Towards the ICRF3: astrometric comparison of the USNO 2016A VLBI
  solution with ICRF2 and Gaia DR1
  The VLBI USNO 2016A (U16A) solution is part of a work-in-progress effort by
USNO towards the preparation of the ICRF3. Most of the astrometric improvement
with respect to the ICRF2 is due to the re-observation of the VCS sources. Our
objective in this paper is to assess U16A's astrometry. A comparison with ICRF2
shows statistically significant offsets of size 0.1 mas between the two
solutions. While Gaia DR1 positions are not precise enough to resolve these
offsets, they are found to be significantly closer to U16A than ICRF2. In
particular, the trend for typically larger errors for Southern sources in VLBI
solutions are decreased in U16A. Overall, the VLBI-Gaia offsets are reduced by
21%. The U16A list includes 718 sources not previously included in ICRF2.
Twenty of those new sources have statistically significant radio-optical
offsets. In two-thirds of the cases, these offsets can be explained from
PanSTARRS images.
