These are high speed longitudinal instrumentation tape recorders that use 1 inch wide tape on reels 14 inches in diameter. The headstack contains 36 heads, 32 for data and 4 for system information, cross-track parity, or duplicate data (if, for example, a head dies). The headstack can be moved under site computer control transverse to the tape motion. The heads are much narrower than the spacing between heads, so multiple pass recording can be used with 14 passes, or more if not all heads are used in each pass.
As many as 32 data tracks can be written to 1 tape drive, with a record rate per track of 8, 4, or 2 Mbps. This can result in an aggregate bit rate of as much as 256 Mbps for 1 tape drive. A doubling of this aggregate bit rate will be possible once appropriate software is available. However, operational constraints require that a ``sustainable'' limit of 128 Mbps (averaged over 24 hours) be imposed on the aggregate bit rate. This can be achieved either by recording at 128 Mbps or by arranging that the duty cycle (ratio of recording time to total allocated time) be less than unity. A thin (17600-foot) VLBA tape lasts 10 h 16 m if recorded continuously at 128 Mbps. Rare, particularly meritorious projects may request exemption from the sustainable bit rate limit.