All of projects on the VLBA that do not involve other antennas or special constraints are scheduled dynamically. That means that they are put into a special queue, along with information about their minimum requirements, and then are run at an appropriate time given the weather and condition of the equipment. This increases the overall quality of VLBA output by avoiding observations at times when it is clear that the results will be poor, even if it also introduces inefficiencies in scheduling that mean that there is some idle time (projects don't mesh together perfectly). For example, there is not much point in observing at 43 GHz when there is bad weather at many sites. But a 1.6 GHz observation at the same time might be fine. Dynamic scheduling also allows more flexible response to targets of opportunity. Galactic sources, in particular, tend to have short periods of enhanced activity so it is best to be able to observe when they are high. Of course, it is possible that some projects in the dynamic scheduling queue will never be scheduled.
The PI for a dynamically scheduled project will be given a window in LST at the VLBA_PT (Pie Town) that will be scheduled, if the observation is done. It is useful slotting projects together if the PI allows some flexibility in the actual start time. The PI should prepare a schedule using the LST parameter with Pie Town as an argument (LST=VLBA_PT).
When a program is selected for a dynamically scheduled time slot, VLBA operations personel will modify the date (calendar or lst day number) to match the scheduled day. This last minute modification is necessary because the stations do not have an LST concept and the machine readable files delivered by SCHED must be in UT. The files will then be loaded to the sites with instructions about the time window to use, which may well be a subset of what is actually in the schedules. Because of this last minute modification of the date, it is best to minimize the use of dates in the SCHED input. Use durations instead.
To provide flexibility in the start time, it is useful to schedule using DWELL to set the scan lengths. Then SCHED can adjust the gaps between scans to allow for the slew times that will be experienced by the actual observations. If doing an astrometric project with ``DELZN'' segments (geodetic type observing all over the sky to solve for the zenith atmospheric delay), it is useful to use the ability of SCHED to construct such segments automatically. Since such segments depend on observing sources near rise and set, they cannot be moved around, so without the automatic construction of the segments, the start time cannot be changed.