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Zn = Z if SIN (LHA) < 0
Zn = 360 – Z if SIN (LHA) > 0
AFPAM11-216 1 MARCH 2001 241
Figure 11.2. Line of Position Computed by Intercept Method.
11.3.2. The calculations may be performed quickly, using a programmable calculator, or they may be
extracted from the appropriate volume of Pub. No. 249. This method enables the observer to use any of
the navigational bodies available at the appropriate fix time. Here is a brief review:
11.3.2.1. Compute a DR for the time of the position, using preflight or in-flight data.
11.3.2.2. Determine the necessary entering values for the Pub. 249 volume being used (Lat, LHA, Dec
contrary or same) and extract all the necessary values of computed altitude (Hc), azimuth angle (Z), etc.
11.3.2.3. After making all the necessary conversions and corrections (Chapter 10), compare the Ho and
corrected Hc. This difference is the intercept. If the Ho equals the corrected Hc, then the circle of equal
altitude passed through the plotting position. If the Ho is greater than the Hc, the difference is plotted in
the direction of the true azimuth (Zn). The Zn represents the azimuth from the observer's position to the
subpoint. If the Ho is less than the Hc, plot the difference 180o from the Zn.
NOTE: If HO is MOre, plot TOward the subpoint ( HO MO TO )
11.3.2.4. Example: The assumed position is 38o N, 121o30' W for a shot taken at 1015Z on Aldebaran.
The Ho is 32o14'. The Hc is determined to be 32o29' and the Zn 120o. A comparison of Ho and Hc
determines the intercept to be 15 NM away (15A).
11.4. Plotting LOP Using Zn Method (Figure 11.3):
11.4.1. Plot the assumed position and set the intercept distance on the dividers.
11.4.2. Draw a dashed line through the assumed position toward the subpoint.
11.4.3. Span intercept distance along dashed Zn line.
11.4.4. Place plotter perpendicular to Zn.
242 AFPAM11-216 1 MARCH 2001
Figure 11.3. Celestial Line of Position Using True Azimuth Method.
11.4.5. Draw LOP along plotter as shown.
11.5. Plotting LOP Using Flip-Flop Method (Figure 11.4):
11.5.1. Plot the assumed position and set the intercept distance on the dividers.
11.5.2. Measure 120o of the Zn with point A of the dividers on the assumed position and place point B
of the dividers down, in this case, away from 120o or in the direction of 300o from the assumed position.
Slide the plotter along the dividers until the center grommet and the 100/200-mile mark are lined up
directly over point B of the dividers marking the intercept point.
11.5.3. Remove point A of the dividers from the assumed position, keeping point B in place. Flip point
A (that was on the assumed position) across the plotter, at the same time expanding the dividers so that
point A can be placed on the chart at the 90o/270o mark of the plotter.
11.5.4. Flop the plotter around and place the straight edge against the perpendicular, which is established
by the dividers.
11.5.5. Draw LOP along the plotter as shown.
11.6. Summary of Intercept Method. When using the intercept method, remember:
11.6.1. For some assumed position near the DR position, find the Hc and Zn of this body for the time of
the observation. This is done with the aid of celestial tables, such as Pub. No. 249, or a programmable
calculator.
AFPAM11-216 1 MARCH 2001 243
Figure 11.4. Plotting Celestial Line of Position Using Flip-Flop Method.
244 AFPAM11-216 1 MARCH 2001
11.6.2. Obtain needed corrections, sextant correction, refraction, etc., and apply these to the Hc by
reversing the sign (remember, we are striving to derive a precomputed value to ensure the correct body
is shot). Measure the altitude (Ho) of the celestial body with the sextant and record the midtime of the
observation.
11.6.3. Find the intercept, which is the difference between Ho and Hc. Intercept is toward the subpoint if
Ho is greater than Hc and away from the subpoint if Ho is smaller than Hc.
11.6.4. From the assumed position, measure the intercept toward or away from the subpoint (in the
direction of Zn or its reciprocal) and locate a point on the LOP. Through this point, draw the LOP
perpendicular to the Zn.
11.7. Additional Plotting Techniques. The preceding techniques involve the basic plotting procedures
used on most stars and the bodies of the solar system. However, there are certain techniques of plotting
that are peculiar to their own celestial methods; for example, the plotting of LOPs obtained by using
Polaris, which is discussed later. Also, certain precomputation techniques lend themselves more readily
to other plotting techniques, such as preplotting the true azimuths or plotting the fix on the DR
computer.
11.7.1. These last plotting techniques are discussed in Pub. No. 249 in the section on precomputation.
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