Plain English Meaning
A two-in-one ground station near the start of the final approach to a runway. It sends out a steady radio signal that helps the pilot fly toward it, and it also sends a short upward signal that flashes a light and makes a tone in the cockpit when the aircraft passes directly over it.
Full Definition
A combined navigation aid located at the outer marker position of an Instrument Landing System (ILS) approach. It pairs a low-power non-directional beacon (), which transmits a signal that an () in the cockpit can to, with an that transmits a narrow vertical signal triggering a indication as the aircraft passes overhead. The is typically located 4 to 7 from the along the .