Definition
An airborne and ground-based radio navigation system that measures the slant-range distance in nautical miles between an aircraft and a selected ground station. The aircraft transmits paired pulses to the ground station, which replies after a fixed delay; the airborne receiver measures the round-trip time and converts it into a distance readout in the cockpit. DME ground stations are typically co-located with VOR or ILS facilities and operate in the UHF band.
Plain English
A radio tool in the aircraft that constantly tells the pilot how far they are, in nautical miles, from a chosen ground station.
Context Anchor
Seen on navigation displays, instrument approach procedures, and route descriptions that use DME distances from a navigation station.
Derivation
The name is literal — equipment that measures distance — but it helps to know the distance is measured by timing radio pulses, not by GPS or by anything visual. The number on the cockpit display is built from how long a radio signal takes to fly to the ground station and back.
Why Pilots Care
Gives precise distance to waypoints for timing, fuel planning, and identifying fixes on charts.
Analogy
It is like asking a fixed point on the ground, “How far away am I?” and getting the answer back in nautical miles.
Grounding Statement
If the aircraft is directly above the station, DME may still show a distance because it measures the straight-line distance from the aircraft to the station, including height.
Intuition Check
Do not assume Distance Measuring Equipment always shows distance over the ground. It shows slant-range distance: the straight-line distance from the aircraft to the station.
Example Sentence 1
The approach chart called for crossing the final approach fix at 6.4 DME, so the pilot tuned the paired VOR frequency and watched the distance count down.
Example Sentence 2
On the approach plate the pilot used the Distance Measuring Equipment reading to confirm passage of the 5-mile fix.