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