DME

DME is a radionavigation system that measures the slant range distance between an aircraft and a ground station. The pilot or autopilot uses DME to determine exact distance to a known ground facility, typically a VOR station or airport. The system works by the aircraft transmitter sending a pulse, the ground station receiving it and responding with a return pulse, and the aircraft receiver measuring the round-trip time. Since radio signals travel at a constant speed, time elapsed converts directly to distance in nautical miles.

DME operates in the ultrahigh frequency band, usually between 960 and 1215 MHz. A single ground station can service up to 100 aircraft simultaneously. The system provides slant range, not horizontal distance, meaning the readout includes vertical separation between the aircraft and station; at 5000 feet altitude over a station 10 nautical miles away horizontally, DME reads slightly more than 10 miles. Accuracy is typically within 0.25 nautical miles or 5 percent of distance, whichever is greater.

Aircraft receive DME signals passively, but the ground station must transmit back. This interrogate-respond design means a broken aircraft DME receiver cannot trigger a station to broadcast; the station only answers questions. Ground stations are usually collocated with VOR facilities and identified by their own call sign during voice identification. In modern practice, DME has become less critical as GPS and inertial systems have matured, but it remains a legal standby for instrument approaches and holds regulatory standing in many regions.

Common failures on the aircraft side include aging antenna connections at the fuselage, worn coaxial cables in the installation, and receiver tuning drift. Ground-based issues center on transmitter downtime and occasional antenna pattern degradation. Older DME equipment requires periodic certification; some operators decommission installed DME receivers entirely and rely on other nav sources, though regulatory requirements vary by airspace and operation type.