What distance does DME measure, and how accurate is it?

1-1-7. Distance Measuring Equipment (DME)
a. In the operation of DME, paired pulses at a specific spacing are sent out from the aircraft (this is the
interrogation) and are received at the ground station. The ground station (transponder) then transmits paired
pulses back to the aircraft at the same pulse spacing but on a different frequency. The time required for the round
trip of this signal exchange is measured in the airborne DME unit and is translated into distance (nautical miles)
from the aircraft to the ground station.
b. Operating on the line-of-sight principle, DME furnishes distance information with a very high degree of
accuracy. Reliable signals may be received at distances up to 199 NM at line-of-sight altitude with an accuracy
of better than 1/2 mile or 3 percent of the distance, whichever is greater. Distance information received from DME
equipment is SLANT RANGE distance and not actual horizontal distance.
c. Operating frequency range of a DME according to ICAO Annex 10 is from 960 MHz to 1215 MHz. Aircraft
equipped with TACAN equipment will receive distance information from a VORTAC automatically, while
aircraft equipped with VOR must have a separate DME airborne unit.
d. VOR/DME, VORTAC, Instrument Landing System (ILS)/DME, and localizer (LOC)/DME navigation
facilities established by the FAA provide course and distance information from collocated components under a
frequency pairing plan. Aircraft receiving equipment which provides for automatic DME selection assures
reception of azimuth and distance information from a common source when designated VOR/DME, VORTAC,
ILS/DME, and LOC/DME are selected.
e. Due to the limited number of available frequencies, assignment of paired frequencies is required for certain
military noncollocated VOR and TACAN facilities which serve the same area but which may be separated by
distances up to a few miles.
f. VOR/DME, VORTAC, ILS/DME, and LOC/DME facilities are identified by synchronized identifications
which are transmitted on a time share basis. The VOR or localizer portion of the facility is identified by a coded
tone modulated at 1020 Hz or a combination of code and voice. The TACAN or DME is identified by a coded
tone modulated at 1350 Hz. The DME or TACAN coded identification is transmitted one time for each three or
Navigation Aids 1-1-5

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four times that the VOR or localizer coded identification is transmitted. When either the VOR or the DME is
inoperative, it is important to recognize which identifier is retained for the operative facility. A single coded
identification with a repetition interval of approximately 30 seconds indicates that the DME is operative.
g. Aircraft equipment which provides for automatic DME selection assures reception of azimuth and distance
information from a common source when designated VOR/DME, VORTAC and ILS/DME navigation facilities
are selected. Pilots are cautioned to disregard any distance displays from automatically selected DME equipment
when VOR or ILS facilities, which do not have the DME feature installed, are being used for position
determination.