Aeronautical Information Manual (AIM), page 57

Index   56 -- Page 57 -- 58


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
of transmission. The owner, operator or representat-
ive 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 must not be
attempted without first correcting the source of the
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 Chart
Supplement U.S.

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
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
airborne equipment and does not operate through
conventional VOR equipment.

1-1-6. VHF Omni-directional

Range/Tactical Air Navigation (VORTAC)

a. A VORTAC is a facility consisting of two
components, VOR and TACAN, which provides
three individual services: VOR azimuth, TACAN
azimuth and TACAN distance (DME) at one site.
Although consisting of more than one component,
incorporating more than one operating frequency,
and using more than one antenna system, a VORTAC
is considered to be a unified navigational aid. Both
components of a VORTAC are envisioned as
operating simultaneously and providing the three
services at all times.

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

1-1-7. Distance Measuring Equipment


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

Navigation Aids 1-1-3

Page 57 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   56 -- Page 57 -- 58