Definition
The radio frequency, 75 megahertz, used by marker beacon transmitters on instrument approaches. Ground-based marker beacons transmit a fan-shaped signal straight up on 75 MHz, and the aircraft's marker beacon receiver detects this signal as the aircraft passes directly overhead, providing a visual and aural indication of position along the approach course.
Plain English
75 MHz is the single radio frequency used worldwide for marker beacons — small ground transmitters that send a signal straight up so the pilot knows the moment the aircraft passes over them on an approach.
Context Anchor
Seen in discussions of marker beacon receivers and instrument approach equipment.
Derivation
MHz stands for megahertz, meaning one million cycles per second. Hertz is named after Heinrich Hertz, the physicist who first demonstrated radio waves. So 75 MHz simply means a radio signal oscillating 75 million times per second — one specific point on the radio dial reserved for marker beacons.
Why Pilots Care
Provides pilots with both audible tones and cockpit lights that confirm distance from the runway threshold during an ILS approach.
Analogy
It is like tuning a radio to one exact station. The marker beacon receiver listens on 75 MHz so it can recognize signals sent on that frequency.
Intuition Check
75 MHz is not a voice communication frequency in this context. It is the fixed frequency used by marker beacon equipment.
Example Sentence 1
The marker beacon receiver operates on 75 MHz and triggers a light and tone when the aircraft passes over the outer marker.
Example Sentence 2
Tuning the marker beacon receiver to 75 MHz lets the pilot receive position fixes while flying the localizer course.