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What is distance measuring equipment that is used within a flight planner?
Distance measuring equipment is a transponder based DP2400 two way radio navigation technology that measures slant range distance by timing the propagation delay or VHF or UHF radio signals. Developed in Australia, it was invented by Edward George Bowen while employed as Chief of the Division of Radio physics of the Commonwealth Scientific and Industrial Research Organisation (CSIRO).
Another engineered version of the system was deployed in the early 1950s operating in the 200 MHz VHF band. The Australian domestic version was referred to by the Federal Department of Civil Aviation as DME (D) and the later international version adopted by ICAO as DME.
DME is similar to secondary radar, except in reverse. This system was a post war development of the IFF systems of World War II. To maintain compatibility, DME is functionally identical to the distance measuring component of TACAN used within a flight planner.
Aircraft use DME to determine their distance from a land based transponder by sending and receiving pulse pairs. These are two pulses of fixed duration and separation. The ground stations are typically located with VORs. A typical DME ground transponder system for en-route or terminal navigation will have a 1 kW peak pulse output on the assigned UHF channel.
A low power DME can also be co-located with an ILS glide slope antenna installation where it provides an accurate distance to touchdown function, similar to that otherwise provided by ILS Marker Beacons.
Distance calculation and accuracy used within a flight planner
When using a flight planner, a radio pulse takes around 12.36 microseconds to travel 1 nautical mile to and from and it is also referred to as a radar mile. The time difference between interrogation and reply 1 nautical mile minus the 50 microsecond ground transponder delay is measured by the interrogator’s timing circuitry and translated into a distance measurement (slant range) which is stated in nautical miles and then displayed on the cockpit DME display.
The accuracy of DME ground stations is 185m. Its important to know that DME provides the physical distance from the aircraft to the DME transponder. This distance is often referred to as ‘slant range’ and depends trigonometrically upon both the altitude above the transponder and ground distance from it in a flight planner.