the detected velocity change is due to a speed
detailed description of an INS follows later in the
change or a direction change or both. It does not
chapter.
matter what forces cause the velocity change.
Neither can the accelerometer distinguish between
the acceleration of the vehicle and the pull of
gravity. Therefore, if the accelerometer tilts off
REVIEW SUBSET NUMBER 1
its level, its output will include a component of
gravity as well as vehicle acceleration. To get the
Q1. A point that is defined by stated or implied
.
correct vehicle acceleration in the horizontal
coordinates is known as a
plane, the sensitive axis of the accelerometer must
be perpendicular to the gravitational field.
Of course, the earth is not flat, and not
exactly round either. Its radius at the poles is less
Q2. The intended horizontal direction of travel
.
than its radius at the equator. It also spins about
is known as
its polar axis. A spinning gyro in gimbals tries to
maintain a fixed direction in relation to space
rather than to any point on earth. Consider a gyro
at the equator with its spin vector direction east,
Q3. In what two reference directions can you
toward the morning sun. After 6 hours of earth
express bearings?
rotation, the spin vector would be up in relation
to the earth's surface. After 12 hours it would be
west. After 18 hours it would be down; and after
24 hours it would be east again. Also, consider
Q4. The east/west geographical coordinate is
.
a spinning gyro with its spin axis parallel with the
known as
earth's axis of rotation. At the equator it is
parallel with the earth's surface. However, as it
moves to the North Pole, it becomes vertical to
the earth's surface. You must take all of these
Q5. You measure longitude 180 east or west
items into account and correct for them. So, to
from what point?
navigate on the earth requires a highly complex
inertial system, each component of which is
capable of extreme accuracy.
A four-gimbal system allows the platform to
Q6. The angle between true north and the
retain the original orientation regardless of what
direction of the earth's magnetic field is
.
maneuvers the aircraft makes. This allows the
known as
platform to serve as a level mount for the
accelerometers. The stable platform contains two
identical floated, two-degree-of-freedom gyros.
They mount with their spin axes horizontal and
Q7. How do you label variation?
at right angles to each other. Using the gyroscopic
principle of precession, it is possible to apply a
continuous torque to the appropriate axes. This
action reorients the gyros to maintain the stable
Q8. Magnetic influences cause what type of
platform horizontal to the earth's surface and
error in magnetic compasses?
pointed north. An electronic analog computer
develops the signals necessary to properly torque
the gyros. The correct ions for earth rate depend
Q9. The net result of both variation and
on the aircraft's position on the earth's surface.
.
Some system's use as many as three acceler-
deviation is known as
ometers. Two are horizontal with one sensitive to
north-south acceleration and the other sensitive
to east-west acceleration. The third accelerometer
Q10. When variation and deviation have the
mounts to determine vertical acceleration. A
computer subtracts the gravity component from
same name, do you add or subtract to
the output of the vertical accelerometer. A more
obtain compass error?
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