Directly connecting the Very Long Baseline Array

At present, the signals received by the 10 antennas of the Very Long Baseline Array (VLBA) are recorded on instrumentation tapes. These tapes are then shipped from the antenna locations-distributed across the mainland USA, the US Virgin Islands, and Hawaii-to the processing center in Socorro, New Mexico. The Array operates today at a mean sustained data rate of 128 Mbps per antenna, but peak rates of 256 Mbps and 512 Mbps are also used. Transported tapes provide the cheapest method of attaining these bit rates. The present tape system derives from wideband recording techniques dating back to the late 1960s, and has been in use since the commissioning of the VLBA in 1993. It is in need of replacement on a time scale of a few years. Further, plans are being developed which would increase the required data rates to 1 Gbps in 5 years and 100 Gbps in 10 years. With the advent of higher performance networks, it should be possible to transmit the data directly to the processing center. However, achieving this connectivity is complicated by the remoteness of the antennas-"the last mile problem." In addition, it is not clear that the data rates involved can be obtained at a reasonable cost.