Error sources for an Inertial System

Error sources for an Inertial System

Many factors contribute to errors in an inertial navigation system. In-flight errors arise from imperfections in gyros, accelerometers and computers. Initial misalignment may cause additional errors.

The most common error that occurs with any inertial system is Human Error.  If the pilot fails to input the correct data then the information the inertial gives out will be wrong: "Rubbish in ...Rubbish out"

The INS platform is held in alignment by gyroscopes and gyros have two important properties:

  • Rigidity.  The spin axis of a gyroscope will always point in the same direction even if the gyroscope is moved.
  • Precession.  If a force is applied to the spin axis the gyroscope will move in a direction different to the force axis or the spin axis.

A spinning gyroscope should always point in the same direction because of rigidity. However, if you monitor a gyroscope for a period of time you will observe that the spin axis drifts away from the direction in which it was originally pointing.  This movement is called wander. There are two types of wander:

Real Wander. Caused by mechanical errors normally in the manufacturing stage such as the gyro not being perfectly balanced and friction at the gimbals.  These factors cause precession and the spin axis moves.  Real wander can be reduced, but not completely removed, by improved manufacturing tolerances.

Apparent Wander.  Even if Real Wander was fully eliminated and the spin axis is maintained pointing in a single direction (rigidity), the spin axis is maintained relative to space and not the Earth’s surface.  Therefore, as the Earth rotates or the gyro is moved the direction of the spin axis will appear to wander (change slowly).  There are two reasons for Apparent Wander, as follows:

  • Earth Rate.  A gyro is pointing to a direction in space. A man measures the direction from North.  The Earth is spinning, rotating once every 24 hours.  The gyro will stay pointing to the same direction in space.  However the direction of the gyro measured from North has changed.  The gyro has appeared to wander because of the Earth's rotation.  The Earth Rate depends on the latitude of the observer.
  • Transport Wander.  When a gyro is moved in an Easterly or Westerly direction the angle between the spin axis and the local meridian will change.  Therefore, the gyro will have appeared to wander.  This is known as transport wander.  Transport Wander can also be calculated and corrected.

The Apparent Wander of the gyros is corrected by the equipment. However, the Real Wander of the gyros will degrade the INS information over a period of time.

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