Appendix: the spacecraft attitude reconstruction

In this Appendix we investigate in some details the problems that affected the absolute attitude reconstruction of BeppoSAX during the first two years of the mission.

In summer 1997, during the commissioning of the BeppoSAX Single Gyro Pointing Mode, an error of about 20 arcsec in the misalignment matrices of the three star-trackers, used to control the spacecraft attitude, was discovered.

As the following investigation revealed, the error originated because the coordinates of the stars used by the spacecraft star-trackers were not corrected for the annual aberration due to the heliocentric motion of the Earth.

This error was soon corrected (August 1997), but this led to even greater problems: rather large movements of the spacecraft (up to about 2 arcmin) was in fact observed when the attitude control was switched between different star-trackers. The inspection of this problem, in spring 1998, revealed that the correction for the annual aberration was applied with the wrong sign. The error was finally fixed on May 6 1998 with a new and exact computation of star coordinates and star-trackers misalignment matrices.

To summarize, we can define 3 distinct time intervals with respect to BeppoSAX attitude reconstruction:
 

• Period 1 (May 1996 - May 1997): ~20 arsec error on both star-trackers alignment matrices and target star positions (due to a missing correction for annual aberration).
• Period 2 (August 1997 - May 6 1998): ~40 arcsec error on both star-trackers alignment matrices and target star positions (due to a wrong correction for annual aberration).
• Period 3 (May 6 1998 - present): the spacecraft attitude data is unaffected by errors related to the annual aberration.

 

The effects of the satellite attitude reconstruction problems on the positioning accuracy of BeppoSAX NFI instruments is discussed in Section 3, 4 and 5 of this document.