New Developments
This edition of the VLBA Observational Status Summary coincides closely with the 20th anniversary of the VLBA's inauguration on 1993 August 20. It can also be taken to mark the completion of the VLBA Sensitivity Upgrade project, with the beginning of observing semester 2013B, the first for which the VLBA's original observing system was no longer available for new proposals. That legacy equipment will be decommissioned following completion of some ongoing long-term monitoring projects, for which a change in instrumentation is undesirable.
The Sensitivity Upgrade project can trace its origins to a two-year internal NRAO study of the scientific goals (Romney 1999; VLBA Sensitivity Upgrade Memo # 2) and derived technical requirements (Romney 2000; VLBA Sensitivity Upgrade Memo # 3), for an upgrade of the VLBA. These concepts were subsequently cited among the initial steps recommended in Phase II of the VLBI Future Committee report "Mapping the Future of VLBI Science in the U.S." (Taylor, Lonsdale, et al. 2004; VLBA Sensitivity Upgrade Memo # 4), upon which the initial Sensitivity Upgrade proposal (Walker et al. 2007; VLBA Sensitivity Upgrade Memo # 15) was based. Funding to begin implementation of these goals became available in 2007.
The Sensitivity Upgrade instrumentation now coming into full operational use addresses, finally, many of the VLBI Future Committee recommendations. The primary discrepancy lies in the achieved maximum recording rate being limited to 2 Gigabits/sec (Gbps), rather than the desired minimum 4-Gbps capability. It should be noted, however, that the Sensitivity Upgrade equipment was designed to support a 4-Gbps rate if sufficient resources can be found to buy a second recorder for each station, and to double the pool of recording media. The 2-Gbps rate, corresponding to a continuum bandwidth of 256 MHz per polarization, is nevertheless a 16-fold increase over the standard "sustainable rate" at which the VLBA operated for many early years.
Overviews of the new instrumentation are presented in this document in the sections on the ROACH Digital Backend and Mark 5C Recorder. More detailed information is available in the Sensitivity Upgrade memo series. Questions can be submitted via the NRAO Helpdesk.
