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VOR test signal continuously. The owner or operator of the
aircraft must make arrangements with the repair station to
have the test signal transmitted. A representative of the repair
station must make an entry into the aircraft logbook or other
permanent record certifying to the radial accuracy and the
date of transmission.
Certified Checkpoints
Airborne and ground checkpoints consist of certified radials
that should be received at specific points on the airport surface
or over specific landmarks while airborne in the immediate
vicinity of the airport. Locations of these checkpoints are
published in the A/FD.
Should an error in excess of ±4° be indicated through use of
a ground check, or ±6° using the airborne check, IFR flight
shall not be attempted without first correcting the source of
the error. No correction other than the correction card figures
supplied by the manufacturer should be applied in making
these VOR receiver checks.
If a dual system VOR (units independent of each other except
for the antenna) is installed in the aircraft, one system may
be checked against the other. Turn both systems to the same
VOR ground facility and note the indicated bearing to that
station. The maximum permissible variation between the two
indicated bearings is 4°.
Distance Measuring Equipment (DME)
When used in conjunction with the VOR system, DME makes
it possible for pilots to determine an accurate geographic
position of the aircraft, including the bearing and distance
TO or FROM the station. The aircraft DME transmits
interrogating radio frequency (RF) pulses, which are received
by the DME antenna at the ground facility. The signal triggers
ground receiver equipment to respond to the interrogating
7-17
Figure 7-17. DME Arc Interception.
aircraft. The airborne DME equipment measures the elapsed
time between the interrogation signal sent by the aircraft and
reception of the reply pulses from the ground station. This
time measurement is converted into distance in nautical miles
(NM) from the station.
Some DME receivers provide a groundspeed in knots by
monitoring the rate of change of the aircraft’s position relative
to the ground station. Groundspeed values are accurate only
when tracking directly to or from the station.
DME Components
VOR/DME, VORTAC, ILS/DME, and LOC/DME
navigation facilities established by the FAA provide course
and distance information from collocated components under
a frequency pairing plan. DME operates on frequencies
in the UHF spectrum between 962 MHz and 1213 MHz.
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.
Some aircraft have separate VOR and DME receivers, each
of which must be tuned to the appropriate navigation facility.
The airborne equipment includes an antenna and a receiver.
The pilot-controllable features of the DME receiver
include:
Channel (Frequency) Selector
Many DMEs are channeled by an associated VHF radio, or
there may be a selector switch so a pilot can select which
VHF radio is channeling the DME. For a DME with its
own frequency selector, use the frequency of the associated
VOR/DME or VORTAC station.
On/Off/Volume Switch
The DME identifier will be heard as a Morse code identifier
with a tone somewhat higher than that of the associated VOR
or LOC. It will be heard once for every three or four times
the VOR or LOC identifier is heard. If only one identifier is
heard about every 30 seconds, the DME is functional, but
the associated VOR or LOC is not.
Mode Switch
The mode switch selects between distance (DIST) or distance
in NMs, groundspeed, and time to station. There may also
be one or more HOLD functions which permit the DME to
stay channeled to the station that was selected before the
switch was placed in the hold position. This is useful when
you make an ILS approach at a facility that has no collocated
DME, but there is a VOR/DME nearby.
Altitude
Some DMEs correct for slant-range error.
Function of DME
A DME is used for determining the distance from a ground
DME transmitter. Compared to other VHF/UHF NAVAIDs,
a DME is very accurate. The distance information can be
used to determine the aircraft position or flying a track that
is a constant distance from the station. This is referred to as
a DME arc.
DME Arc
There are many instrument approach procedures (IAPs) that
incorporate DME arcs. The procedures and techniques given
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