Starting in October 2011, the VLBA will be 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 will be a geodetic style run of duration up to 1.5 hours using the Pie Town to Mauna Kea baseline. The observations will happen within about 4 hours of 18 hours UT, but will likely shift 12 hours from there after confidence is gained in the results.
These observations will cause some disruption to normally scheduled projects. The plan is to schedule fixed and dynamic projects in the normal manner. Then, if possible, the USNO observations will be inserted in gaps between other projects. If that is not possible, the USNO observations will preempt the normal project for the two stations involved. An effort to minimize the impact on the normal project will be 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 two allowed arguments -- 'NO' and 'OK'. The default, 'OK', means that the scan be preempted. 'NO' means that it should be protected. 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.