The JPL Extragalactic Radio Reference Frame: Astrometric Results of 1978-96 Deep Space Network VLBI

Abstract

Celestial reference frames determined from measurements of extragalactic radio sources are used in interplanetary navigation, Earth orientation measurements, geodesy, and astrometry. The JPL 1997-3 celestial reference frame is derived from very long baseline interferometric (VLBI) measurements of 291 compact extragalactic radio sources. The observations were done principally at 8.4 GHz, supplemented by simultaneous 2.3-GHz observations needed to calibrate the charged-particle effects of the solar plasma and the Earth's ionosphere. The radio source positions that constitute this frame have median formal precisions of 123 and 188 microarcseconds (μas) in α cos δ and δ, respectively. Within the presently available 17.5-year span of observations, these sources are characterized by coordinate drifts that have median uncertainties of approximately 70 μas/yr. Only a few of these rates are statistically significant, and they probably are caused by variations in the internal structure of the sources. In agreement with the general relativistic theory of gravity, the parameterized post-Newtonian γPPN factor is determined to be γPPN = 1.001 ± 0.001. On the basis of internal consistency tests and comparisons to independent celestial frame determinations, we estimate that the formal uncertainties must be increased by a factor of 2 in order to more realistically describe the accuracy of the source positions. The dominant error comes from inaccurate modeling of the troposphere and, to lesser extents, from the lack of radio source structure models and imperfectly calibrated instrumentation. We briefly describe models of the observation covariances caused by these classes of errors and assess the size of remaining unmodeled errors. The absence of an all Southern-Hemisphere baseline makes the positions of southern sources especially sensitive to tropospheric mismodeling. As a result, zonal errors in the south may approach 1 milliarcsecond. The JPL frame provides an important independent verification of the International Astronomical Union's (IAU's) new International Celestial Reference Frame at the level of a few hundred μas. Finally, we review the work done to link the VLBI extragalactic radio frame to the planetary ephemeris frame and the Hipparcos optical frame, thereby creating a unified system that is much more valuable than the sum of the separate constituent frames.