and an audio tone, telling the pilot exactly where they are along the approach path.
Plain English
A small ground transmitter that sends a signal straight up. When you fly over it, a light and tone in the cockpit tell you you've just passed a known point on the approach.
Context Anchor
Seen in instrument flying, older approach charts, and cockpit marker beacon lights or tones.
Derivation
Marker means 'something that marks a position.' Beacon comes from Old English beacen, meaning a signal or sign. So a marker beacon is literally a signal that marks a place — in this case, a point in the sky above the transmitter.
Why Pilots Care
Gives exact position fixes during low-visibility approaches so the pilot can time descent steps, configure the aircraft, and decide on continuing or going missed.
Grounding Statement
As the aircraft flies over the beacon’s ground location, the signal is received briefly, like crossing an invisible line in the sky.
Intuition Check
A marker beacon is not a physical sign or a flashing airport light. In this context, it is a radio signal that marks a known point for the aircraft.
Example Sentence 1
As the aircraft crossed the outer marker, the blue light flashed and a steady tone sounded in the headset.
Example Sentence 2
The middle marker beacon tone sounded as we configured for landing on the ILS.