  New functionality to process Very Long Baseline Interferometry (VLBI) data
has been implemented in the CASA package. This includes two new tasks to handle
fringe fitting and VLBI-specific amplitude calibration steps. Existing tasks
have been adjusted to handle VLBI visibility data and calibration meta-data
properly. With these updates, it is now possible to process VLBI continuum and
spectral line observations in CASA. This article describes the development and
implementation, and presents an outline for the workflow when calibrating
European VLBI Network or Very Long Baseline Array data in CASA. Though the CASA
VLBI functionality has already been vetted extensively as part of the Event
Horizon Telescope data processing, in this paper we compare results for the
same dataset processed in CASA and AIPS. We find identical results for the two
packages and conclude that CASA in some cases performs better, though it cannot
match AIPS for single-core processing time. The new functionality in CASA
allows for easy development of pipelines or Jupyter notebooks, and thus
contributes to raising VLBI data processing to present day standards for
accessibility, reproducibility, and reusability.
