PREEMPT is used to allow, or not, scans to be preempted for other purposes. There are two cases. PREEMPT = EXTRA tells /schedb and VLBA operations to treat the scans as extras that can be used to help fill gaps between projects that often occur with dynamic scheduling. PREEMPT = NO tells operations not to use the Pie Town and Mauna Kea antennas at the time of the scan for daily USNO Earth Orientation Parameter (EOP) observations. PREEMPT = OK means that the time of the scan can be used for the EOP observations on PT and MK.
The PREEMPT = EXTRA option should only be used for a block of scans at the beginning of the project and another at the end of the project. These scans can, and normally will, extend outside the time assigned by the Time Allocation Committee. The scans that do not have PREEMPT = EXTRA are considered to be the ``core'' and should stay within the time allocation. If SCHED finds any ``EXTRA'' scans in the ``core'', it will complain and stop. The EXTRA scans should be designed to enhance the goals of the project but should not be critical to it as they may well not get observed. Also, they should not be used to do what is effectively a different project.
Scans that have PREEMPT = EXTRA will not be written to the output files other than the summary file unless they are included in the range of scans specified with DOSCANS. The files affected are the .vex, .oms, crd.xx, sch.xx, and .flag files. These include all the antenna control files, so as far as the telescopes are concerned, these scans do not exist unless explicitly included in DOSCANS. Generally DOSCANS should only be used by VLBA schedulers.
Starting in October 2011, the VLBA has been used to provide daily Earth Orientation Parameter (EOP) observations for the U.S. Naval Observatory in return for financial support for VLBA operations. These observations are a geodetic style run of duration up to 1.5 hours using the Pie Town to Mauna Kea baseline. The observations were happening within about 4 hours of 18 hours UT. In early June 2013, they were shifted to within about 4 hours of 7 UT, during the night.
These observations cause some disruption to normally scheduled projects. The fixed and dynamic projects are scheduled in the normal manner. Then, if possible, the USNO observations are inserted in gaps between other projects. If that is not possible, the USNO observations preempt the normal project for the two stations involved. An effort to minimize the impact on the normal project is made. Parameter PREEMPT allows the PI to help with that minimization by specifying portions of the project that can be preempted and portions that should be protected. For example, the parameter can be used to protect key calibration observations.
PREEMPT can be specified for each scan and has three allowed arguments -- 'EXTRA', 'NO' and 'OK'. The default, 'OK', means that the scan be preempted and is a ``core'' scan. 'NO' means that it should be protected from preemption for the EOP observations. 'EXTRA' means that this is an extra scan at the beginning or end. 'EXTRA' implies 'OK' as far as preemption for EOP observations is concerned. The parameter need only be specified when it changes.
SCHED will examine the PREEMPT specifications and identify time periods of at least 1.7 hours when all scans allow preemption. Those times are listed in the .sum file and the new .preempt file. Shorter periods that include the start and/or end of the project are also listed as those would allow a partial overlap with the USNO observations.
If there is a period of more than 4 hours between the 
hour
preemptable periods, SCHED will complain. If such periods are left
in the schedule, the user requests for scan protection may be ignored
when chosing when to insert the USNO observations. If the whole project
is less than 4 hours long and is all protected, a warning will be
issued, but preemption would probably only occur if the project is
adjacent to a higher priority project that also cannot be preempted.
If PREEMPT is not specified for any scans, but the project uses automatic generation of geodetic segments, the geodetic segments will have PREEMPT default to 'NO'. This helps avoid problems for legacy schedules.