  One major problem of phase-referencing VLBI observations are phase errors due
to the unknown tropospheric zenith delay at each antenna. These errors degrade
the quality of the phase-referenced image and limit the achievable astrometric
accuracy. We will present and compare two independent methods to estimate the
zenith delay offset at each antenna. The zenith delay offsets can then be used
to correct the data. These corrections improve the quality of the
phase-referenced image and an astrometric accuracy of 10 $\mu$as can be
achieved. With this accuracy, measurements of proper motions in the Local Group
become feasible.
