NRAO is nearing completion of a VLBA Sensitivity Upgrade project that significantly enhances the instrument's capabilities. The project's goal is to expand the VLBA's continuum bandwidth to 512 MHz per polarization, and thereby improve the signal/noise ratio of a typical continuum observation by factors of 2.8-5.6. The latter is equivalent to nearly two optical magnitudes. The corresponding recorded data rate of 4 Gigabits per second (Gbps) is a 32-fold increase over the sustainable rate of 128 Megabits per second (Mbps) that was available when the instrument was inaugurated, and 8 times the current standard data rate. New technologies for digital signal processing, data transmission and recording, and correlation, make this an achievable goal for a fairly modest investment. Overviews of the VLBA sensitivity upgrade are given by Romney (2007) and Walker et al. (2007b).
The first of the Sensitivity Upgrade developments, the DiFX software correlator described in Section 7, has been in routine operational use for over a year. Two additional elements, introduced more recently, are described in Sections 6.9 and 6.10. The new instrumentation, which has now been installed across the VLBA, enables an interim bandwidth of 256 MHz per polarization (2 Gbps recording rate). Further information on the Sensitivity Upgrade project is available in a memo series, at http://www.vlba.nrao.edu/memos/sensi/ . Questions can be submitted via the NRAO Helpdesk, at https://help.nrao.edu/ .
A new project, initiated recently, will replace the VLBA's 6 cm receivers. The new systems, with an expanded tuning range of 4.1 - 7.9 GHz, will enable observations of the 6.7 GHz transition of methanol. The first receiver has been upgraded to the new, wider band and higher sensitivity configuration, and re-installed on the array, where a noise temperature of about 22 Kelvin was measured on the sky. Since the downconverter module required to support the entire range of frequencies is not yet available, temporary filters currently limit the operation of the new receiver to the old system's range. An expedited schedule foresees installation proceeding at about one per month, with the upgrade reaching completion by mid-2012. During this period, the VLBA's complement of 6 cm receivers will be limited to 8 or 9 at most times. Proposals for 6 cm observations that require an array of 9 or 10 stations should include a careful justification.