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Chapter 3 / Inertial Navigation
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3.3 Inertial Navigation and Airframe Stabilisation
Before dealing with individual Inertial Navigation System (INS) sensors it
is useful to consider how they combine to provide navigation data for
aircraft and long-range missiles. Typical accuracy requirements for ship,
aircraft and cruise missile navigation are 0.1, 1 and 10 nm/hr respectively.
An INS comprises an Inertial Measurement Unit (IMU), containing
gyroscopes, accelerometers and temperature measuring devices for thermal
calibration, and an autonomous height reference such as a barometer. GPS
can be used instead but the importance of height stabilisation is such that
only an autonomous instrument can satisfy safety critical applications. IMU
data is also required for stabilising the highly manoeuvrable airframes used
in modern weapon systems, as well as for terrestrial referenced positioning.
Conceptually, gyroscopes provide a space stable reference frame in which
the accelerometer outputs can be integrated. For navigation the temperature
inside the IMU case is continuously monitored and used to correct the
sensor output using manufacturer’s calibration data. Coriolis corrections are
then applied so that the multiplexed, non-coincident output from each sensor
is referred to a single instant in time and a common reference point. For
airframe stabilisation and inertial seeker pointing, where accuracy
requirements may be relaxed, the IMU output is often used without these
corrections. The primary IMU outputs are increment angle and velocity, i.e.
the angle the IMU has turned through since the last output. Thus the
orientation of the IMU case is maintained with respect to the space stable
frame, and the change in velocity along the IMU reference axes. In an INS
these data are combined to produce the following vertically stabilised,
geodetic data:
• Longitude, latitude and geodetic height above the World Geodetic System
(WGS) 84 ellipsoid
• Geodetic velocity
• Orientation of the IMU case (vehicle body axes) with respect to Local
Geodetic Axes (LGA)
In addition to these data incremental sensor output is often processed to
obtain:
• Angular rate of the IMU axes (vehicle body axes) with respect to LGA
• Specific acceleration along the IMU axes (vehicle body axes)
For navigation the incremental data is used directly, whereas data derived
from it is often adequate and simpler to use for less accurate applications.
Under no circumstances should both sets of data be used to update a state
observer as the derived data contains no new information. Low bandwidth
barometer data is mixed with the high bandwidth accelerometer output to
prevent vertical acceleration biases from integrating into a rapidly
increasing height error.
3.3-1