The VLBA correlator is situated in the DSOC, at the end of the data path. Its role is to reproduce the signals recorded at the VLBA stations and any others involved in the observation, and to combine them in two-station baseline pairs, to yield the visibility function which is the fundamental measurement produced by the VLBA. VLBA observations are processed using the DiFX software correlator. DiFX was developed at Swinburne University in Melbourne, Australia (Deller et al. 2007), and adapted to the VLBA operational environment by NRAO staff (Brisken 2008). Subsequent references to "DiFX" apply specifically only to this VLBA implementation.

We encourage users to include the following text in the Acknowledgments section of any publication arising from VLBA observations made since December 2009:

This work made use of the DiFX software correlator developed at Swinburne University of Technology as part of the Australian Major National Research Facilities program.

... and to cite the following paper by the developers: Deller, et al. 2011, PASP, 123, 275.

Software correlation is especially well suited to applications like VLBI with bandwidth-limited data-transmission systems and non-realtime processing. Among its several advantageous aspects are: (1) flexible allocation of processing resources to support correlation of varying numbers of stations, frequency and time resolution, and various special processing modes, with no fundamental fixed limits other than the finite performance of the processing cluster; (2) optimization of resource usage to minimize processing time; (3) integration of control and processing functions; (4) continuously scalable, incremental upgrade paths; and (5) relatively straightforward implementation of special modes and tests. These and other virtues of software correlation are discussed in more detail by Deller et al. (2007).

Despite the absence of fixed limits cited in item (1) above, guidelines have been established for the extremes of spectral resolution, integration period, and output rate, for routine DiFX processing, as specified in the appropriate sections below. Exceptions will be considered for proposals including a sufficiently compelling scientific justification.

The VLBA DiFX correlator is not configured to process data from a single antenna, nor is a multi-station autocorrelation-only mode available.

Operation of DiFX is governed primarily by an observation description in VEX format (currently vex1.5). This format is used for both station and correlator control functions in a number of VLBI arrays, and VLBA program SCHED (Walker 2011) has been producing it for many years.

The VLBA and HSA stations currently record data exclusively on Mark 6 disk modules and DiFX fully supports Mark 6 data. DiFX also has limited support for data recorded on Mark 5 disk modules, as recorded by a Mark 5A, Mark 5B, Mark 5B+, or Mark 5C recorder. Support for VDIF format is currently incomplete but includes those versions created by the VLBA RDBE and the VLA WIDAR correlator. Modes recorded at EVN stations are also fully supported.

Correlator output is written according to the FITS Interferometry Data Interchange Convention (FITS-IDI; Greisen 2009). In addition to the fundamental visibility function measurements and associated meta-data, the FITS files include amplitude and phase calibration measurements, weather data, and editing flags, all derived from data logged at the observing stations. An up-to-date release of AIPS is required to handle DiFX data properly.

Conversion of DiFX correlator output to the Mark 4 format that is used primarily in analysis of geodetic observations is also available. To enable this additional output, a SCHED parameter CORDFMT=MARK4 should be specified.