INSTRUMENT FLIGHT RULES WORKBOOK
CDI oscillates from side to side, and
TO/FROM indicator switches from "TO" to "FROM."
Frequencies and Ground Equipment
1. Frequencies. TACAN operates in the UHF (1000 MHz) band with a total of 126 two-way
channels in the operational mode (X or Y). The DME air-to-ground frequencies for these
channels are in the 1.025 to 1150 MHz range and the associated ground-to-air frequencies are in
the 962 to 1213 MHz range.
2. Ground equipment. TACAN ground equipment consists or a rotating type antenna for
transmitting bearing information and a receiver-transmitter (transponder) for transmitting
distance information. The TACAN station is identified by an international Morse-coded tone
modulated at 1350 Hz with a reception interval of approximately 37.5 seconds. Permanent
TACAN ground stations are usually dual transmitter equipped (one operating and one on
standby), fully monitored installations which automatically switch to the standby transmitter
when a malfunction occurs. The ground monitor, set to alarm at any radial shift of plus or minus
one degree, is usually located in the base control tower or approach control and sets off a light
and buzzer to warn the ground crew when an out-of-tolerance condition exists. Sometimes
TACAN reception might be suspected of being in error or bearing/distance unlock conditions
might be encountered in flight. When this occurs, the status of the ground equipment can be
checked by calling ATC. When ground equipment is undergoing tests or repairs which might
cause it to transmit erroneous signals, its identification is silenced. Therefore, always listen for
identification signals during flight.
The signal pattern for bearing information is formed by varying the nondirectional pattern sent
types of bearing signal patterns are the coarse and fine azimuth patterns.
1. Coarse azimuth pattern. The pattern is created by rotating a plastic cylinder around the
central element of the antenna at 15 revolutions per second (RPS). A metal wire embedded
vertically in the cylinder distorts the radiated signal into a cardioid (heart-shaped) pattern. Its
rotation causes the cardioid pattern to also revolve at 15 RPS. This resulting rotating pattern
(Figure 1-11) is referred to as the coarse pattern. From this, the aircraft receives an amplitude
modulation of 15 Hz. This means that the strength of the signal goes from maximum to
minimum and back to maximum at the rate of 15 times per second.
2. Fine azimuth pattern. To produce the fine pattern, another larger plastic cylinder
containing nine wires is mounted around the central element and the smaller cylinder and also
rotates at 15 RPS. This is the fine antenna which superimposes nine lobes on the already formed
coarse pattern (Figure 1-12). This forms a 135 Hz signal.
1-14 INTRODUCTION TO AIRBORNE NAVIGATION AND COMMUNICATIONS
EQUIPMENT AND PRINCIPLES OF OPERATION