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made more accurate, the Ho needs to be made larger, thus the correction is added to the Ho to make the
Ho value increase. If the LOP needs to be moved away from the Zn, the correction will be subtracted
from the Ho to make the Ho less. In Figure 13.16, the LOP needs to be moved 10 miles toward the Zn in
order to be accurate; thus, the sextant error correction is +10 to the Ho and can be used on subsequent
shots obtained from the same sextant.
AFPAM11-216 1 MARCH 2001 295
13.15.4. An important thing to remember is that the sextant error correction assumes conditions will be
consistent. As a technique, it is wise to obtain several LOPs with a sextant, noting the sextant errors on
each, before establishing a value to be carried on the precomp. Once using that correction, make sure
you use the same sextant.
13.16. Summary. The first half of this chapter described the parts and operation of the sextant, and the
second half explained sextant errors. Remember to apply parallax, semidiameter, and refraction errors
on every applicable shot. Corrections for acceleration errors can be applied only if you know the track
and groundspeed before and after each shot, so be aware of your speed and direction when shooting.
Time permitting, always try to evaluate the accuracy of your sextant on the ground.
296 AFPAM11-216 1 MARCH 2001
Chapter 14
GRID NAVIGATION
Section 14A— Introduction
14.1. Basics. The original purpose of grid navigation was to ease the difficulties facing the navigator
during high latitude flights. But grid can be used at all latitudes; particularly on long routes because grid
uses a great circle course for a heading reference. Grid is simply a reorientation of the heading reference,
and does not alter standard fixing techniques.
Section 14B— Problems Encountered in Polar Navigation
14.2. Basics. Two factors peculiar to polar areas which make steering more difficult than usual are (1)
magnetic compass unreliability and (2) geographic meridians converging at acute angles. The combined
effect of these two factors makes steering by conventional methods difficult if not impossible. Each
factor is examined below.
14.3. Unreliability of Magnetic Compass. Maintaining an accurate heading in high latitudes is difficult
when a magnetic compass is used as the heading indicator. Built to align itself with the horizontal
component of the earth's magnetic field, the compass instead must react to the strong vertical
component, which predominates near the magnetic poles. Here, the horizontal component is too weak to
provide a reliable indication of direction. As a result, compass performance becomes sluggish and
inaccurate. The situation is further aggravated by the frequent magnetic storms in the polar regions,
which shift the magnetic lines of force.
14.3.1. But even if these conditions did not exist, the mere proximity to the magnetic pole would sharply
reduce compass usefulness. While the aircraft may fly a straight course, the compass indicator would
swing rapidly, faithfully pointing at a magnetic pole passing off to the left or right.
14.3.2. To cope with the unreliable magnetic compass, we use gyro information for our heading inputs.
14.4. Problem of Converging Meridians. The nature of the conventional geographic coordinate system
is such that all meridians converge to the pole. Each meridian represents a degree of longitude; each is
aligned with true north and true south. On polar charts, the navigator encounters 1 degree of change in
true course for each meridian crossed; thus, the more closely the aircraft approaches a pole, the more
rapidly it crosses meridians. Even in straight-and-level flight along a great circle course, true course can
change several degrees over a short period of time. You are placed in the peculiar position of constantly
altering the aircraft's magnetic heading in order to maintain a straight course. For precision navigation,
such a procedure is clearly out of the question. Notice in Figure 14.1 that the course changes 60o
between A and B and much nearer the pole, between C and D, it changes 120o. To cope with the
problem of converging meridians, we will use the USAF Grid Overlay.
AFPAM11-216 1 MARCH 2001 297
Figure 14.1. Converging Meridians.
14.5. Grid Chart Projections. The three polar projections most commonly used in polar areas for grid
navigation are the transverse Mercator, the polar stereographic and the polar gnomonic. The transverse
Mercator and polar stereographic projections are used in-flight, the polar gnomonic is used only for
planning. The Lambert conformal projection is the one most commonly used for grid flight in subpolar
areas. The division between polar and subpolar projections varies among the aeronautical chart series.
　
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