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A bug was found in the timing of individual events and was due to the preprocessing software. The origin of the problem was the spacecraft clock reset which followed the spacecraft tumbling incident of 1991 January 25. All PSPC data after that time will be affected (this problem occurs after each clock reset, which is done annually to accommodate roll-over of the counter).
The problem leads to relative shifts of 1 s between adjacent PSPC events. Although the problem is not cumulative, throughout the data there may be relative time shifts of the order of 1 second. The effect of the problem is to invalidate some timing analyses which are done on seconds or subsecond time scales. The bug has negligible impact on spectral and spatial analyses. Unfortunately there is no algorithm for correcting bad data. The bug has been corrected in SASS (Version SASS5_7, 1992 April 22, the SASS version numbers in this document refer to the entry on the front page of the SASS paper output and in the PUBLIC_CONTENTS file). Unfortunately, there is no algorithm for correcting bad data. All data will be correct after REV2 processing.