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If the dividers will not reach between the two points, set the dividers at a fixed distance (30
NM is a good distance), and "walk off" this fixed distance along the course.
Set the dividers for 30 NM using any meridian.
Place the dividers along the course line with one tip on the departure point. Rotate
the dividers by lifting one point off the departure and keeping the other point on the
course line. Lay the first tip on the course ahead of the other. Continue "walking" the
dividers in this manner until the point of the dividers ends up past the destination
point. Count each "step" of the walk in multiples of 30 (30, 60, 90, etc.). Now
squeeze the dividers closed to measure off this remaining distance and add it to the
multiples of 30.
Recall the discussion in lesson 4.1 concerning the operation of the TACAN. If the aircrew
knows what radial of the TACAN the aircraft is currently on and the distance from the
station, then the position of the aircraft relative to the station can be determined. This
ultimately determines the aircraft's position over the earth. The information relative to the
station is displayed in the cockpit on an instrument called the Bearing Distance Heading
Indicator (BDHI). Figure 4.2-23 contains a typical BDHI found in most military aircraft.
The information concerning the TACAN is displayed on the #2 needle. The point of the
needle (called the head) gives a magnetic bearing to the station. The tail displays the
current radial. In figure 4.2-23, the aircraft is on the 135 radial and is 7.5 nm from the
station. The distance displayed is actually a slant range. For purposes of this course the
slant range is equal to the ground range.
To determine our position we must first determine the magnetic variation of the station.
This is found in the enroute supplement under the name of the TACAN or under the
NAVAID section of an airfield (for a TACAN located on an airfield). If the aircrew had
selected the Lake Charles TACAN to fix their position, they would have had to look under
Lake Charles to find that the magnetic variation is 7 east.
This 7 must be ADDED to the 135 radial in order to plot the true radial (Refer back to the
section in this unit on variation. Because we are going from magnetic to true, the formula is
reversed). This produces a True radial of 142. This is plotted from the station using the
techniques described previously in the plotting section. The last thing to do, is measure the
distance from the station, and mark the point on the radial drawn. The circle in Figure 4.2-
24 is the TACAN position fix.

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