Revisiting the VLBA Calibrator Surveys for ICRF3

he original VLBA Calibrator (VCS) surveys were a set of 24 24-hr VLBA sessions between 1994 and 2007 in which ~2600 compact radio sources were observed in X/S bands to obtain precise positions and snapsot images. Most were observed in only one session but they vastly expanded the pool of calibrator sources available for phase-referencing VLBI. These sources were later incorporated into ICRF2, but within ICRF2 they became a second class compared to the other ICRF2 sources observed in regular Mark3/Mark4/RDV VLBI geodesy and astrometry sessions over nearly 30 years. The VCS sources represented 2/3 of ICRF2, but they had formal errors approximately 5 times greater, on average, than the other 1/3. In order to reduce this 2 class distinction, the VCS-II team was formed to re-observe these sources for ICRF3 and improve their usefulness as phase-referencing calibrators. We report here on the re-observations of 2400 of these sources in 8 VLBA sessions in 2014 and 2015. The new observations were made at 2 GBits/sec versus the 128 MBits/sec of the original VCS sessions, and thus are considerably more sensitive. Some 2000 'old' VCS sources have been re-observed and their formal position errors have improved by an average factor of 3.6 in RA and Dec. And ~300 'new' sources (not detected in the original VCS analysis) have also now been detected. Mapping of these sources is also being performed and we anticipate being able to compute structure indices for them as well