The recent Senior Review commissioned by the National Science
Foundation's Division of Astronomical Sciences lauded the VLBA as
a unique facility, but recommended that outside funding assistance
be obtained for operations by 2011, or else that the VLBA be closed.
NRAO is confident of its ability to attract this funding, and has
embarked on a program to significantly enhance the sensitivity
of the VLBA by substantially increasing the data rate supplied from
the telescopes to the correlator. The goal of this increase is
to achieve a 4 Gigabit per second (4 Gbps) capability by 2011,
a 32-fold increase over the present sustainable data rate of
128 Megabit per second (128 Mbps). This will increase the
standard observing bandwidth from 16-32 MHz per polarization to
500 MHz per polarization (a factor of 5 above the current VLA)
and will enhance the
signal/noise ratio of the typical continuum observation by a
factor of
, the equivalent of nearly two optical
magnitudes. New technologies for digital backends, data
transmission and recording, and data correlation make this an
achievable goal for a fairly modest investment over the next
several years. In the nearer term, an upgrade of the 1 cm receivers
at the VLBA recently has been completed, with financial assistance
from the Max Planck Institut für Radioastronomie; see
Section 5. Overall descriptions of the VLBA
sensitivity upgrade are given by Romney (2007) and Walker et
al. (2007b).