It is normally positioned 0.5 to 0.8 nautical miles (1 km) before the runway threshold. When the aircraft is above the middle marker, the receiver's amber middle marker light starts blinking, and a repeating pattern of audible morse code-like dot-dashes at a frequency of 1,300 Hz in the headset. This alerts the pilot that they are descending through the CAT I decision altitude (typically 200 feet (60 m) above the ground level on the glideslope) and should have already initiated the missed approach if one of several visual cues has not been spotted.