Antenna Axis Offsets and Their Impact on VLBI Derived Reference Frames

We have estimated the antenna axis offsets of VLBI (Very Long Baseline Interferometry) telescopes using VLBI data from 1995 to 2013. These were compared to values estimated in terrestrial surveys. We generally found an agreement of a few millimeters, however, occasionally the differences were larger than 1 cm. Furthermore, we investigated the impact of the axis offset on the terrestrial and the celestial reference frames derived by VLBI. This was done by comparing the reference frames estimated simultaneously with the axis offset with those estimated when the axis offsets were fixed to their a priori values. For a telescope with azimuth-elevation mount we found that an error in the axis offset of 1 cm causes an error in the station height of about 1.3 cm.