DETERMINATION O THE EXTRAGALACTIC-PLANETARY FRAME TIE FROM JOINT ANALYSIS OF RADIO INTERFEROMETRIC AND LUNAR LASER RANGING MEASUREMENTS

Very Long Baseline Interferometry (VLBI) observations of extragalactic radio sources provide the basis for defining an accurate non-rotating reference frame in terms of the angular positions of the sources. Mea surements of the distance from the Earth to the Moon and to the inner planets provide the basis for defining an inertial planetary ephemeris reference frame. The relative orientation, or frame tie, between thes e two reference frames is of interest for combining Earth orientation measurements, for comparing Earth orientation results with theories re ferred to the mean equator and equinox, and for determining the positi ons of the planets with respect to the extragalactic reference frame. This work presents an indirect determination of the extragalactic-plan etary frame tie from a combined reduction of VLBI and Lunar Laser Rang ing (LLR) observations. For this determination, data acquired by LLR t racking stations since 1969 have been analyzed and combined with 14 ye ars of VLBI data acquired by NASA's Deep Space Network since 1978. The frame tie derived from this joint analysis, with an accuracy of 0.003 '', is the most accurate determination obtained so far. This result, c ombined with a determination of the mean ecliptic (defined in the rota ting sense), shows that the mean equinox of epoch J2000 is offset from the x-axis of the extragalactic frame adopted by the International Ea rth Rotation Service for astrometric and geodetic applications by 0.07 8'' +/- 0.010'' along the y-direction and by (0.019'' +/- 0.002'' alon g the z-direction.