The Upgraded VLBA
By Jon Romney (NRAO-Socorro)
This year marks 20 years since the VLBA's dedication. The instrument has just completed a major 6-year upgrade. This talk will concentrate on technical aspects of the upgrade, but also reviews some areas of recent scientific progress.
Wideband data-acquisition systems using modern digital signal processing techniques are the centerpiece of the upgrade. Output data rates up to 2048 Mbps required development of a wideband recording system, and the original correlator was replaced by a flexible, software-based system. A new C-band receiver now covers 4-8 GHz, including the methanol maser line at 6.7 GHz.
Enhancements of operational modes have increased the VLBA's scientific productivity substantially, and a "High Sensitivity Array" offers a unified mechanism for requesting time including several large single-dish apertures. The GBT is the best integrated of these HSA stations.
Scientifically, an early concentration on AGNs continues, while astrometric observations have expanded significantly from early efforts. Accurate distances to star-forming regions are being measured throughout the Milky Way galaxy, and observations of distant galaxies are yielding an improved value for the Hubble Constant.
