Constructing the EVLA while operating the VLA

Begun in 2001 with a total budget of around $100M, the Expanded Very Large Array (EVLA) project is the only major upgrade to the VLA undertaken since the interferometer was dedicated in 1980. The goal of this 11-year long project is to improve all the observational capabilities of the original VLA - except for collecting area and spatial resolution - by at least an order of magnitude. To achieve this, the 28 VLA antennas have been modernized with new digital data transmission systems that link to a new, wideband, fiber optic digital LO/IF system, and eight new sets of cooled receivers are under construction that will offer full frequency coverage from 1 to 50 GHz, with instantaneous bandwidths up to 8 GHz provided by two independent dual polarization frequency pairs. The new WIDAR correlator provided by NRAO's Canadian EVLA partner replaced the old VLA correlator in early 2010 and is currently undergoing commissioning. The long duration of the EVLA construction project coupled with the need to maintain the scientific productivity and user base of the telescope obviously precluded shutting down the old array while new infrastructure was built and commissioned. Consequently, the construction plan was based on the fundamental assumption that the old VLA would continue to operate as new EVLA capabilities gradually came online; in some cases, additional complexity had to be designed into new hardware in order to maintain transitional interoperability between the old analog and new digital systems as the latter were installed and commissioned. As construction has advanced, operations has increasingly had to coexist side by side with EVLA commissioning and verification. Current commissioning plans attempt to balance making new EVLA capabilities available to the user community as soon as they have been installed and verified, and maintaining a stable and robust end-to-end data acquisition and delivery process for the user community.