Frequencies and Ground Equipment

CHAPTER ONE

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.

Signal Patterns

The signal pattern for bearing information is formed by varying the nondirectional pattern sent

from the stationary central element of the TACAN transmitter antenna (Figure 1-10). The two

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