Distance measuring equipment or DME is really a transponder-based radio navigation technology that calculates distance by timing the propagation delay of VHF or UHF radio signals.
DME is identical to secondary radar, with the exception of reverse. The system had been a post-war progression of the IFF (identification friend or foe) programs of World War II. To keep up compatibility, DME is functionally similar to the distance measuring element of TACAN.
Airplanes use DME to find out their distance from a land-based transponder by transmitting and acquiring pulse pairs - two pulses of fixed duration and separation. The ground stations are typically co-located with VORs. A normal DME ground transponder system for en-route or terminal navigation should have a 1 kW peak pulse output on the assigned UHF channel.
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