Distance measuring equipment
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).
An interrogator (airborne) initiates an exchange by transmitting a pulse pair, on an assigned 'channel', to the transponder ground station.
A common combination is a DME colocated with a VHF omnidirectional range (VOR) transmitter in a single ground station.
[4] Developed in Australia, DME was invented by James "Gerry" Gerrand[5] under the supervision of Edward George "Taffy" Bowen while employed as Chief of the Division of Radiophysics of the Commonwealth Scientific and Industrial Research Organisation (CSIRO).
Another engineered version of the system was deployed by Amalgamated Wireless Australasia Limited in the early 1950s operating in the 200 MHz VHF band.
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.
In its first iteration, a DME-equipped airplane used the equipment to determine and display its distance from a land-based transponder by sending and receiving pulse pairs.
A typical DME ground transponder for en-route or terminal navigation will have a 1 kW peak pulse output on the assigned UHF channel.
A radio signal takes approximately 12.36 microseconds to travel 1 nautical mile (1,852 m) to the target and back.
Slant range error is most pronounced at high altitudes when close to the DME station.
To assure this is the case, flight inspection organizations check periodically critical parameters with properly equipped aircraft to calibrate and certify DME precision.
A typical DME ground-based transponder beacon has a limit of 2700 interrogations per second (pulse pairs per second – pps).
The U.S. FAA has installed three DME transponder types (not including those associated with a landing system): Terminal transponders (often installed at an airport) typically provide service to a minimum height above ground of 12,000 feet (3,700 m) and range of 25 nautical miles (46 km); Low altitude transponders typically provide service to a minimum height of 18,000 feet (5,500 m) and range of 40 nautical miles (74 km); and High altitude transponders, which typically provide service to a minimum height of 45,000 feet (14,000 m) and range of 130 nautical miles (240 km).
However, many have operational restrictions largely based on line-of-sight blockage, and actual performance may be different.
There are assignment 'holes' centered on 1030 and 1090 megahertz to provide protection for the secondary surveillance radar (SSR) system.
Precision DME (DME/P), a component of the Microwave Landing System, is assigned to Z channels, which have a third set of interrogation and reply pulse spacings.
However, ground-based navigation will continue, for three reasons:[citation needed] One advantage of the fifth-generation equipment proposed in 2020 is its ability to be function-checked by drone flights, which will significantly reduce the expense and delays of previous manned certification flight tests.
