Definition
A radio navigation system that uses paired pulses of microwave-frequency radio signals between an aircraft transceiver and a ground station to measure and continuously display the slant-range distance, in nautical miles, from the aircraft to the station.
Plain English
A piece of avionics that constantly tells the pilot how far the aircraft is from a ground navigation station, in nautical miles. It works by sending out radio pulses and timing how long the reply takes to come back.
Context Anchor
You may see DME on a cockpit display, in avionics equipment lists, in navigation discussions, and in maintenance or preventive maintenance material dealing with installed radio equipment.
Derivation
The phrase is straightforward — it measures distance — but "microwave frequency" matters here. Microwaves are very short radio waves (in DME's case, around 1 GHz, in the UHF band). They travel in straight lines and need line-of-sight to the ground station, which is why DME range depends on altitude.
Why Pilots Care
Gives accurate distance data needed for position fixing, holding patterns, instrument approaches, and fuel planning.
Analogy
Think of DME like an electronic tape measure stretched in a straight line from the airplane to the ground station. It measures that straight line, not the road-like distance across the ground below.
Grounding Statement
When the aircraft sends a signal to the DME ground station, the system times the reply and converts that time into a displayed distance.
Intuition Check
Do not assume DME shows distance along the ground. It shows straight-line distance from the aircraft to the station.
Example Sentence 1
Crossing the fix at 3,000 feet, the pilot confirmed the position by checking the DME, which read 12.4 nautical miles from the VOR.
Example Sentence 2
With the DME locked on, the crew maintained the required distance from the station during the hold.