Distance measuring equipment

This article is about the aviation equipment. For more general usage, see Distance measurement.

In aviation, distance measuring equipment (DME) is a radio navigation technology that measures the slant range (distance) between an aircraft and a ground station by timing the propagation delay of radio signals in the frequency band between 960 and 1215 megahertz (MHz). Line-of-sight between the aircraft and ground station is required. An interrogator (airborne) initiates an exchange by transmitting a pulse pair, on an assigned 'channel', to the transponder ground station. The channel assignment specifies the carrier frequency and the spacing between the pulses. After a known delay, the transponder replies by transmitting a pulse pair on a frequency that is offset from the interrogation frequency by 63 MHz and having specified separation.

  • DME systems are used worldwide, using standards set by the International Civil Aviation Organization (ICAO), RTCA, the European Union Aviation Safety Agency (EASA) and other bodies. Some countries require that aircraft operating under instrument flight rules (IFR) be equipped with a DME interrogator; in others, a DME interrogator is only required for conducting certain operations.

While stand-alone DME transponders are permitted, DME transponders are usually paired with an azimuth guidance system to provide aircraft with a two-dimensional navigation capability. A common combination is a DME co-located with a VHF omnidirectional range (VOR) transmitter in a single ground station, designated as VOR/DME. When this occurs, the frequencies of the VOR and DME equipment are paired. Such a configuration enables an aircraft to determine its azimuth angle and distance from the station. A VORTAC (a VOR co-located with a TACAN) installation provides the same capabilities to civil aircraft but also provides 2-D navigation capabilities to military aircraft.

Low-power DME transponders are also associated with some instrument landing system (ILS), ILS localizer and microwave landing system (MLS) installations. In those situations, the DME transponder frequency/pulse spacing is also paired with the ILS, LOC or MLS frequency.

ICAO characterizes DME transmissions as ultra high frequency (UHF). The term L-band is also used.

Developed in Australia, DME was invented by James "Gerry" Gerrand..

In 1945, after Edward George Bowen became Chief of the Division of Radiophysics at CSIRO, Brian Cooper developed DME. Based upon the Rebecca/Eureka transponding radar, the 200 MHz system weighed less, and used a dial instead of a screen. In 1946, the Provisional International Civil Aviation Organization adopted the VOR and DME airways model. In 1947, the system was deployed on commercial aircraft, and by 1953, all commercial aircraft in Australia were so equipped.

DME is similar in principle to secondary radar ranging function, except the roles of the equipment in the aircraft and on the ground are reversed. DME was a post-war development based on the identification friend or foe (IFF) systems of World War II. To maintain compatibility, DME is functionally identical to the distance measuring component of TACAN.

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