As part of the VLBA sensitivity upgrade, NRAO currently is integrating the DiFX software correlator into the operational environment of the VLBA, and performing tests to validate its results by comparison with those of the original VLBA correlator. The DiFX system was developed at Swinburne University in Melbourne, Australia (Deller et al. 2007). We anticipate that DiFX will become the operational VLBA correlator in 2009. At that time, the present section will become obsolete.
The VLBA correlator, located at the DSOC, accommodates the full range of scientific investigations for which the array was designed. The correlator supports wideband continuum, high-resolution spectroscopy, bandwidth synthesis, polarimetric, and gated observations.
The correlator is designed to process all observations involving VLBA stations. With its 20-station capacity and sub-arraying capabilities, it is designed to correlate an extended array combining the VLBA with as many as 10 other stations. At present, the VLBA correlator has available 17 data inputs from Mark 5A recorders, so the rare observations requiring correlation of more than 17 stations including the VLBA require multiple correlator passes.5
Each station input comprises 8 parallel ``channels'' (as defined in Section 6.13), which operate at a fixed rate of 32 Msamples per second, for either 1- or 2-bit samples. Observations at lower sample rates generally can be processed with a speed-up factor of 2 (for 16 Msamples per second) or 4 (for 8 Msamples per second or less) relative to observe time. Special modes are invoked automatically to enhance sensitivity when fewer than 8 channels are observed, or when correlating narrowband or oversampled data. The correlator accepts input data recorded in VLBA or Mark 4 longitudinal format, or on Mark 5A disk modules, and plays these data back on tape or disk drives similar to the station recorders (see Section 6.16).
Each input channel can be resolved into 1024, 512, 256, 128, 64, or 32 ``spectral points,'' subject to a limit of 2048 points per baseline across all channels. The correlator cannot process maximally (16-fold) oversampled data at the highest spectral resolution, which effectively prohibits 1024-channel resolution at the narrowest bandwidth of 62.5 kHz. Adjacent, oppositely polarized channels can be paired to produce all four Stokes parameters; in this case correlator constraints impose a maximum spectral resolution of 128 points per polarization state.
The correlator forms cross-spectral power measurements on all
relevant baselines in a given sub-array, including individual antenna
``self-spectra.'' These can be integrated over any integral multiple
of the basic integration cycle, 131.072 milliseconds (
microsec). Adjacent spectral points may be averaged while integrating
to reduce spectral resolution (see Sections 12 and
13).
Correlator output is written in a ``FITS Binary Table'' format, and includes editing flags plus amplitude, weather, and pulse calibration data logged at VLBA antennas at observe time (Flatters 1998; Ulvestad 1999). All results are archived on digital-audio-tape (DAT) cassettes. The output data rate is limited to 1.0 Mbytes per second (MB/s), which must be shared among all simultaneous correlator sub-arrays. Data are copied from the archive for distribution to users on a variety of media, with DAT and Exabyte currently given primary support. Observations since approximately 1998 (or earlier, depending on when you read this document!) can be retrieved directly from the NRAO archive at http://archive.nrao.edu.
Operation of the correlator is governed primarily by information obtained from the VLBA control system's monitor data or from foreign stations' log files. A few additional items, all of which have been mentioned above, will be specified by the user prior to correlation. Supervision of the correlation process is the responsibility of VLBA operations personnel, and requires no participation by the observers. As described in Section 25, however, users are encouraged to visit the DSOC after correlation for post-processing analysis.
Consult Benson (1995) and Romney (1995, 1999), respectively, for more information on the VLBA correlator and on VLBI correlation in general.