Accuracy
The accuracy of DME ground stations is 185 m (±0.1 nmi). It's important to understand 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 the ground distance from it.
For example, an aircraft directly above the DME station at 6076 ft (1 nmi) altitude would still show 1.0 nmi (1.9 km) on the DME readout. The aircraft is technically a mile away, just a mile straight up. Slant range error is most pronounced at high altitudes when close to the DME station.
Radio-navigation aids must keep a certain degree of accuracy, given by international standards, FAA, EASA, ICAO, etc. To assure this is the case, flight inspection organizations check periodically critical parameters with properly equipped aircraft to calibrate and certify DME precision.
ICAO recommends accuracy of 0.25 nmi plus 1.25% of the distance measured.
Read more about this topic: Distance Measuring Equipment
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