The VLBA currently has limited ``scaled-array'' capability compared to that of the VLA. Because of its small number of antennas, the image quality deteriorates if any antennas are removed at one frequency to ``match'' the coverage at another. It is also not reconfigurable. Multi-frequency coverage at a fixed angular resolution can be obtained only by adding data from baselines shorter than 400 km at the higher frequencies ( Figure 4.2). Cross-linking an expanded VLA with the VLBA will extend the scaled-array capability of both instruments to cover a much greater domain of angular resolution, providing well-filled coverage that can be scaled with wavelength over a wide frequency range. For example, when studying relativistic jets in AGN's or galactic X-ray transients, we must be able to image total and polarized emissions at the same resolution over a wide range of frequencies to determine spectral energy distributions, Faraday rotations and magnetic field directions, as well as the collimation and proper motion information that are available from single frequencies.
Figure: VLBA images of the nucleus of the active galaxy
3C84 at five frequencies. Note the differing sensitivities. The
extent to which the detailed differences between these images are
really frequency-dependent properties of the source (rather than
differences in the coverage of the VLBA at these frequencies)
would be unclear even with matched sensitivities. The high-resolution
43-GHz image clearly under-samples the broader (>5 milli-arcsecond)
structure in the lower-frequency images, however and accounts for only
about 3/4 of the known total flux density. These uncertainties could
be much reduced by cross-linking the expanded VLA and the VLBA to
provide a scaled-array capability, especially at the highest
frequencies.