Note that in this instance, the position of the aircraft has not changed (i.e. it is still on the
260° radial at 163 NM form the TACAN station ). The only thing that has changed is the
heading, which is now 360°.
Computation of distances is made easy for us by the DME portion of the TACAN.
Anytime your TACA N is in the T/R mode and tuned to a functional TACAN station that is
within reception range of your aircraft, DME will be displayed.
The DME will always be a slant range from the aircraft's position to the TACAN station.
Consider the following diagrams.
Both diagrams depict an aircraft at 36,000 feet (6nm) above the Earth. The aircraft in
diagram (A) has 10 DME displayed, yet in just 8nm it will be overhead the TACAN station.
The aircraft in diagram B at the same altitude, displays 60 DME. It will pass over the
TACAN station in 59.7nm. It should be apparent that we are simply solving for the third side
of a right triangle. As the distance away from the TACAN increases DME will nearly equal
actual ground range to the TACAN.
Distance Between Radials
In the diagram on the following page, the symbol for a VORTAC is displayed with selected
radials emanating from it. A circle is drawn around the VORTAC and its radials at a point
60nm from its center. Utilizing geometry this time, we can determine that the circumference
of the circle is 2 R or 2 x 3.1415 x 60 = 377. Recalling now that each TACAN (and VOR)
possesses 360 radials we can establish this relationship: at 60nm from any TACAN station the
distance between each radial is:
377 or 1.05nm
2-6 NAVAID POSITIONING