Distance Measuring Equipment (DME)

Distance Measuring Equipment (DME) is a radio navigation system that provides slant-range distance from an aircraft to a ground station. The airborne DME interrogator transmits a pulse; the ground transponder (co-located with a VOR or ILS) responds; the airborne unit measures the round-trip time and converts it to distance in nautical miles.

Slant Range vs. Ground Distance

DME measures slant range — the straight-line distance from the aircraft to the antenna, not the horizontal ground distance. At low altitudes far from the station, slant range and ground distance are nearly identical. At high altitudes directly over the station, slant range equals altitude AGL and ground distance is zero. For practical navigation, slant range is used without correction.

VOR-DME and ILS-DME

DME is co-located with most VORs (called VOR-DME or VORTAC). It is also co-located with ILS glideslopes (DME-ILS), where it provides a distance cross-check for the approach. DME distance can substitute for some marker beacons on ILS approaches when authorized by the approach procedure.

DME Arcs

DME arcs are instrument approach transitions that involve flying at a constant DME distance from a VOR while intercepting the final approach course. They require active manual control or FMS input to maintain the arc. DME arc procedures are common on many Canadian instrument approaches.

Related Terms

See also: VOR, ILS, Marker Beacon.