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b. Transmitted signals of VOR and TACAN are
each identified by three-letter code transmission and
are interlocked so that pilots using VOR azimuth with
TACAN distance can be assured that both signals
being received are definitely from the same ground
station. The frequency channels of the VOR and the
TACAN at each VORTAC facility are “paired” in
accordance with a national plan to simplify airborne
operation.
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
AIM 2/14/08
1-1-4 Navigation Aids
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
1350Hz. The DME or TACAN coded identification
is transmitted one time for each three or 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.
1-1-8. Navigational Aid (NAVAID) Service
Volumes
a. Most air navigation radio aids which provide
positive course guidance have a designated standard
service volume (SSV). The SSV defines the reception
limits of unrestricted NAVAIDs which are usable for
random/unpublished route navigation.
b. A NAVAID will be classified as restricted if it
does not conform to flight inspection signal strength
and course quality standards throughout the
published SSV. However, the NAVAID should not be
considered usable at altitudes below that which could
be flown while operating under random route IFR
conditions (14CFR Section 91.177), even though
these altitudes may lie within the designated SSV.
Service volume restrictions are first published in
Notices to Airmen (NOTAMs) and then with the
alphabetical listing of the NAVAIDs in the A/FD.
c. Standard Service Volume limitations do not
apply to published IFR routes or procedures.
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