Mark 4 VLBI Correlator

The Mark 4 VLBI correlator of the MIT Haystack Observatory, located in Westford, MA, is supported by the NASA Space Geodesy Program. Production correlator time is dedicated to processing both geodetic and astronomical VLBI data. In addition to its role as an operational correlator, the Haystack Correlator also serves as a development system for testing new correlation modes, e-VLBI, hardware improvements, and for diagnosing problems at identical correlators located at the U.S. Naval Observatory or Max Planck Institute for Radio Astronomy in Germany. This flexibility is made possible by the on-site presence of the team that designed the correlator hardware and software.

The Mark 4 correlator is capable of simultaneously processing 1024Mbps data streams from up to 16 stations, performing cross-correlations on all 120 baselines. For continuum processing, 32 lags are normally processed, but the 131,000 lags available in the correlator may be flexibly reconfigured to trade number of baselines with number of lags. The original design of the correlator was targeted for the use with the tape-based Mark 4 VLBI data system, but has recently been updated to support the disk-based Mark 5 VLBI data system as well. In addition, the correlator supports real-time correlation of data transmitted electronically from stations (so-called ‘e-VLBI’), and has been used in this mode with data transmitted in real-time at up to 512Mbps from stations distributed globally (link to e-VLBI).

The Mark 4 correlator was developed by the International Advanced Correlator Consortium (IACC), which divided development responsibilities between the U.S. and Europe, in a project which started in 1993 and concluded in late 1999. MIT Haystack Observatory developed the signal-processing core of the correlator, including a large ASIC chip specifically designed for high-speed VLBI processing. The Joint Institute for VLBI in Europe (JIVE), along with the Jodrell Bank Observatory, developed the ‘Station Unit’, which is responsible for reconstituting data from tape and pre-applying a model delay before transmission to the correlator core.

Mark 4 VLBI correlators are now placed at MIT Haystack Observatory, the U.S. Naval Observatory in Washington, D.C., JIVE in The Netherlands, and at the Max Planck Institute in Bonn, Germany. In addition, large connected-element correlators based on the Mark 4 correlator ASIC chip and board are in place at the Sub-Millimeter Array (SMA) at Mauna Kea, Hawaii, and at Westerbord, The Netherlands.