VLBI difference astrometry at 43 GHz

From 43 GHz VLBA observations of the pair of radio sources QSO 1928+738 and BL 2007+777 we have demonstrated the feasibility of precision phase-delay differential astrometric techniques at millimeter wavelengths. For a pair o f sources with 5 degrees separation and high antenna elevations, we have sh own that present astrometric models and millimeter arrays are advanced enou gh to model the differential phase-delay to within 2 picoseconds, less than one tenth of a phase-cycle at 43 CHz. The root-mean-square of the differen tial phase-delay residuals is dominated by the fluctuations of the atmosphe ric water vapor. We have determined the relative position of the observed s ources with a precision twofold better than previous determinations at lowe r frequencies and, more importantly, largely free from ambiguous definition s of the reference point on the structure of the radio sources. Our result makes 43 GHz VLBI phase-delay differential astrometry an ideal tool to stud y the absolute kinematics of the highly variable structures of regions near the core of extragalactic radio sources.