accelerometers on an unstabilized platform sense
the system must torque the gyro opposite and
an acceleration due to gravity. Since the
equal to the apparent precession. The earth's
accelerometers cannot distinguish gravitational
rotation affects the upper (XZ2) and lower (YZ1)
accelerations from horizontal accelerations, the
gyros. Corrections for earth rotation go to the
integrators develop a fictitious velocity with a
Y and Z1 torquing coils (Z1 and Z2 are caged
together and both respond accordingly). The
corresponding distance error. It is essential,
therefore, that the system maintains the accel-
system does not use the X torquing coil for earth
erometers in a truly horizontal reference plane.
rate corrections.
This plane must be independent of aircraft
To establish an earth frame of reference, the
attitude at all times.
gyroscopes are controlled by continuously
computed signals. These signals introduce forces
This is a basic requirement of the inertial
navigation system. The accuracy with which the
(torque) to cause the gyro spin axes to precess in
horizontal reference is maintained determines the
the desired direction. This torque is in the form
overall performance capabilities of the system. A
of direct current signals applied to torquing coils
gyrostabilized platform in a gimbal structure
mounted on the gyro float assembly. It creates a
magnetic field that aids or opposes the magnetic
serves as an inertial reference. Also, it accurately
fields of the permanent magnets mounted on the
defines directional reference for the coordinate
end bells. This circuit effectively torques the gyro,
system.
With the platform installed in the aircraft and
causing the spin axis to precess to the desired
aligned along the y-axis, it will remain level
orientation.
The first step in establishing a frame of
regardless of aircraft attitude. The instant the
reference is leveling the stable element. To level
aircraft begins to change pitch attitude (fig. 7-44),
the platform gyro senses this angular movement.
the stable element, you align it to the local
The gyros then begin to precess at a rate
vertical (gravity vector). This is done by
proportional to the pitching rate. A pickoff coil
stable element until the x and y accelerometers
on the gyro axis senses this movement and changes
cease to sense any acceleration caused by gravity.
it to a voltage. After amplification, the voltage
This says that the outputs from the accelerometers
goes to the pitch gimbal servo drive motor. The
motor rotates the stable element exactly equal and
provide the torquing signals for the gyros.
opposite to the aircraft angular motion. As a
During this time, and while operating, computed
result, it continuously precesses the gyro to its
earth rate torquing signals continuously go to the
neutral or level position. This action maintains
y and z axes torquing coils of the lower gyro. The
the gyro output signal at null.
size of these earth rate torquing signals is
resolved by computing the vertical and horizontal
Regardless of any new pitch attitude the
aircraft assumes, the gyro will keep the stable
components of earth rate as a function of latitude.
After establishing the stable element in a rough
element level. This is the only position that allows
the gyro's output signal to be at null. In actual
level position, the x-axis torquing signal drives the
practice, the stable element is maintained level.
stable element in azimuth to null this signal (fine
It uses a similar method to maintain its azimuth
alignment). This signal consist of earth rate
acceleration only, which is a measure of stable
element unlevelness. At this time, the X gyro's
spin axis is aligned to true north establishing the
frame of reference. This alignment condition will
remain until you manually sequence the inertial
navigation system to the navigate position.
As previously shown, the earth's rotation does
not affect the upper X-axis gyro. Therefore, no
compensating earth rate torquing signal goes to
this gyro.
MAINTAINING THE HORIZONTAL REF-
ERENCE. --When the aircraft remains stationary
or moves at a constant velocity, the accelerometer
outputs are zero. If the aircraft attitude
Figure 7-44.-Gyro maintaining inertial platform level.
changes while maintaining a constant speed, the
7-35
