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observer is called the vertical component. The tendency of the spin axis to drift around laterally; that is,
to change in azimuth, is called the horizontal component. Generally, when earth rate is mentioned, it is
the horizontal component which is referred to, since the vertical component is of little concern.
14.15.2. Figure 14.10. A gyro located at the North Pole, with its spin axis initially aligned with a
meridian, appears to turn 15.04o per hour in the horizontal plane because the earth turns 15.04o per hour.
As shown in A of Figure 14.10, the apparent relationship between the Greenwich meridian and the gyro
spin axis will change by 90o in 6 hours, though the spin axis is still oriented to the same point in space.
Thus, apparent precession at the pole equals the rate of earth rotation. At the equator, as shown in B of
Figure 14.10, no earth rate precession occurs in the horizontal plane if the gyro spin axis is still aligned
with a meridian and is parallel to the earth's spin axis.
14.15.3. Vertical Component. When the gyro spin axis is turned perpendicular to the meridian (Figure
14.11), maximum earth rate precession occurs in the vertical component. But the directional gyro does
not precess vertically because of the internal restriction of the gyro movement in any but the horizontal
plane. Thus, for practical purposes, earth rate precession is only that precession which occurs in the
horizontal plane. Figure 14.11 illustrates earth rate precession at the equator for 6 hours of time.
14.15.4. Precession Variation. Earth rate precession varies between 15.04o/hr at the poles and 0o/hr at
the equator. It is computed for any latitude by multiplying 15.04o times the sine of the latitude. For
example, at 30o N, the sine of latitude is 0.5. The horizontal component of earth rate is, therefore,
15o/hour right x 0.5 or 7.5o/hour right at 30o N as shown in Figure 14.12.
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Figure 14.10. Initial Location of Gyro Affects Earth Rate Precession.
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Figure 14.11. Direction of Spin Axis Affects Earth Rate Precession.
Figure 14.12. Earth Precession Varies According to Latitude.
14.15.5. Steering by Gyro. Obviously, if the gyro is precessing relative to the steering datum of GN or
TN, an aircraft steered by the gyro will be led off heading at the same rate. To compensate for this
precession, an artificial real precession is induced in the gyro to counteract the earth rate. At 30o N
latitude, earth rate precession is equal to 15o X sin lat = 15 X .5 or 7.5o per hour to the right.
14.15.6. Offsetting Each Rate Effect. Hence, if at 30o N latitude, a real precession of 7.5o left per hour
is induced in the gyro, it will exactly balance and offset earth rate effect. In ordinary gyros, a weight is
used to produce this effect but, since the rate is fixed for a given latitude, the correction is good for only
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one latitude. The latitude chosen is normally the mean latitude of the area in which the aircraft will
operate. The N-1 and AHRS compass systems have a latitude setting knob, which you can use to adjust
for the earth rate corrections.
14.16. Earth Transport Precession (Horizontal Plane). This form of apparent precession results from
transporting a gyro from one point on the earth's surface to another. The gyro spin axis appears to move
because the aircraft, flying over the curved surface of the earth, changes its attitude in relation to the
gyro's fixed point in space (Figure 14.13). Earth transport precession causes the gyro spin axis to move
approximately 1o in the horizontal plane for each true meridian crossed. This effect is avoided by using
GN as the steering reference.
Figure 14.13. Earth Transport Precession.
14.17. Grid Transport Precession. Grid transport precession exists because meridian convergence is
not precisely portrayed on charts. The navigator wants to maintain a straight-line track, but the gyro
follows a great-circle track, which is a curved line on a chart. The rate at which the great-circle track
curves away from a straight-line track is grid transport precession. This is proportional to the difference
between convergence of the meridians as they appear on the earth and as they appear on the chart and
the rate at which the aircraft crosses these meridians.
14.18. Summary of Precession. Real precession is caused by friction in the gyro gimbal bearings and
dynamic unbalance. It is an unpredictable quantity and can be measured only by means of heading
checks.
14.18.1. Earth rate precession is caused by the rotation of the earth. It can be computed in degrees per
hour with the formula: 15.04 x sin lat. It is to the right in the Northern Hemisphere and to the left in the
308 AFPAM11-216 1 MARCH 2001
　
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