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: http://www.atnf.csiro.au/people/Tasso.Tzioumis/sched/Adjusting_Scan_or_Recording.html
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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. The more modern systems, especially the Mark5C recorders and the software correlators, approach this ideal and the recordings are started at the time specified in the VEX file for the start of good data. That time is calculated based on the expected slew rates and on any extra time, specified using the TSETTLE, MINSETUP and TLEVSET parameters in the station catalog. The start can also be postponed with GAP or PRESCAN. With fixed scheduling (DUR, START, etc), the nominal scan starts are forced by SCHED , but the time of good data, if later than the scheduled scan start, is still set by the actual expected slews and additions and that is believed by the Mark5C control software and the correlator. Parameters MINPAUSE and PRESTART, discussed below have no actual effect if the derived start time is before the expected start of good data.
With older systems, including the MARK5A systems that are still in use at least part time at the beginning of 2013, it can take a small amount of time to get the the recording going at the stations and the correlation fully synced up. With previous generation correlators and tape recorders, 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 scans. 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 with the older systems could run into this issue so the default MINPAUSE (see below) has been set to prevent recording gaps during the fast-switching in appropriate cases. This section describes the tools in SCHED to allow the recordings to be started early.
The schemes described in the 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. For the legacy (Mark5A) systems, those times are in the crd.xx files for VLBA systems and as the uncommented scan-wide start time in the $SCHED section of the VEX file. For the RDBE (VLBA etc) and WIDAR (VLA) systems, it is the data good time in the VEX file, shown for each telescope as an offset from the start time.
There are two primary parameters that can affect the recording start time used for the legacy systems. 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 removed and now MINPAUSE applies to the record time.
The default value for PRESTART is 5 seconds when the DAR is the legacy VLBA system or the MKIV system. Otherwise it is zero, which will be the case with the modern digital systems. The default value for MINPAUSE is 10 seconds when using the Field System or the legacy VLBA DAR. Otherwise it is zero which is the case for the new VLBA digital system. The Field System with the DBBC may be switched to zero in the future.
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.
Another way to shift the recording start time is to use the parameter PRESCAN. This parameter was the original way of introducing gaps between scans, but was considered obsolete in recent years. However, it may have a use as a way of increasing the time allowed between scans beyond what would be given with the station parameter TSETTLE, although a negative PRESTART can do the same thing. PRESCAN shifts the start time in either direction (positive shifts to later times) after the other parameters discussed above set the time. If a positive PRESCAN is given, and the DURation or DWELL is increased by the same amount, the whole scan can be shifted farther from the period of the slew, allowing somewhat increased confidence that the recording will start with good data.