A fact of life with VLBI has been that the data are recorded on tapes and tapes have various limitations. Tapes are not infinitely long and they cannot be stopped and started instantaneously. Whenever they are stopped, they must be resynchronized at the correlator, which takes a few to a few tens of seconds. Also tapes have finite bandwidth and capacity. The instantaneous bandwidth cannot be exceeded. This section provides some detail on various aspects of tape management.
Many of the concerns described here do not apply to the disk based recording systems (mainly Mark5) that began to be deployed in 2003. The disk systems can start and stop very quickly and take less time to be synchronized on the correlators. There still are limitations on the total bit rate and recording volume. But disk management is be much less visible to the user than tape management. For the rest of this section, we are talking about tapes. Hopefully by late 2005, this whole section can be relegated to a historical appendix to this manual.
The scheduler always has to worry about not exceeding the amount of tape that has been allocated for the project. For projects only including stations that use automatic tape allocation (eg VLBA, VLA, and GBT), that should be the only concern other than providing occasional gaps for readback tests, as described later, and not stopping the tape too often. Automatic tape handling is described in more detail below in the section on automatic tape allocation and in the description of the AUTOTAPE input parameter. If there are any stations at which automatic tape handling is not being used, the scheduler has a lot more to worry about. Tape reversals, without automatic tape handling, can only occur at scan boundaries. For efficient tape usage, the schedule must have scan breaks at appropriate intervals given the tape type and recording speed. More is said about this below, especially in the section on tape lengths.
The schedules sent to the antennas will also have to specify such details as track assignments, head positions etc unless automatic tape allocation is used. The user, however, should not need to worry about these details since SCHED has defaults that work fine in all but odd test observations. For the masochistic, the details of the SCHED defaults are given in Appendix B.4.
The maximum bit rate that can be recorded on a VLBA tape is 256 Mbps. The systems at the VLBA antennas and at the VLA and Green Bank have two drives and can use both simultaneously to achieve 512 Mbps. Mark IV systems can have 2 head stacks on a single drive to achieve 512 Mbps. In addition, they can record at 16 Mbps per track, twice the normal maximum rate. With both heads recording at 16 Mbps per track, the Mark IV systems can reach 1024 Mbps.
When part of an project uses the two drive (VLBA) or two head (Mark IV) mode, all of that project will be recorded in that mode. If some scans use narrower bandwidth, the tape drives will be slowed. This avoids lots of confusion in keeping track of tape usage.