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prior to flight by the accelerometers that measure acceleration in the horizontal plane. The stable
element is moved until the output of the x and y accelerometers is zero, indicating that they are not
measuring any component of gravity and that the platform is level. Azimuth alignment to TN is
accomplished before flight by starting with the magnetic compass output and applying variation to
roughly come up with TN reference. From this point, gyrocompassing is performed. This process makes
use of the ability of the gyros to sense the rotation of the earth. If the stable element is misaligned in
azimuth, the east gyro will see the wrong earth rate and will cause a precession about the east axis. This
precession will cause the north accelerometer to tilt. The output of this accelerometer is then used to
torque the azimuth and east gyro to ensure a TN alignment and a level condition.
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16.17.6. In the more modern gyroscopes, the gyro cannot be physically torqued because the gyro is
either not moving or the gyro is electronically suspended. In these systems, the stable platform is leveled
mathematically using gyro data. The precise orientation of the x and y accelerometers on the stable
platform is less critical since the computer can mathematically correct for any orientation. The next
generation of INS may work without a stable platform, with orientation and stability maintained
mathematically from accelerometer inputs.
16.18. Integrator. Simply stated, the processing of acceleration is done with an integrator. An integrator
integrates the input to produce an output: it multiplies the input signal by the time it was present.
Accurate navigation demands extremely accurate integration of both acceleration and velocity. One of
the most used analog integrators is the DC amplifier, which uses a charging current stabilized to a
specific value proportional to an input voltage. Another analog integrator is the AC tachometergenerator,
which uses an input to turn a motor, which physically turns the tachometer-generator,
producing an output voltage. The rotation of the motor is proportional to an integral of acceleration.
16.19. Computer. The computer changes the integrators' outputs into useful navigation information. To
do this, one accelerometer is mounted aligned to north and another is mounted 90° to the first, to sense
east-west accelerations. Any movement of this system indicates distance traveled east-west and northsouth.
The INS maintains a local vertical reference and measures distance traveled over a reference
spheroid perpendicular to the local vertical. On this spheroid, the latitude and longitude of the present
position are continuously measured by the integration of velocity. In Figure 16.7, q represents latitude
and l represents longitude. The axes are designated x, y, and z, corresponding to east, north, and local
vertical. This defines their positive directions. References to velocities, attitude angles, and rotation rates
are about the x, y, and z axes. The local vertical (z) is established by platform leveling. This is the most
fundamental reference direction. To complete platform alignment, the INS uses gyrocompassing to
establish true north (y). Gyrocompassing establishes platform alignment to the earth's axis of revolution
or North Pole. The INS is capable of doing this to an accuracy of 10 minutes of arc or less. After
alignment, the platform remains oriented to TN and the local vertical, regardless of the maneuvers of the
aircraft.
16.19.1. GS components of velocity in track (V) are measured by the system along the x and y axes, as
shown in Figure 16.8. These components, Vx and Vy include all effects on the aircraft, such as wind,
thermals, engine accelerations, and speed brake decelerations. Some form of digital readout usually
displays the GS (V).
16.19.2. The angles between the aircraft attitude and the platform reference attitude are continuously
measured by synchros. The aircraft yaws, rolls, and pitches about the platform in a set of gimbals, each
gimbal being rotated through some component of attitude. TH is measured as the horizontal angle
between the aircraft's longitudinal axis and platform north. Roll and pitch angles are measured by
synchro transmitters on the platform roll and pitch gimbals.
338 AFPAM11-216 1 MARCH 2001
Figure 16.7. Geographic References.
Figure 16.8. Measurement of Aircraft Groundspeed.
16.20. Summary. INS technology has advanced very rapidly within the past few years. Advanced
navigation systems are commonly designed with INS as an essential component. INS reliability is
AFPAM11-216 1 MARCH 2001 339
exceptional and INS accuracies are second only to GPS. Traditional INS design has capitalized on the
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