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reducing the chance of math errors on the precomp. To accomplish a three-LHA fix, you must plan 4
minutes between the midtime of each shot (Figures 12.4 and 12.5). Because LHA changes 1 degree for
every 4 minutes, the precomp will have three successive LHAs, 1 degree apart. To correct for off-time
motion, adjust the assumed position based on true course and groundspeed. If a shot is planned earlier
than fix time, the assumed position is advanced (down-track). For shots planned later than fix time, the
assumed position is retarded (up-track).
12.4.1. The example in Figures 12.4 and 12.5 shows the LHA method for a 12-8-4 early shooting
schedule. This shooting schedule allows the fix and/or MPP to be resolved before the fix time. To adjust
the assumed positions, plot the fix time assumed position and then advance it for 4 minutes of track and
groundspeed for each body. This will satisfy motion of the observer. When shooting the selected bodies,
take care to shoot them exactly on the prescribed times. This will eliminate motion of the body.
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Figure 12.4. Typical Example of Three LHA Method.
260 AFPAM11-216 1 MARCH 2001
Figure 12.5. Plotting Three LHA.
12.4.2. A variation of advancing the assumed position is to use half motions. This will enable you to plot
all three LOPs from one assumed position. Table 1 from Pub. No. 249 lists corrections to position of the
observer. Each correction is for 4 minutes of time. To use it, enter with your relative Zn (Zn-Track) and
groundspeed. Now look at the bottom of the table and note you can apply this correction to your
tabulated altitude or observed altitude. It doesn't matter which you choose but note that the sign will
change dependent on where you apply it. Now take the number and multiply it by the 4-minute
increment of the shot. For example, Figure 12.6 shows the precomp for a 0300 fix using 3 LHAs and
half motions. The 0248 shot, Alpheratz, relative Zn, and groundspeed were used to extract a +20
correction from Table 1. Because this shot is 12 minutes early we need to multiply +20 by three before
we apply it to the shot. Note the +60 correction was applied to the observed altitude and therefore kept
its positive sign. The benefit of doing this is a reduction in plotting. See Figure 12.7 for the plotted
LOPs. This technique can be applied to day celestial as well.
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Figure 12.6. Half Motions Three LHA Format.
262 AFPAM11-216 1 MARCH 2001
Figure 12.7. Plotting a Half Motions Observation.
Section 12C— Daytime Celestial Techniques
12.5. Basics. Daytime fixing, using celestial techniques, is rather limited because often only one body,
the sun, is visible. Ordinarily, three LOPs cannot be obtained for a fix from one body because the LOPs
plot nearly parallel to each other.
12.6. The Sun Heading Shot at High Noon. The azimuth of the sun changes very rapidly when the
subpoint of the sun is directly over the longitude of the observer, i.e., called the time of transit. The LHA
at transit time is 360o. This phenomenon is more pronounced at lower latitudes as the subpoint of the sun
passes closer to the observer. This makes it extremely difficult to get an accurate celestial heading shot
at the transit time. Therefore, if you need a heading shot near the time of transit, you must take extra
precaution to get the heading observation exactly at the precomputed fix time. If the moon or Venus is
available, consider using these bodies for an accurate celestial heading. If using the sun, you should
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weigh the increased possibility of an inaccurate heading shot. If the accuracy is questionable, get another
heading shot as the sun's rate of azimuth change slows enough to allow a more accurate shot.
12.7. Intercept Method. The intercept method is normally used in obtaining a noonday fix. If the sun
passes close to the observer's position, within about 4o, the subpoint method of plotting the fix may be
used. This method differs from normal procedures in that three different precomps for three different
times are computed. Because of the rapid change of the sun's azimuth at or near transit, this variation is
necessary. The procedure is:
12.7.1. Determine the time of transit.
12.7.2. Select the LHA before and after transit for which the change in azimuth is 30o or more. Since 1o
of LHA is equal to 4 minutes of time, the difference in transit LHA and the new LHA can be converted
to time in minutes. Thus, the time preceding and following transit can be determined.
12.7.3. Plot the DR positions for times determined in 12.7.2. Select the appropriate assumed positions
necessary for the computation and plotting of the LOPs. The assumed position for time of transit is also
　
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