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It has been found that in the Standard Processing, some of the data delivered to the observers may still contain time intervals during which the applied attitude solution shows unacceptably large errors. The aspect error in the attitude file, which is screened against, has been lowered successively from 120'' to 2'' (SASS5_4_8, 1992 December 20) and 1'' (SASS5_6, 1992 March 17) for the PSPC and to 1'' (SASS5_5_2, 1992 March 10) for the HRI. As long as the formal aspect error is below 1'' the GSOC attitude solution is reliable within about 10''. For aspect errors the range 1''-10'', the situation is not unique because the star sensor is usually tracking only one star, which cannot be identified unambiguously. We urge the user to assure himself of the integrity of the data by inspecting the attitude error entries in the attitude file.
Users analyzing data summed over from observations taken several months apart should be aware that the boresighting and irreproducible errors may produce position differences of up to 20'' between the observations. These can be corrected for by first analysing the datasets separately and then shifting them to a common reference frame using the detected X-ray sources.