Although a separate ``100-km'' array (MERLIN) can provide some of
the ``missing coverage'' at 
>6cm for northern sources, the
high-frequency coverage that is critical to much of the astronomical
program, and the 
``baseline synergy'' with the VLA or VLBA can be
obtained only with antennas in the Southwestern United States.
Furthermore, MERLIN has poorer image fidelity and sensitivity (having
only 4 to 7 antennas) and less frequency flexibility, than the
proposed .
Ultimately, to study how the properties of astronomical sources change with frequency without changing resolution, we should be able to select the antenna combinations needed to create the most appropriate set of matched spatial filters for any given multi-frequency experiment.
We could do this at high angular resolution if we were able to choose ``sub-arrays'' that include VLA and VLBA baselines almost interchangeably. The NRAO prepared for this by siting the VLBA antennas around the VLA as their centroid, and by co-locating the main VLA and VLBA operations centers. The is needed to exploit the unique opportunity that this creates. It gives us a way to transform the scientific capabilities of two major instruments significantly at once.
To take full advantage of this, we must outfit the new antennas and a few existing VLA antennas with VLBA back-ends. The new antennas would be operated as part of the VLBA when the VLA is in its more compact configurations, and would need VLBA data acquisition systems. It is also desirable to have enough hardware at the VLA site itself to record signals from four VLA antennas at once. An upgrade of recording and formatter systems to reach 1 or 2 Gbps is also desirable. Astrometric VLBI observations would benefit from a 2.4 GHz/8.4 GHz (S/X) dual frequency system. The VLBA correlator was designed to handle 20-station experiments, so no new correlator capacity is required specifically for the new antennas proposed as part of the .