Precise absolute astrometry from the VLBA imaging and polarimetry survey   at 5 GHz
  We present in this paper accurate positions of 857 sources derived from the astrometric analysis of 16 eleven-hour experiments from the Very Long Baseline Array imaging and polarimetry survey at 5 GHz (VIPS). Among observed sources, positions of 430 objects were not determined before at a milliarcsecond level of accuracy. For 95% of the sources the uncertainty of their positions range from 0.3 to 0.9 mas, with the median value of 0.5 mas. This estimate of accuracy is substantiated by the comparison of positions of 386 sources that were previously observed in astrometric programs simultaneously at 2.3/8.6 GHz. Surprisingly, the ionosphere contribution to group delay was adequately modeled with the use of the total electron contents maps derived from GPS observations and only marginally affected estimates of source coordinates. 
Leonid Petrov, Gregory Taylor
astro-ph.CO