9303527 Shapiro VLBI Astrometry. The technique of very-long-baseline-interferometry (VLBI) has been used with radio telescopes girdling the globe to determine the sky positions of compact radio sources with uncertainties of one milliarcsecond or less and the relative positions of neighboring sources with uncertainties as low as 12 microarcseconds. Dr. Shapiro and his colleagues at Harvard University will continue using this technique to: (a) determine galactic and extragalactic distances and to estimate the Hubble constant; (b) measure, or place bounds on, the proper motions of several extragalactic radio sources in order to improve our understanding of the nature of these sources and to improve the extragalactic reference frame; and (c) tie this frame to the solar-system reference frame.
