Limitations of ad hoc "SKA+VLBI" configurations & the need to extend SKA to trans-continental dimensions

The angular resolution of the proposed Square Kilometre Array, SKA, must be extended towards the milliarcsecond scale if it is to resolve the distant starburst galaxies that are likely to dominate the radio source counts at micro and sub-microJy flux levels. This paper considers the best way of extending SKA's angular resolution towards the milliarcsecond scale. Two possible SKA-VLBI configurations have been investigated and simulated SKA and SKA-VLBI visibility data sets generated. The effects of non-uniform data weighting on the associated images are considered. The results suggest that the preferred option is for SKA to be extended to trans-continental dimensions. By retaining 50% of the array's collecting area within a region no larger than 50 km, the surface brightness sensitivity of the array at arcsec resolution is hardly compromised. In this configuration SKA's capabilities are impressive: in a single 12 hour run, between 100-1000 sources will be simultaneously detected and imaged with arcsecond, sub-arcsecond and milliarcsecond resolution.