The Very Large Array has transformed many areas of radio astronomy with a powerful combination of angular resolution and brightness sensitivity. Even so, twenty years of research have also exposed many ways in which the VLA's limitations direct our observations. Depending on the field of study, we may be biased to objects that are unusually luminous examples of their class, or unusually nearby, or not too extended, or in a particular redshift range, because of constraints on the VLA design that were accepted in the 1970's. Many of these constraints can be greatly relaxed today.
The scientific capabilities of the VLA can again be transformed by returning it to the state of the art in sensitivity, frequency coverage, angular and spectral resolution. An expanded VLA could be over one hundred times faster at high frequencies, several times more frequency-agile, and one hundred times better at resolving details at all frequencies, for substantially less than the replacement cost of the instrument.
Many important observing programs are limited (or prohibited!) by sensitivity. The VLA's intermediate-frequency transmission system and correlator were designed with a maximum total bandwidth of 200 MHz. This limits the sensitivity, particularly at frequencies above 2 GHz, where wider bandwidths are permitted by the antennas, the feeds, and the level of interference. The higher frequencies are very attractive for many fields of research, including studies of the Sun and solar system objects, of thermal (stellar and circumstellar) sources in the Milky Way, of young supernovae in other galaxies, of polarimetry of the jets from active galactic nuclei, of the dense magnetized media in gas-rich clusters of galaxies, and of ``radio-quiet'' source populations at high redshifts. Fiber optic signal transmission, combined with a new correlator, can increase the maximum total bandwidth to 16 GHz, enabling a huge improvement in the VLA's capability in many astrophysical arenas.
Similarly, the VLA correlator is based on a custom ECL circuit that
was the state of the art in the 1970's. The correlator limits the
number of frequency channels and frequency resolution in ways that are
particularly onerous for spectral studies at high frequencies, and for
wide-field surveys at low frequencies. Modern designs allow much
greater spectral resolution, wider bandwidths, and increased
flexibility. A new state-of-the-art correlator would, for example,
allow complete surveys of the neutral hydrogen gas in the Universe out
to 
0.2, avoiding bias in present surveys of the local
structure that are based on cataloging galaxies. We could also
independently and simultaneously observe multiple spectral transitions
in one or two frequency bands. An expanded correlator would let us
exploit the sensitivity of broad-band systems to image wide fields of
view in the continuum, by using bandwidth synthesis to image
nearby galaxies and to inventory their stellar and interstellar
emissions with less bias toward the brightest or most compact
features.
The VLA antennas can be used up to roughly 50 GHz, and we have experience with holography and reference pointing to optimize their high-frequency performance. These techniques justify outfitting the VLA more fully for frequencies above 20 GHz, where we can explore thermal processes around galactic stars, image the structures of protoplanetary disks, detect young supernovae in nearby galaxies, and study CO and other molecules in galaxies at high redshifts.
The VLA's maximum baseline of 35 km sets an angular resolution limit at any frequency. The VLBA's inner baseline scale (about 300 km) similarly sets a largest angular scale that it can image efficiently at any frequency. The disparity, or ``gap'', between these two scales leaves a wide range of interesting astrophysical phenomena, including stellar winds, star-forming regions, and the inner regions of bright extragalactic jets, inaccessible to imaging by either instrument. These phenomena are, in effect, unresolved to the VLA but over-resolved by the VLBA, even when existing VLA antennas are used in VLBA observations. In many cases, the VLA has the sensitivity, but not the resolution, to make useful observations -- a situation which will be much more general if we upgrade only its sensitivity. The ``coverage gap'' also seriously restricts the range of observing frequencies at which angular resolutions between one arcsecond and tens of milli-arcseconds are available. This frequency ``inagility'' at fixed angular resolution limits our ability to explore the frequency dependence (and hence the physics) of phenomena with angular scales of tens of milli-arcseconds.
Bridging the ``VLA-VLBA'' imaging gap with new antennas in New Mexico and Arizona will greatly enhance the scientific productivity of both major centimeter-wave instruments. Operating the ``intermediate-baseline'' antennas as part of either instrument would let us match spatial filters to the needs of a wide range of astrophysical studies that cannot be pursued satisfactorily with either instrument alone. Furthermore, the intermediate baseline antennas would themselves perform superbly as a standalone array, giving permanent, high sensitivity, high fidelity imaging capability on small angular scale phenomena such as novae, GRBs, and expanding galactic jets.
Finally, some planned VLA subsystems were never built for lack of development (infrastructure) funding, although there are strong scientific reasons for adding them. For example, the 2.4 GHz system, initially of interest for planetary radar and for Faraday depth studies of galactic and extragalactic sources, is now of interest to all continuum projects because RFI from mobile communications systems is proliferating at lower frequencies.
It has long been clear that the VLA could be transformed into a much more powerful tool for astrophysics at only a modest cost relative to the original investment (in today's dollars). Ideally, its construction would have been followed by timely upgrades of the technology in its infrastructure, allowing a steady increase in its scientific capabilities. In fact, the NRAO's operating budget has not kept pace with inflation since the construction of the VLA, so only a few high-priority, but still incremental, improvements have been possible, and several of these have been done with non-NSF funds. The imbalance between our technical ability to increase the VLA's scientific productivity and the ability to fund even modest improvements to the instrument has grown steadily. The accumulated imbalance can now be addressed only by pervasive upgrades to the VLA technology.
In January 1995, the NRAO hosted a workshop at which VLA users and NRAO staff reviewed technical aspects and scientific benefits of an upgraded VLA. The conclusions of six topical working groups (on Solar System research, on galactic astronomy (2 groups), on extragalactic astronomy (2 groups) and on technical issues) during and after this meeting were published as the
in July 1995. Shortly thereafter, the NRAO formally organized a VLA Upgrade Project to prepare a more detailed design document summarizing the technical and engineering goals of a , the technical means and costs to achieve them, and an ongoing review of the scientific benefits to be obtained.
The general goal of the is to improve, by at least an order of magnitude, every key instrumental characteristic of the VLA. The resulting scientific benefits are enormous, as the existing infrastructure and sound basic design of the VLA now let us exploit modern technologies at modest cost.
It is important to note that much of this greatly enhanced scientific potential can be achieved by using newer, but well established, well understood, technologies. And because MMA development will implement many of these technologies, their incorporation in the can be done at very modest extra cost. Furthermore, the operations cost of the Expanded VLA should be only slightly increased over current levels. Indeed, for the improvements at the VLA site, we expect no additional cost in operations. The only anticipated increase will be due to the operation of the new antennas.