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equals grid azimuth. The result will allow the use of the grid lines for plotting the LOPs. When using
grid azimuth for plotting, apply Coriolis to the assumed position (in this case, the pole). Precession or
nutation corrections are not necessary since current SHA and Dec are used. Motion of the observer
tables may also be used in precomputation, since grid azimuth relative to grid course may be
determined. Motion of the body is zero at the poles.
12.19.4. Note the exact GMT of the celestial observation. From the Air Almanac, extract the proper Dec
and GHA. Plot the azimuth. Compare Ho and Hc to obtain the intercept. When the observed altitude
(Ho) is greater than the Dec (Hc), it is necessary to go from the pole toward the celestial body along the
azimuth. If the observed altitude is less than the Dec, as is the case with the sun in Figure 12.15, it is
necessary to go from the pole away from the body along the azimuth. Draw the LOPs perpendicular to
the azimuth line in the usual manner. Don't be concerned about large intercepts; they have no bearing on
the accuracy of this type of fix. Observations on well-separated bearings give a fix that is as good close
to the pole as it is anywhere else.
274 AFPAM11-216 1 MARCH 2001
Figure 12.15. Using Pole as Assumed Position.
Section 12F— Adjusting Assumed Position
12.20. Adjusting Assumed Position for Off-Time Shot. There will be times when the observer does
not start the shot at the prescribed time for various reasons. For example, the observer may struggle to
find the body due to cloud cover. If a shot is taken off time, you can use the FEAST (Fast EAST) rule: a
shot taken too fast or too early has the assumed position moved 15' of longitude east for each minute
early to compensate for body motion (see example in Figure 12.16). Apply the reverse of the FEAST
rule for late shots (move the assumed position west). This adjusted position is then advanced or retarded
for track and GS to account for motion of the observer, applying the same concept used in the
three-LHA method (Figure 12-5). This technique for solving motions is also discussed in Chapter 10.
EXAMPLE: Original assumed position: 23o-50' N 120o-00'W
Move 15' of longitude west for 1 minute late
Retard 6NM from track of 360o TH
New assumed position 23o-44' N 120o-15' W
AFPAM11-216 1 MARCH 2001 275
Figure 12.16. Corrections for Off-Time Shooting.
12.21. Longitude Adjustment Principle. You will occasionally make errors in your precomputations.
Possibly the most common would be an extraction error of the GHA or math error while computing the
LHA. If one of these numbers is incorrect, then all the extractions from the Pub. No. 249 would be based
on erroneous information and the result would be an LOP error. Fortunately there is a way of
compensating for this type of error without having to reenter the table and retrieving the correct data.
This method is called the Longitude Adjustment Principle (LAP). You need only adjust the assumed
longitude (up to 2 1/2o) to correct for a GHA extraction error, or a math error. Moving the assumed
position beyond the 2 1/2o induces some error in the plotting LOP. Suppose you wanted the GHA for
1410Z (Figure 12.17), you extracted the value for 1400Z and applied it to the longitude. The resultant
LHA was used and the precomp completed before you realized your error. To do the LAP first, extract
the correct GHA (031-20), keep the old LHA, and adjust the longitude so that the math is correct (Figure
12.18). A math error can occur in solving for the LHA (Figure 12.19). Once you have corrected the
precomp, use the adjusted longitude for your assumed longitude to plot the LOP.
276 AFPAM11-216 1 MARCH 2001
Figure 12.17. LAP Using Incorrect GHA.
Figure 12.18. LAP Using Correct GHA.
Figure 12.19. LAP Correcting a Math Error.
AFPAM11-216 1 MARCH 2001 277
12.22. Summary. Any of the techniques discussed here, if used on a regular basis, can be just as
accurate as normal precomping procedures and save some time as well. These techniques are not all
inclusive. There are many commercial publications available as a source for celestial navigators: for
example, American Practical Navigator by Bowditch (available through the NIMA) and the Journal of
the Institute of Navigation (available through the Institute of Navigation).
278 AFPAM11-216 1 MARCH 2001
Chapter 13
SEXTANTS AND ERRORS OF OBSERVATION
Section 13A— Sextants
13.1. Introduction. For hundreds of years, mariners have navigated the seas keeping track of their
positions by use of the sextant. This instrument measured the altitude of celestial bodies (angular
distance above the horizon) and the information derived from this measurement was used to determine
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