The technique of very long baseline interferometry (VLBI) is used with radio telescopes girdling the globe to image compact radio sources with an angular resolution reaching 100 microarcseconds and to determine their relative positions with an accuracy approaching 10 microarcseconds. This grant will be used by Dr. Shapiro at Harvard University to continue using the VLBI technique (a) to determine galactic and extragalactic distances and to estiamte Hubble's constant Ho, (b) to measure and place bounds on proper motions of radio sources, and (c) to improve the extragalactic radio source reference frame.
