Definition
The two pieces of equipment that make a Distance Measuring Equipment (DME) system work: an airborne interrogator installed in the aircraft, and a ground-based transponder located at a navigation facility such as a VOR or ILS. The interrogator sends paired pulses on a UHF frequency; the ground transponder receives them and replies with its own paired pulses on a different UHF frequency. The aircraft equipment measures the round-trip time of the signals and converts it into slant-range distance in nautical miles, which is displayed in the cockpit along with groundspeed and time-to-station.
Plain English
DME has two parts that work together: a radio in your aircraft that asks 'how far am I?' and a radio on the ground that answers. The aircraft times how long the round trip takes and turns that into a distance reading on your instrument panel.
Context Anchor
Seen in instrument flying when learning how DME equipment is built, selected, tested, and used with navigation facilities.
Derivation
DME stands for Distance Measuring Equipment. 'Interrogator' comes from Latin interrogare, 'to question' -- the aircraft unit literally questions the ground station. 'Transponder' is a blend of transmitter and responder -- it transmits a response when interrogated. The names describe exactly what each component does.
Why Pilots Care
Knowing that DME relies on two cooperating radios explains why a DME reading can fail: the airborne interrogator may be unserviceable, the ground transponder may be out, or the aircraft may be out of line-of-sight range. It also explains why the displayed distance is slant range, not distance over the ground -- the signal travels directly between aircraft and station, including the vertical component.
Intuition Check
Do not think of DME components as only the box or display in the cockpit. DME depends on both aircraft equipment and a replying ground station working together.
Example Sentence 1
Before departure, the pilot confirmed both DME components were working by tuning a nearby VORTAC and checking that the cockpit display showed a valid distance.
Example Sentence 2
Maintenance checked the ground DME components after reports of inaccurate distance readings from aircraft.