Figure 1-17. TACAN Azimuth Cone of Confusion

INSTRUMENT FLIGHT RULES WORKBOOK

CHAPTER ONE

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Figure 1-17 TACAN Azimuth Cone of Confusion

Range indicator fluctuations. Slight oscillations up to approximately NM are normal for

range indicator operation. When a usable signal is lost, the memory circuit maintains the

indicated range for about 10 seconds. If the signal is regained during this period, the indicator

will "jump" to the correct reading.

4. Erroneous TACAN indications. Several forms of malfunction of airborne equipment or

interference between ground stations can give false or erroneous TACAN navigational

information to an aviator. These discrepancies are easier to recognize and guard against if the

aviator is aware they can occur. The more common erroneous indications are:

A 40 azimuth error lock-on/lock-off. The construction of the TACAN ground

antenna is such that it transmits a series of nine signal lobes (eight auxiliary and one

main reference pulse) 40 apart. With the airborne receiver working correctly, the

main reference pulse (which occurs when the peak of the rotating cardioid pattern is

at the 090 magnetic direction) locks on at the 090 slot of the receiver. With a weak

airborne receiver, the reference pulse may "slide over" or miss the 090 slot and lock

on at one of the auxiliary positions. When this occurs, azimuth indications will be 40

or some multiple of 40 in error. Rechanneling (retuning) the airborne receiver to

deliberately cause unlock may cause the receiver to lock on properly. When other

bearing information such as VOR or ADF is available, it should be used to verify the

position periodically. This type error is unusual, but possible in present day TACAN

sets.

Co-channel interference. Co-channel interference occurs when a aircraft is in a

position to receive TACAN signals from more than one ground station on the same

frequency. Normally this occurs only at very high altitudes when distance separation

INTRODUCTION TO AIRBORNE NAVIGATION AND COMMUNICATIONS

EQUIPMENT AND PRINCIPLES OF OPERATION 1-21