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precomputation have been available since 1940; however, there was no operational requirement for
precomputation at that time. With present day high-speed aircraft, however, the picture has changed
radically. By "postcomping," a great deal of work must be done after the last celestial observation. The
fix could easily be 15 minutes old by the time it is plotted on the chart. At 450 knots groundspeed (GS),
a fix that is 15 minutes old is over 100 miles behind the aircraft and is of questionable value. Another
factor necessitating precomputation in high-speed aircraft lies in the method of shooting celestial. With
the limited field of view of the sextant, the correct star is difficult to find unless you know where to look.
10.2. Presetting the Sextant. Precomputation greatly reduces both of the problems just mentioned. By
completing most of the computations before shooting, the navigator reduces the time necessary to plot
the fix after the last observation. Also, the problem of finding the star in the optics of the sextant is
simplified. The procedure for finding the star is similar to the heading check performed with the
periscopic sextant, using the true bearing (TB) method as explained in Chapter 12. In this case the true
azimuth (Zn) is set into the sextant mount and the computed altitude (Hc), which will approximate the
sextant altitude (Hs), is set into the sextant. Now, instead of sighting the body to determine the true
heading (TH), set the TH under the vertical crosshair to find the selected body, hopefully very close to
the crosshairs in the sextant field of view. Use the inverse relative bearing (IRB) method to avoid
erroneous settings in the azimuth window and to increase speed in setting up the sextant. In this method,
the azimuth window remains permanently at 000.0o and the IRB is computed by the formula: IRB = TH
– Zn. The body should be found at its computed altitude when its IRB appears under the crosshairs.
Section 10B— Precomputation Techniques
10.3. Basics. There are many acceptable methods of precomputation in general usage. However, these
methods are basically either graphical, mathematical, or a combination of both methods. Selection is
largely based on individual navigator preference and assigned command.
10.3.1. Celestial corrections which are used in precomputation include atmospheric refraction, parallax
of the moon, instrument and acceleration errors, Coriolis and rhumb line, precession and nutation,
motion of the observer, and wander. With precomputation, new corrections and terminology are
introduced, which include fix time, solution time, observation time, scheduled time, and motion of the
body adjustment.
10.3.2. Fix time is the time for which the LOPs are resolved and plotted on the chart. Solution time is the
time for which the astronomical triangle is solved. Observation time is the midtime of the actual
observation for each celestial body. Scheduled time is the time for which the astronomical triangle is
solved for each LOP in the graphic method. Motion of the body correction is used to correct for the
changing altitude of the selected bodies from shot to fix time and may be applied either graphically or
mathematically.
230 AFPAM11-216 1 MARCH 2001
10.4. Motion of the Body Correction. Motion of the body correction can be applied graphically by
moving the assumed position eastward or westward for time. This is possible because the Greenwich
hour angle (GHA) and the subpoint of the body move westward at the rate of 1o of longitude per 4
minutes of time. In the graphic method, a scheduled time of observation is given to each body. If
shooting is off schedule, the following rules apply: For every minute of time that the shot is taken early,
move the assumed position 15' of longitude to the east; for every minute of time that the shot is taken
late, move the assumed position 15' of longitude to the west.
10.4.1. When the latitude of the assumed position and the Zn of the body are known, the motion of the
body can be computed mathematically. For 1 minute, the formula is: 15(cos lat)/(sin Zn). This correction
is shown in tabular form in Figure 10.1. In Pub. No. 249, the local hour angle (LHA) increases 1o in 4
minutes of time. Thus, the Hc for an LHA that is 1o less than the LHA used for precomputation is the Hc
for 4 minutes of time earlier than the solution time. The difference between the two Hcs is the value to
apply to the Hc or Hs to advance or retard the line of position (LOP) for 4 minutes of time. If the Hc
decreases (Zn greater than 180o), the body is setting and the sign is minus to advance the LOP if the
value is applied to the Hs. If the Hc increases (Zn less than 180o), the body is rising and the sign is plus
to advance the LOP if the value is applied to the Hs.
　
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