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readings when located at greater distances where
weaker signal areas exist. The likelihood of this
deterioration varies between receivers, and is
generally considered a function of time. The best
assurance of having an accurate receiver is periodic
calibration. Yearly intervals are recommended at
which time an authorized repair facility should
recalibrate the receiver to the manufacturer’s
specifications.
d. Federal Aviation Regulations (14 CFR Section
91.171) provides for certain VOR equipment
accuracy checks prior to flight under instrument
flight rules. To comply with this requirement and to
ensure satisfactory operation of the airborne system,
the FAA has provided pilots with the following means
of checking VOR receiver accuracy:
1. VOT or a radiated test signal from an
appropriately rated radio repair station.
2. Certified airborne check points.
3. Certified check points on the airport surface.
e. A radiated VOT from an appropriately rated
radio repair station serves the same purpose as an
FAA VOR signal and the check is made in much the
same manner as a VOT with the following
differences:
1. The frequency normally approved by the
Federal Communications Commission is
108.0 MHz.
2. Repair stations are not permitted to radiate the
VOR test signal continuously; consequently, the
owner or operator must make arrangements with the
repair station to have the test signal transmitted. This
service is not provided by all radio repair stations.
The aircraft owner or operator must determine which
repair station in the local area provides this service.
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
2/19/04 AIM
Air Navigation Aids 1−1−3
of transmission. The owner, operator or representative
of the repair station may accomplish the
necessary checks in the aircraft and make a logbook
entry stating the results. It is necessary to verify
which test radial is being transmitted and whether you
should get a “to” or “from” indication.
f. Airborne and ground check points 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.
1. Should an error in excess of plus or minus
4 degrees be indicated through use of a ground check,
or plus or minus 6 degrees using the airborne check,
Instrument Flight Rules (IFR) flight shall not be
attempted without first correcting the source of the
error.
CAUTION−
No correction other than the correction card figures
supplied by the manufacturer should be applied in
making these VOR receiver checks.
2. Locations of airborne check points, ground
check points and VOTs are published in the A/FD and
are depicted on the A/G voice communications
panels on the FAA IFR area chart and IFR enroute low
altitude chart.
3. 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 variations between the two
indicated bearings is 4 degrees.
1−1−5. Tactical Air Navigation (TACAN)
a. For reasons peculiar to military or naval
operations (unusual siting conditions, the pitching
and rolling of a naval vessel, etc.) the civil
VOR/Distance Measuring Equipment (DME) system
of air navigation was considered unsuitable for
military or naval use. A new navigational system,
TACAN, was therefore developed by the military and
naval forces to more readily lend itself to military and
naval requirements. As a result, the FAA has
integrated TACAN facilities with the civil VOR/
DME program. Although the theoretical, or technical
principles of operation of TACAN equipment are
quite different from those of VOR/DME facilities, the
end result, as far as the navigating pilot is concerned,
is the same. These integrated facilities are called
VORTACs.
b. TACAN ground equipment consists of either a
fixed or mobile transmitting unit. The airborne unit in
conjunction with the ground unit reduces the
transmitted signal to a visual presentation of both
azimuth and distance information. TACAN is a pulse
system and operates in the Ultrahigh Frequency
(UHF) band of frequencies. Its use requires TACAN
　
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