Calibration of atmospheric effects for either imaging or astrometric observations can be improved by the use of multiple phase calibrators that enable multi-parameter solutions for phase effects in the atmosphere. See AIPS Memos 110 (task DELZN, Mioduszewski 2004) and 111 (task ATMCA, Fomalont & Kogan 2005), available from the AIPS web page (Section 20.1), for further information.
Walker & Chatterjee (1999) have investigated ionospheric corrections. Such corrections can even be of significant benefit for frequencies as high as 5 GHz or 8 GHz (Ulvestad & Schmitt 2001). These corrections may be made with the AIPS task TECOR, as described in AIPS Cookbook Appendix C (NRAO 2006), or the procedure VLBATECR. In addition, it is strongly recommended that the most accurate Earth-Orientation values be applied to the calibration, since correlation may have taken place before final values were available; this may be done with AIPS task CLCOR or more easily with the AIPS procedure VLBAEOPS.
The rapid motion of VLBA antennas often can lead to very short time intervals for the slew between target source and phase reference source. Some data may be associated with the wrong source, leading to visibility points of very low amplitude at the beginnings of scans. Application of the AIPS program QUACK using the `TAIL' option will fix this problem.