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The alignment of a strapdown inertial system consists in:
  • A
    positioning the platform relative to the local vertical and true north.
  • B
    positioning the accelerometers.
  • C
    positioning the gyroscopes and accelerometers relative to the fuselage axis.
  • D
    measuring the earth rotation and local gravitation to position the reference co-ordinate system.

The initial alignment and leveling of the IRS need to be carried out when the aircraft is stationary and on the ground. A set of three accelerometers will sense gravitational acceleration that will derive the local vertical of the platform. Once the vertical is found thanks to the gravity, any change of attitude in space will be detected, but they will only come from the Earth’s rotation (on the ground when stationary). Moreover, by detecting the drift due to the earth’s rotation, the Inertial Reference Unit (IRU) will be able to internally calculate the True North.

Latitude can also be internally calculated, but because this calculation is not sufficiently precise, a manual position input is required from the flight crew for this alignment sequence. Therefore, the system will compare the inserted latitude with the calculated one and compute precisely the aircraft’s position. On the other hand, the IRS cannot calculate/measure longitude, therefore the only reference is the one provided by the pilot during the alignment process.

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