Status Report # 19
Feb 25th 1992
Since Monday Feb 24th 1992, the ROSAT Spacecraft has been back on the Mission Timeline. Evaluation of the Z-gyro malfunction on Feb 5th proved that the measurement outputs of the Z-gyro are still valid, however the scale factor has changed by a constant factor of 2.8. The most convincing explanation of the available data implies that 1 of 3 phases of the gyro motor failed, and consequently the motor current increased.
To compensate for the decreased gyro output, a new change of the onboard software was developed and tested on the GSOC simulator over Feb 18-19th. This software patch contains the change of the Z-gyro scale factor of 280%. This software update means that the scale factors of the other gyros cannot be commanded anymore. This should not affect mission operations as the other gyros appear to be stable.
The software patch was successfully uplinked on Feb 20th and passed a first test in the form of a slew to the north ecliptic pole on the same day. After the slew, the attitude was correct and reference stars were identified. On Monday morning, the nominal timeline maneuvers were started and the first three slews successfully performed. This success indicates that the new y-gyro scale factor was correctly determined and appears to be quite stable. If further analysis of the dumped data confirms that the scale factor is stable, then the nominal pointing mission will continue.
Data processing status:
The US ROSAT Science Data Center has overcome some of the problems with the mission timeline and has been processing reduced phase data from AO1. Processing started with day 351 (May 13, 1991 - when reduced phase began due to gyro loss) and has reached day 445 (Aug 13,1991) as of Feb 24,1992. These data are more difficult to process due to longer exposure times and problems with the mission timeline therefore requiring a lot of additional manual work to complete processing.
Status Report #20
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