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Is attitude estimation error always large in the direction of the star tracker boresight?

Aerospace Control Systems

Spacecraft GNC Systems Engineering

 

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Is attitude estimation error always large in the direction of the star tracker boresight? PDF Print E-mail

As is well known, the attitude measurement error from a star tracker is larger in the boresight direction of the star tracker than in two orthogonal directions perpendicular to the boresight. For single star tracker operation with a three-axis IMU, attitude estimation error is larger in the direction of the star tracker boresight, when the spacecraft is not maneuvering.

When the spacecraft maneuvers, information from the other directions is moved into the previous inertial direction of the boresight by virtue of the gyro. When the maneuvering stops, process noise (due to the IMU error) will cause the covariance in the boresight direction to grow to a larger steady-state value. The movement of information into the boresight direction depends on the rate of rotation.

Here we assume the IMU has high accuracy compared to the star tracker, which is generally true. Boresight accuracy will be less with a low accuracy IMU.