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The USNO Flagstaff Station has been participating in NASA's Galileo mission by providing astrometric positions of two asteroids, Gaspra and Ida, and CCD frames of the Galilean Moons. Successful encounters with the asteroids have provided direct measures of the accuracy of the ground-based astrometry and an unusual opportunity to evaluate potential sources of error in astrometric measurement. The effects of various error sources are evaluated, including astronomical seeing, exposure duration, geometry of reference stars in the field of view, type of reference catalog, and mode of observing (scan, stare, or multisnap). Three different types of reference catalogs were used in the reductions: (1) a star catalog based on astrograph observations, (2) the VLBI Reference Frame catalog, based on Very Long Baseline radio Interferometry of QSOs and other compact radio sources, and (3) a mixed reference frame containing astrograph catalog stars and a few HIPPARCOS stars. A comparison of reductions based on each type of catalog provides further insight into sources of astrometric errors.
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