One way in which the performance of the recording system is monitored is to read back some data from the tapes and check the playback quality. On the VLBA and Mark IV systems, this is done any time there is a gap of longer than about 2 minutes per tape (actually somewhat less). If a really bad head is found, it is possible to make a copy of the data from that head on one of the system tracks (recall that the headstacks have 36 heads). Some day, the correlator will be able to use this data, although not yet. On the VLBA, where there are two tape drives available, it may be possible to switch to the other drive to avoid the problem. It is in the interests of both the individual project and of overall system maintenance for occasional gaps to be inserted in all projects for readback tests to be done. These gaps are often inserted at the ends of passes when automatic tape allocation is not being used.
Readback tests on the VLBA systems cannot be done on both drives at once so, when using the 2 tape, 512 Mbps mode, a gap of a bit less than 4 minutes will be required to complete a set of tests.
SCHED will look for gaps long enough for readback tests. It will report the number for each station in the summary file. If there are less than one every 2 hours, it will complain, but go ahead and make schedules anyway.