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The VLBA correlator has been operational for somewhat more than a year. Since the beginning of 1995, it has operated at 54\% efficiency (i.e., that fraction of scheduled production time leads directly to finished results). After correcting for effects of the playback speedup and the part-time production schedule, this corresponds to a 39\% net operational efficiency of the VLBA instrument over the same interval.
The current operational system includes most of the capabilities originally planned for the correlator's initial configuration. Two important exceptions could not be included: narrowband (less than 250~kHz bandwidth) spectroscopy, and various forms of sub-arraying. On the other hand, two capabilities not planned until later have been achieved: cross-polarized correlation and an extremely accurate fringe model. Parts of the real-time control software are currently being rewritten. The goals of this effort are both scientific, including support for narrowband and subarrayed observations, and technical, providing a more secure basis for development of additional capabilities in future phases. Among the most promising of these later features is a frequency-dependent signal gate, which will enable gated, de-dispersed interferometry of pulsars.
The correlator is a 20-station system, capable of processing data rates up to the VLBA's maximum of 512~Mbit/s, with spectral resolution up to 2048~points per baseline. Its design is based on an ``FX'' architecture, performing a fast Fourier transform operation on segmented input signal streams, followed by a pointwise cross-multiplication operation and subsequent integration. This approach makes possible a relatively inexpensive, flexible yet simple hardware organization, and in turn a fairly simple structure of the control software. It provides several enhancements of sensitivity relative to a conventional lag correlator. Facilities included in the VLBA array and correlator for dealing with the less favorable aspects of the FX architecture have also been shown to function effectively.
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