There are a variety of ways to set the time of a scan. A START time must always be set for the first scan of a schedule -- SCHED obviously has to know when to start. After that, the ways to set the scan times are:
The above schemes 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 additional parameters that can affect the recording start time. They are PRESTART, and MINPAUSE. You may want the recording start time to differ from the scan start time to make correlation proceed more robustly. If the recordings are started somewhat before the scan start time, the chances that they will be synced up on playback by the time of the scan start time are enhanced. PRESTART can be 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 time required to sync playback on the VLBA hardware correlator is 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. It is expected that the software correlator being developed for the VLBA (expected in 2008) and other instruments will not have a delayed sync, at which time attempts to adjust the recording start time relative to the scan start time can be abandoned.
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 is in units of playback time, so it is multiplied by the speed up factor to get the effect at 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=0, MINPAUSE=10 should be ok in most situations. 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.
A factor to consider when planning scan times is formatter reconfigurations. These happen when the internal switching and setup of the formatter at the station has to be changed. Such changes happen when any of a number of parameters, including the number of channels, the sample rate, the BBC assignments, the BBC sidebands, and the pulse cal detector setup, are modified. A formatter reconfiguration takes about 8 seconds on the VLBA and during that time the formatter is not sending valid time codes to the recorder. If this happens during recording, it knocks the correlator out of sync for both the duration of the reconfigure plus the time to resync. In practice, it also seems to confuse the VLBA correlator somehow for maybe one or two stations and they can take over a minute to resync. It is best to avoid reconfigurations if possible. In the rare cases where that is not possible, try to provide a gap between scans of sufficient length to do the reconfiguration. The formatter configuration requested during a gap is the same as that during the following scan, so this only requires using the GAP command (or any other mechanism for having one scan start a while after another ends). Common reasons that reconfigurations occur in schedules are changes in the sample rate, changes in the BBC sideband (remember for net upper sideband, the 20cm and 13cm systems on the VLBA use lower sideband at the BBC), and changes in the kHz part of the frequency which changes the pcal detector frequency (changes in the MHz part will not change the pcal setup and will not trigger a reconfigure).
Concerns about reconfigurations will likely change when the digital backends under development in 2008 are deployed.
The date specification for a scan is for the scan stop time, regardless of how the scan times are specified. If there is a scan that crosses midnight, this can cause some confusion, especially if it is the first scan of the experiment and the date is being specified along with START. If a schedule crosses a day boundary and START or STOP times are being specified, the new day should be specified. However, if midnight is crossed during any form of duration scheduling, the day will be incremented automatically.
All scans for a given station must be specified in time order. However, it is not necessary for scans for different stations to be in time order. This allows, for example, for the scans for one antenna to be specified separately from the scans for another antenna. While this works, it is not recommended bacause SCHED does not try to identify scans that can be correlated together. Anything that depends on knowing what the whole array is doing is likely to fail. Dwell time scheduling is one such item because SCHED must know how long the slews are for all antennas in a scan. Plotting of u-v coverage is another because SCHED only plots u-v points for baselines between antennas in the same scan. The estimates of data volumes and rates from the correlator, given in the summary file, are yet another because they depend on counts of baselines. Finally, any VEX file produced in such a way is likely to cause problems at a correlator that depends on it (not the VLBA correlator).
SCHED allows sidereal time scheduling by means of the LST parameter. For VLBI, the concept of LST needs a bit more specification since it is different for each element of the array. LST can take an argument which is the station whose LST is to be used. If there is no argument, that station is assumed to be the VLA since LST scheduling was most commonly used for VLA schedules when the capability was first added to SCHED. Now that dynamic scheduling is being used on the VLBA, many users will want to set LST=VLBA_PT to conform to the style of scheduling requested for such projects.
If LST is specified, there are two ways to set the date. With the original method, the year and month are ignored and the day is assumed to be the (modified?) julian sidereal day number. One way to determine what this number is to find a VLA schedule and read it off the side. Another is to make a dummy run of SCHED with the VLA as a station and look at the fifth line in the exptobs.y file where the day is specified as the ``Observation day''. For comparison, LST day 56789 was 1995 Dec. 12. Of course the LST day increments at a slightly different rate that the UT day. The second method is to specify the UT day in the usual manner. SCHED will attempt to figure out the LST day number, taking into account the fact that 0 hours LST is sometime in the middle of the UT day. It will also check if your start time is in the approximately 3.9 minutes where the result is ambiguous (LST days are shorter than UT days) and request specification of the LST day number -- giving you the options.
If sidereal time scheduling is requested, most times and durations are assumed to be in sidereal units. Some exceptions are PRESCAN and MINPAUSE.
It is a very good idea, when using LST scheduling, to check the final output schedules, which are all in UT, to be sure that they are for the right day.