Figure 7 - P-8010028

INSTRUMENT NAVIGATION

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.

Distance computation

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.

Slant Range

The DME will always be a slant range from the aircraft's position to the TACAN station.

Consider the following diagrams.

Figure 7

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

360

2-6 NAVAID POSITIONING