T-45C TS, ADV & IUT ENG-24
GINA Operating Characteristics
(a) Apparent precession is not a physical
movement of the gyro’s inertial orientation,
which remains fixed
(b) Apparent precession is a precession
“perceived” in relationship to the earth and
its coordinate system
(c) Apparent precession is a major factor that
must be accounted for mechanically or
mathematically when using a gyro as an
attitude reference platform
INSTRUCTOR NOTES
Apparent Earth-Rotation precession can be exemplified by
mentally visualizing yourself on the equator viewing a fully
stabilized 3-axes gyroscope at 0600 hours in the morning. Its
primary spin axis is pointing due east. Due to the earth’s
rotation, half way through the day at 1800 hours, the gyro’s spin
axis “appears” to have precessed through the vertical so as to,
then, point due west. In another 12 hours, once again 0600
hours, the gyro will again appear to point due east. In fact
through the entire 24 hours, the gyro has maintained its inertial
orientation towards the same point in space, but due to the
earth’s rotation has “appeared” to precess; hence, the name
Apparent precession.
Apparent Transport precession occurs, when a gyro is
transported across the earth’s surface, either physically or
apparently due to earth rotation. By further example, a rapid
movement (transport) of a gyro from Chicago to Paris will result
in the same apparent precession as though the earth had
rotated an equal number of degrees longitude.
Original (9-98)
Page 24-6
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