Introduction
The correlator is situated in the SOC, 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 Swinburne University of Technology software correlator, developed as part of the Australian Major National Research Facilities Programme and operated under licence.
... and to cite the following recent paper by the developers: Deller, et al. 2011, PASP, 123, 275.
Software correlation has become feasible in recent years, and 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, NRAO has established guidelines 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.
DiFX processes 2-bit samples with substantially greater efficiency than 1-bit samples over double the bandwidth, basically because only half as many samples must be correlated. Since these two cases have nearly equivalent sensitivity, 1-bit sampling is no longer supported by the VLBA's data systems.
DiFX also does not process data from a single antenna. Specifically, autocorrelation-only mode is not available.
Operation of DiFX is governed primarily by an observation description in VEX format. This format is used for both station and correlator control functions in a number of VLBI arrays, and NRAO program SCHED (Walker 2011) has been producing it for many years.
DiFX only accepts data on Mark5 disk modules, as recorded by a Mark5A, Mark5B, Mark5B+ or Mark5C recorder. It can process data in a variety of formats including VLBA, Mark4, and Mark5B. Support for VDIF format is currently incomplete but includes those versions created by the VLBA RDBE and the VLA WIDAR correlator.
Correlator output is written according to the FITS Interferometry Data Interchange Convention (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, derived from data logged at the VLBA stations. A recent AIPS release is required to handle DiFX data properly; 31DEC12 is recommended to support all features of the current DiFX version.
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.
