The details of the IF channelization are still to be decided. A possible design has the GHz of instantaneous bandwidth broken into 2 pairs of oppositely polarized IF channels, each 500 MHz wide. The two IF channels in each polarization need not be contiguous in frequency. At higher frequencies, where much more than 1 GHz of receiver tuning range is available, four different frequencies could be observed simultaneously. For full flexibility, this scheme will require 4 synthesizers per IF conversion in each antenna, and will return 4 independent IF channels to the correlator. It is possible that the number will be further upgraded, to 8 independent IFs. To avoid serious bandwidth smearing of the maps the widest bandwidth to be correlated will probably be no more than a few MHz.
Analog transmission is preferred for the simplicity of the equipment at the antennas. Stability of the frequency response may drive us to the use of digital transmission, especially on runs longer than 20 km. Further study, including laboratory testing of a fiber link, is required. As well as increasing the instantaneous bandwidth, this new system will strive to provide better bandpass stability leading to better spectral dynamic range.
The antennas in the Pie Town and Los Alamos ``rings" must be connected to the VLA by wide-band data links. Many technical and economic aspects of the long runs of optical fiber being contemplated for this, as described in §1.2.5, require detailed study before the cost and feasibility of the A+ configuration can finally be assessed.
For links that the NRAO might install and use on an exclusive basis, we need to explore the cost of obtaining right-of-way and of installation (estimated as about $5 per foot, considerably more than the cost of the fiber cables themselves). The main technical issue for such links would be the stability of the frequency response of the IF channels; this should be evaluated over a link to the Pie Town VLBA antenna as soon as possible.
The proposed links to Bernardo and to the ``Los Alamos ring'' antennas are unlikely to be economic to install and operate on an exclusive basis. It may however be practical to lease long wide-band fibers and suitable booster amplifiers from commercial carriers when these links are needed for A+ configuration operation. The NRAO should plan to get early experience in using commercial fiber links for real-time interferometry using the nearer VLBA antennas. The commercial-link environment should be simulated using a fiber to the Pie Town antenna as soon as possible.
It will be important to evaluate the logistical and technical problems of using commercial fiber before the feasibility and operating cost of the full A+ configuration can be determined. If commercial links can be used successfully, however, this approach may be extensible beyond the baselines considered here as part of the A+ configuration, e.g., to Kitt Peak and Fort Davis.
The final locations for the A+ configuration antennas should be chosen by combining the above factors with the need for good imaging properties when the antennas are used with both the VLA and the VLBA. These areas require further study, which should begin as soon as possible.