Impact of Hipparcos Data on Astrometric Reduction of Solar System Bodies

We investigated the improvement of photographic astrometric observations of solar system bodies by use of Hipparcos Catalogue stars, PPM Catalogue stars, and advanced plate reduction procedures. In the plate reductions, we rst computed apparent, topocentric coordinates of the date for the reference stars, taking into account eeects of refraction, aberration , precession, nutation, etc. In this way, the 6 plate constants, of our linear reduction model, need account for only the instrumental eeects of scale, orientation , and zero point. This method is very accurate even if few reference stars are available on the plates. We tested our reduction methods with photographic plates of Jupiter's Galilean satellites, taken with the 26-inch refractor of the U.S. Naval Observatory. The position of Jupiter on the plates was determined from measurements of the Galilean satellites, combined with ephemerides accurate to better than 10 milliarcseconds (mas). The resulting observations were compared to DE403. Our preliminary results gave an average (O{C) of (0:014 0:013) arcsec in right ascension, (?0:005 0:014) arcsec in declina-tion.