Automatic tape allocation is provided on the VLBA. In fact, its use is required for most observations. With automatic tape allocation, the only thing the user needs or can worry about is the total tape consumption. It much be kept below that allocated for the experiment. There are two reasons for the use of automatic tape allocation. The first is to relieve the user of all the worry about tape handling. It is no longer necessary to schedule everything in units of passes. The second is to decouple the tape change times from the times allocated to each project. This allows the tape changes at the stations to be done on a fixed schedule that is the same every week and which fits well with the work schedule for the site techs. This does cause tapes to contain data from more than one project, but the bookkeeping is in place to insure that they don't get erased until all the projects are correlated. Recall that the VLBA stations are unmanned most of the time. The second reason is why the use of automatic allocation is required.
Automatic tape allocation is not available for all stations, including all that use VEX files. SCHED knows which stations can use it and schedules accordingly. For global observations to be correlated on the VLBA correlator, the scheduler needs to worry about tape handling at the stations without auto allocation, but can ignore it for the ones that have it. For any observations to be correlated on other correlators (including JIVE), automatic tape allocation cannot be used and SCHED will not allow it.
Two levels of automatic tape handling are available on the VLBA. They are invoked using the parameter AUTOTAPE. The lower level allows the on-line system to decide where on the tape to start the next scan. However if the scan runs out of tape going in the current direction, the recorder will stop and the rest of the scan will be lost. After the first forward pass, any scan starting less than 450 feet from the end of the tape will be started in the opposite direction. The first forward pass always runs to the low tape sense to determine the tape length. It is the author's opinion that, because of the variable length of tapes, projects scheduled with this scheme are very likely to exhibit unexpected behavior with possible considerable loss of data. It should work ok for long scans that intentionally try to drive well beyond the end of the tape, but this is almost worse than worrying about specifying the tape behavior at schedule time. This level of automation is not recommended.
The higher level of automation is the method of choice for tape scheduling. It would be even better if the speed of synchronization on the correlator were improved, but that project doesn't seem to be getting any priority. With this mode, whenever the tape gets to an end, it stops and reverses. The minimum time lost is the time required to reverse the tape at the correlator, which is about 5 seconds times the speed up factor. However some time to resync after the reversal is required and the actual time is more like 10 seconds times the speed up factor and somewhat variable. For most experiments the data loss should not be a significant problem. Scans of arbitrary length can be scheduled. The user only need to worry about not exceeding his/her total tape allotment. The total amount of tape used in a schedule is reported in the summary file. Note that the same behavior, where scans starting within 450 feet of the expected end of tape after the first pass will start in the opposite direction, also applies with this level of automation. 450 feet is 67.5 seconds at 80ips -- the speed that gives a speed up factor of 2 on the VLBA and is used by a large fraction of observations.
When automatic tape handling is requested, as is required for most VLBA projects, it is futile to worry about coordinating scans with tape passes. Don't bother to try. There are two reasons for this. First, the tape position at the start of the project will depend on where it was left by the previous project. It will not necessarily be at an end. Second, the autoallocation software uses the real length of the tape as measured on the first pass. Tapes vary in length by several hundred feet depending on their history of abuse. So you cannot know ahead of time where a scan will be on the tape so there is no point in trying to schedule passes unless some antennas in the array are not using autoallocation.
If a station that is using automatic tape allocation is scheduled in a scan on a source that is below the antenna hardware limits (not just the horizon), that antenna will be removed from the scan by SCHED. This will not happen when or where automatic allocation is not in use because the tape management could be messed up. This action is provided mainly to help the Mauna Kea site technicians who have a very long drive to change tapes and whose antenna is often scheduled in scans for which the source has not yet risen. Mark5 stations that are not using tape will also be scheduled in this manner. This behavior can be overridden using DODOWN