Scans have a nominal start time and a stop time that may be set explicitly by the user or derived by SCHED based on a variety of criteria. Scans have a separate start time, often offset from the nominal start time, that SCHED assigns for the start of recording. SCHED also has a reasonably good idea of when good data will start to be available after any required slews and setup times are completed. Throughout most of SCHED, the scan times presented to the user are the nominal start and stop times. On the other hand, the recording start time is what will be given in the files sent to the telescopes and correlators. The VEX file, which is becoming dominant as the telescope and correltor control file, uses the recorder start time as the scan time, but contains the offset from that to the good data start. Some systems, including the VLBA and VLA now use the good data start as the time to start recording or correlation. This section describes how all this is controlled using a number of SCHED input parameters.
There are a variety of ways to set the nominal times 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 nominal scan times are:
The calculation of the time required for an antenna to start delivering good data is based on on information in the station catalog. See the section on that catalog for details of the parmeters mentioned here. The slew time is calculated based on the pointing direction at the previous and next sources along with the slew rates and accelerations for the antenna. An additional time TSETTLE is added to the slew time for the antenna to settle and the electronics to get ready. Additional time might be added for digital systems to set power levels based on TLEVSET for the first time a setup is seen (VLBA and VLA). The total time to be ready will not be less than MINSETUP, even if the slew is fast and the settling time (TSETTLE) is short.
The slew time calculator tries to take into account the need for cable wraps. The algorithm is simply -- the shortest path to the next source will be taken even if that turns out not to be optimal later. Early VLBA experience suggested that a simple, predictable algorithm was preferred over a more optimal, but hard to predict, algorithm.
After the nominal times are determined, the recording start time is adjusted as described in the Adjusting the Scan or Recording Start Time section below using PRESTART, and MINPAUSE. Such adjustments were used to allow time for the recording media to get up to speed, formaters to reconfigure between scans with different setups, and for playback to synchronize on correlation. These were significant issues with the old tape systems, and to a lesser extent with the MARK5A system. The newer MARK5C recording system and DiFX correlators, and perhaps others, are able to record and correlate data beginning right at the assigned start time so such adjustments are not required.
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, such as DiFX (VLBA and many others).
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. Finding the LST day can be a bit painful, but a rough estimate can be had from the fact that LST day 60501 was 2006 Feb. 16. The estimate can be put in SCHED to narrow down to the right date. The second method is much easier and the scheme normally used. It 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.
When dynamic scheduling, it is often difficult to mesh projects together perfectly. This can lead to gaps in scientific observing which causes an under-utilization of the array. SCHED supports two features that are meant to help with this situation. The first is that the user can specify optional scans on the ends of the project by setting PREEMPT = EXTRA for those scans. The scans will appear in the summary file so their properties are clear, but will not be passed to the machine readable files used to control the antennas. To activate them, so they are on the control files, parameter DOSCANS can be used to select the scan range to be used. The second option applies to 24 hour schedules which could be started at times other than in the original schedule and wrapped around the day. Parameter WRAP24 causes SCHED to copy the scans onto the end of the original scans to make a 48 hour schedule. DOSCANS is then used to select the desired 24 hour block. It is expected that DOSCANS and WRAP24 will be used mainly by operations personel.