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Southern Hemisphere. All gyros are corrected to some degree for this precession, many by means of a
latitude setting knob.
14.18.2. Earth transport precession (horizontal plane) is an effect caused by using TN as a steering
reference. It can be computed by using the formula (change longitude/hr x sine mid latitude). The
direction of the precession is a function of the TC of the aircraft. If the course is 0o – 180o, precession is
to the right; if the course is 180o – 360o, precession is to the left. This precession effect is avoided by
using GN as a steering reference.
14.18.3. Grid transport precession is caused by the fact that the great circles are not portrayed as straight
lines on plotting charts. The navigator tries to fly the straight pencil-line course, the gyro a great circle
course. The formula for grid transport precession is change longitude/hr (sin lat-CF), where CF is the
chart convergence factor. The direction of this precession is a function of the chart used the latitude and
the TC. Direct substitution into the formula will produce an answer valid for easterly courses, such as,
0o–180o. For westerly courses, the sign of the answer must be reversed.
14.19. Gyro Steering. Gyro steering is much the same as magnetic steering, except that grid heading
(GH) is used in place of true heading (TH). GH has the same relation to grid course (GC) as TH has to
true course (TC). The primary steering gyro in most aircraft provides directional data to the autopilot
and maintains the aircraft on a preset heading. When the aircraft alters heading, it turns about the
primary gyro while the gyro spin axis remains fixed in azimuth. If the primary gyro precesses, it causes
the aircraft to change its heading by an amount equal to the precession.
Section 14E— Starting a Grid Navigation Leg
14.20. Basics. Grid navigation is normally entered while airborne on a constant heading. A constant
heading is necessary because grid entry is accomplished by resetting the compass from a magnetic to a
grid reference while on the same heading. After obtaining a grid celestial or INS heading check, reset
the compass immediately to the correct grid heading to avoid heading errors (because the precomputed
grid Zn is only good for shot time). Since the exact grid heading is set at the beginning of the navigation
leg, precession is assumed to be zero until subsequent heading checks assess the accuracy of the gyro.
The grid heading is normally obtained using a variant of the true heading method or the INS TH. Using
this method, set a grid Zn in the sextant azimuth counter before collimating on the body. Other heading
shot methods can be used, but would delay resetting the gyro to accomplish math computations after the
heading shot. Although any celestial body may be used, navigators commonly use the sun or Polaris,
depending on the time of day (Figure 14.14.)
14.21. Using a Zn Graph. In order to get an accurate grid Zn for daytime grid entry, the navigator must
compute Zn of the sun for a time and geographic position where the grid navigation leg will begin. If the
geographic position for grid entry is known well in advance, you can prepare a Zn graph for a time
window. A Zn graph makes grid entry easier because it is usable for an extended period of time,
therefore eliminating the need to precomp for a specific time. The graph can be constructed during
mission planning, thus reducing workload in the air.
AFPAM11-216 1 MARCH 2001 309
Figure 14.14. Grid Precomp.
310 AFPAM11-216 1 MARCH 2001
14.21.1. To construct the graph, precomp and plot Zn on one axis and time on the other (Figure 14.15).
Set up the time axis to cover the planned start time and several minutes earlier and later. Plot grid Zn on
the other axis using normal precomp procedures and the start point coordinates. Because the time/Zn
slope is close to linear, precomping at 20-30 minute intervals and connecting the points will give
acceptable accuracy. When the sun is near local noon, precomp Zn at closer intervals because the Zn
changes rapidly. To use the graph when it is finished, enter on the time axis. Then extend a line
perpendicular to the time axis until reaching the time/Zn line. Finally, read the appropriate Zn on the Zn
axis.
14.21.2. The sample in Figure 14.15 demonstrates using a graph to get a grid Zn for the time of 1700Z.
Although preparing a Zn graph takes a while, it pays dividends as long as you actually fly over the
planned geographic point within the time frame covered by the graph.
Figure 14.15. Zn Graph.
14.22. Applying Precession to the DR. The most accurate method for applying precession to the DR is
the "all behind/half ahead method." This method corrects for the "banana" effect most commonly
　
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