Munchen

Distance Measuring Equipment (DME) tells a pilot exactly how far the aircraft is from the station. The airborne unit sends interrogation pulses on a UHF frequency (960–1215 MHz); the ground transponder replies, and the equipment times the round trip to display distance in nautical miles, usually alongside groundspeed and time-to-station.

One important subtlety: DME measures slant range — the straight-line distance from the aircraft to the station — not distance across the ground. Directly overhead at 6,000 ft, a DME reads about one nautical mile even though you are over the station. The error is negligible at normal ranges but grows close in and at high altitude. DME is frequently paired with a VOR or ILS and is used to fly arcs, identify fixes, and check altitude on approach. It is often the component of a VORTAC that civil aircraft actually use for distance.