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CHAPTER ONE
INSTRUMENT FLIGHT RULES WORKBOOK
Distance Measuring Equipment
Distance between the aircraft and the ground station is determined with DME by measuring the
elapsed time between transmission of interrogating pulses of the airborne set and reception of
corresponding reply pulses or the ground station (Figure 1-14). The aircraft transmitter starts the
process by sending out the distance interrogation pulse signals. Receipt of these signals by the
ground station receiver triggers its transmitter which sends out the distance reply pulse signals.
These pulses require approximately 12 microseconds round trip travel time per nautical mile of
distance from the ground beacon. The range indicator displays slant-range distance
(Figures 1-15 and 1-16) to the TACAN beacon in nautical miles. Since a large number of
aircraft could be interrogating the same beacon, any particular airborne set must sort out only the
pulses which are replies to its own interrogations. Interrogation pulses are transmitted on an
irregular, random basis by the airborne set which then "searches" for replies synchronized to its
own interrogations. If the signals are interrupted, a memory circuit maintains the last distance
indication on the range indicator for approximately 10 seconds to prevent the search operation
from recurring. The searching process starts automatically 10 seconds after the airborne set is
tuned to a new beacon or when there is a major interruption in beacon signals. Depending upon
the aircraft's actual distance from the beacon at the time, the searching process may require up to
22 seconds.
Figure 1-14 DME Principles
1-18 INTRODUCTION TO AIRBORNE NAVIGATION AND COMMUNICATIONS
EQUIPMENT AND PRINCIPLES OF OPERATION


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