VLBI difference astrometry at 43GHz

From 43GHz VLBA observations of the pair of radio sources 1928+738 and2007+777 we have demonstrated the feasibility of precision phase-delaydifferential astrometric techniques at millimeter wavelengths. For a pair ofsources with 5 degrees separation and high antenna elevations, we have shownthat present astrometric models and millimeter arrays are advanced enough tomodel the differential phase-delay to within 2 picoseconds, less than one tenthof a phase-cycle at 43GHz. The root-mean-square of the differential phase-delayresiduals is dominated by the fluctuations of the atmospheric water vapor. Wehave determined the relative position of the observed sources with a precisiontwofold better than previous determinations at lower frequencies and, moreimportantly, largely free from ambiguous definitions of the reference point onthe structure of the radio sources. Our result makes 43GHz VLBI phase-delaydifferential astrometry an ideal tool to study the absolute kinematics of thehighly variable structures of regions near the core of extragalactic radiosources