Phase Referencing

If the VLBA target source is not sufficiently strong for self-calibration or if absolute positional information is needed but geodetic techniques are not used, then VLBA phase referenced observations must be employed (Beasley & Conway 1995). Currently, more than half of all VLBA observations employ phase referencing. Wrobel et al. (2000) recommend strategies for phase referencing with the VLBA, covering the proposal, observation, and correlation stages. A VLBA phase reference source should be observed frequently and be within a few degrees of the VLBA target region, otherwise differential atmospheric (tropospheric and ionospheric) propagation effects will prevent accurate phase transfer. VLBA users can draw candidate phase calibrators from the source catalog in use at the VLBA correlator, distributed with the NRAO program SCHED (Walker 2006); easy searching for the nearest calibrators is available on-line through the VLBA Calibrator Survey (Beasley et al. 2002) at http://www.vlba.nrao.edu/astro/calib/ . Most of these candidate phase calibrators now have positional uncertainties below 1 mas, as announced in the NRAO Newsletter dated 2001 October.

Calibration of atmospheric effects for either imaging or astrometric experiments 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 (see Section 28.1), 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.