Figure 1-15. DME Distances

INSTRUMENT FLIGHT RULES WORKBOOK

CHAPTER ONE

SLANT RANGE DISTANCES

SLANT

RANGE

ACTUAL RANGE

Figure 1-15 DME Distances

108.

TACAN CHARACTERISTICS

1. Bearing/distance unlock. TACAN bearing and distance signals are subject to line-of-sight

restrictions because of their utilization of UHF frequencies. Because of the

transmission/reception principles, unlock (indicated by rotating of bearing pointer and/or range

indicator) will occur if these signals are obstructed. Temporary obstruction of TACAN signals

can occur in flight when aircraft fuselage, wing, gear; external stores, or wingmen get between

the ground and the aircraft antenna. Aircraft receiver memory circuits prevent unlock for short

periods (approximately 10 seconds for DME and 2 seconds for azimuth). But beyond this,

unlock occurs and will persist until the obstruction is removed and search cycles are completed.

Unlock may occur during maneuvers, such as procedure turns, which cause the aircraft antenna

to be obstructed for longer than 2 to 10 seconds.

INTRODUCTION TO AIRBORNE NAVIGATION AND COMMUNICATIONS

EQUIPMENT AND PRINCIPLES OF OPERATION 1-19