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