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11.7.2. Other special techniques are discussed in the section on curves, in which the celestial observation
is plotted on a graph rather than on the chart.
Section 11B— Interpretation of an LOP
11.8. Basics. Navigation has two aspects— the mechanical and the interpretive. The mechanical aspect
includes operation and reading of instruments, simple arithmetical calculations, plotting, and log
keeping. The interpretive aspect is the analysis of the data that have been gathered mechanically. These
data are variable and subject to error. You must convert them into probabilities as to the position, track,
and GS of the aircraft and the direction and speed of the wind. The more these data are subject to error,
the more careful the interpretations must be and the less mechanical the work can be. LOPs and fixes
especially require careful interpretation. It is convenient to think of a fix as the true position of the
aircraft and of the LOP as a line passing through this position, but these definitions are optimistic. It is
almost impossible to make a perfect observation and plot a perfect LOP. Therefore, an LOP passes some
place near this position, but not necessarily through it and a fix determined by the intersection of LOPs
is simply the best estimate of this position on the basis of one set of observations. Thus, in reality, a fix
is a most probable position (MPP) and a LOP is a line of MPP.
11.8.1. The best interpretation of LOPs and fixes means they are used, to the best advantage, with DR.
But good interpretation cannot compensate for poor LOPs, nor can good LOPs compensate for careless
DR. To get good results, every precaution must be taken to ensure the accuracy of LOPs and exact DR
calculations.
11.8.2. Intelligent interpretation requires fine judgment, which can only be acquired from experience.
You can be guided, however, by certain well-established, though flexible, rules.
AFPAM11-216 1 MARCH 2001 245
11.8.3. The following discussion pertains especially to celestial LOPs and fixes. It also applies to LOPs
and fixes established by radio and, to some extent, to those obtained by map reading.
11.9. Single LOP. Previous discussions dealt with the basic plotting of an LOP and errors in LOPs, but
they did not show the actual mechanics of the plotted corrections which must be applied. The LOP must
be corrected for Coriolis or rhumb line correction and also for precession and/or nutation correction if it
is based on a Volume 1 star shot. Coriolis or rhumb line correction becomes a very significant correction
at higher speeds and latitudes. For example, suppose the correction determined from the Coriolis or
rhumb line correction table is 9 NM right (of the track). The LOP must be moved a distance of 9 NM to
the right of track. This can be done either by moving the assumed position prior to plotting, or by
moving the LOP itself after it is plotted. (Remember the assumed position is not used in the plotting of
the LOP obtained from a Polaris observation.) Consider Figure 11.5, which shows a track of 90o.
Figure 11.5. Two Methods of Coriolis/Rhumb Line Correction.
11.9.1. Notice that, in both methods, the corrected LOP is in the same place with respect to the original
assumed position and that the intercept value is the same. The resultant LOP is the same regardless of
the method used.
11.9.2. If, in addition to the Coriolis or rhumb line correction, a precession and/or nutation correction of
3 NM in the direction of 60o is required, it would have been further applied as shown in Figure 11.6.
Again, the corrected LOP is the same, using either method, because the intercept and resultant position
of the corrected LOP to the original assumed position are the same. The corrected LOP alone gives very
little information; hence, a position must be arrived at only after considering the LOP and the DR
position for the same time.
246 AFPAM11-216 1 MARCH 2001
Figure 11.6. Two Methods of Coriolis/Rhumb Line and Precession/Nutation Correction.
11.10. Most Probable Position (MPP) by C-Plot. The MPP is just what the name implies. It is not a
fix; however, since it is the best information available, it is treated as such. Notice in A of Figure 11.7
that the DR position and celestial LOP (for the same time) do not coincide.
Figure 11.7. Most Probable Position by C-Plot.
11.10.1. Obviously, the DR information or celestial information, or both is in error. Notice that the prior
fix has no time on it. Suppose this prior fix had been for the time of 1010. It would then be very likely
that most of the error is in the celestial information and the probable position is closer to the DR position
than to the celestial LOP. On the other hand, suppose the prior fix had been for the time of 0900. Since
　
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