There are a number of concerns related to tape handling at scan boundaries. SCHED provides a variety of ways to specify what is to be done. Between scans, tapes can be kept running or can be stopped. If they are stopped, they can be started at the nominal start time, or at some other time, usually earlier. The actions taken can strongly influence how well correlation proceeds. Whenever a tape is stopped, there is some time lost to resynchronize on the correlator. Also, with short segments of tape motions, it is possible for the VLBA correlator to wind up sufficiently far out of sync that it does not recover until either the end of the pass or the end of the correlator job. Basically, if the schedule consists of long scans, it doesn't really matter much what is done. But if it has many short scans, such as when phase referencing, it can be highly advantageous to keep the tape moving between scans.
Obviously, tapes must be stopped at the ends of passes so that their direction of recording can change. They also must stop when they are changed. If there are long gaps (many minutes) between scans, tape use efficiency is enhanced if the tapes are stopped. If automatic tape handling is being used (normally true at the VLBA and VLA, but not elsewhere), tape stoppage at pass and tape ends will happen automatically and the user need not worry about them. The user should, however, provide occasional tape stoppages of at least 2 minutes (4 min in VLBA 512 Mbps mode) to allow for readback tests. These need not be at the ends of passes.
As the gap between scans gets shorter, or the scans become more frequent, it becomes advantageous to keep the tapes rolling between scans, assuming that the correlator does not require that they stop (the older Mark III system had such a requirement). One way to do this is to use DURation rather than DWELL to specify scan times and not give any command such as GAP that will cause an interval to be scheduled between scans. This simply butts the scans together. If there are gaps between the scans, such as when DWELL or GAP are used, the default behavior of SCHED will automatically keep the tape running if the gap is less than 8 seconds times the speed up factor (usually 2). That interval can be adjusted with the parameter MINPAUSE.
One can request, preferably with parameter PRESTART, that recordings be started before the nominal start of a scan. This can cause the recording medium to keep moving if the gap is short. For longer gaps, it can help optimize the amount of correlator output by providing time for the correlator to synchronize.