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36. Ho. Height observed (sextant reading).
37. INT. Intercept distance (NM) is the difference between the final Hc and Ho. Apply the HOMOTO
rule to determine direction (T or A) along the Zn.
38. LAT. Polaris latitude.
39. CONV ANGLE (W/-E). Convergence angle used in grid navigation.
40. GRID Zn. The sum of blocks 17 and 39.
10.7. Corrections Applied to Hc. In some methods of precomputation, corrections are applied in
advance to the Hc to derive an adjusted Hc. When using corrections that are normally applied to Hs, the
signs of the corrections are reversed if applied to Hc. For example:
Corrections Applied to Hs
Hs 31o 05
REFR -01
PERS/SEXT -05
Ho 30o 59
Hc 30o 40
INT 19T
Corrections Applied to Hc
Hc 30o 40
REFR +01
PERS/SEXT +05
ADJ Hc 30o 46
Hs 31o 05
INT 19T
10.7.1. This example demonstrates that corrections may be applied to either Hs or Hc. As long as they
are applied with the proper sign, the intercept remains the same. The following sample precomp uses a
common fix time (though computation times are different) and common observation times to facilitate
comparison. NOTE: Atmospheric refraction correction must be extracted for the actual Hs. It may then
be applied to either Hc or Hs using the proper sign. Extracting the value for Hc may cause large errors,
especially when the body is near the horizon. Figure 10.3 is a sample three-star precomputation using
the mathematical format. Corrections to altitude of the body are applied to the Hc and the sign of the
correction has been reversed in this process, so the fix can be plotted prior to the computation time. All
shots are early shots, allowing the navigator to resolve the fix and alter at fix time. However, any minor
errors in interpolation for motions are multiplied for the two earliest shots and may cause inaccuracies in
the fix.
10.7.2. Figure 10.4 shows a three-star precomputation using a three-LHA or graphical solution. The
assumed position will then be moved for track and groundspeed to accommodate LOPs shot off time.
Each observation is taken on time and then plotted out of its own plotting position. This precomp is
easier and faster to accomplish with relatively few opportunities for math errors to occur. The three
assumed positions required for this solution, on the other hand, often cause large intercepts and may
make star identification difficult if care is not taken in choosing the precomp assumed position.
AFPAM11-216 1 MARCH 2001 235
Figure 10.3. Mathematical Solution.
236 AFPAM11-216 1 MARCH 2001
Figure 10.4. Graphical Solution.
10.8. Limitations. Precomputational methods lose accuracy when the assumed position and the actual
position differ by large distances. Another limiting factor is the difference in time between the scheduled
and actual observation time. The motion of the body correction is intended to correct for this difference.
The rate of change of the correction for motion of the body changes very slowly within 40o of 090o and
270o Zn and the observation may be advanced or retarded for a limited period of time with little or no
error. When the body is near the observer's meridian, however, the correction for motion of the body
AFPAM11-216 1 MARCH 2001 237
changes rapidly due in part to the fast azimuth change and it's inadvisable to adjust such observations for
long (over 6 minutes) periods of time. NOTE: Errors in altitude and azimuth creep into the solution if
adjustments are made for too long an interval of time. Because of these errors, the navigator should
attempt to keep observation time as close as possible to computation time.
Section 10C— Preplotting True Azimuth (Zn)
10.9. Basics. To speed up fix resolution, some navigators preplot the Zns of the bodies. This technique
works best when used on a constant scale chart and using a technique of precomputation that will give
one assumed position. Before making any observations, plot the assumed position, correct it for Coriolis
and precession and/or nutation (if required) and draw the Zns of the bodies through this point. Label
each Zn as the 1st, 2d, or 3d as shown in Figure 10.5, or use the name of the bodies. Use arrowheads to
identify the direction of the body. Suppose the corrected assumed position is 30o40' N, 117o10' W and
the following Zns were computed for the bodies:
1st shot Zn 020o
2d shot Zn 135o
3d shot Zn 270o
The original assumed position of 31o N; 117o08' W has been corrected for precession and/or nutation and
for Coriolis or rhumb line error to obtain the plotting position. When the first intercept is found to be
10A, second intercept 40A, and the third intercept 50T, the fix may be plotted quickly by constructing
　
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