Marker beacon annunciator lights Radio beacons typically used in combination with an ILS instrument approach. A beacon is indicated by a light and an aural tone when the aircraft flies over the it. An ILS may have up to three marker beacons. The outer marker causes a blue light to flash and beeps in a series of Morse code dashes. It typically marks the point where an aircraft can begin its final descent. The middle marker flashes an amber light, and it beeps in a pattern of alternating dashes and dots. It is normally the missed approach point, where the pilot must decide whether he can land safely. The inner marker is used only on certain approaches. Its light is white, and the tone is a series of Morse dots.