The nominal scan start time, if set using DWELL, is the time good data is expected to be available. If recording and correlation could start instantly, that would also be a good time to start the recorders. However in the real world, it can take a small amount of time to the the recording going at the stations and the correlation fully synced up. With previous generation correlators, this was a serious issue and it was advisable to start the recordings at least 20 seconds before good data. Modern (2010) correlators are faster so the total time needed for both starting recording and synchronizing is only a few seconds, or less. Another reason for possibly starting recordings early, or even recording continuously between scans is that the Mark5A disk system can only handle up to 1024 recording. A recording scan is the time between stops of the recording media. There might be several source scans in a recording scan. Fast-switching, phase-referencing observations could run into this issue so the default MINPAUSE (see below) has been set to prevent recording gaps during the fast-switching in most cases. This section describes the tools in SCHED to allow the recordings to be started early.
The schemes described in scan times section (parent of this section) are used to set the times of the scans as reported in the output files meant for human consumption. But the telescope control files actually give the times for the recording to start and stop. There are two primary parameters that can affect the recording start time (ignoring the obsolete PRESCAN). They are PRESTART, and MINPAUSE. PRESTART is used to request that the recording be started the requested amount of time (record time) before the scan start time. If that time is earlier than the previous stop time, the recorder will be left running.
The extreme, and often useful, case of a pre-start is to not stop recording between scans. This is especially useful if you have many short scans with short intervals between them, such as when phase referencing. MINPAUSE sets the smallest gap between scans for which the recording will be stopped. If the gap is smaller, the recorder will be left running. MINPAUSE used to be in units of playback time, so it was multiplied by the speed up factor to get the effect at record time. The speedup factor is no longer a simple concept so that adjustment has been remove and now MINPAUSE applies to the record time.
PRESTART is applied before MINPAUSE. First the recording start time is shifted earlier, then the interval from the last stop time is examined to determine if the recording should be left running. The defaults of PRESTART=5, MINPAUSE=10 should be ok in most situations (they are also in a state of flux as of the end of 2010, so it is possible they have changed since this was written). Users should not need to worry about these parameters most of the time. The offset of the recording start time from the scan start time can be displayed in the summary file by adding PTSTART to the arguments of SUMITEM. The recording start time is also available in the sch. files.
(Obsolete information kept for historical interest:) The time required to sync playback on the VLBA hardware correlator (no longer in use) was empirically about 8, 13, and 25 seconds for speedup factors 1, 2 and 4 respectively (ie, about 8 seconds in correlator wall clock time). These times were determined for tape. For some time, disks (Mark5a) were slower by about 10 seconds, but a quick check in Dec. 2007 suggests that the times are now about the same for disk. The sync time for disk at JIVE, Bonn, Haystack and USNO is around 1 second or less and can be ignored. The software correlator now in use on the VLBA and other instruments do not have a delayed sync so attempts to adjust the recording start time relative to the scan start time are no longer especially important.