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Disposition Letter and Scheduling Constraints

by Justin Linford last modified Aug 17, 2020

Interpreting the Disposition Letter

Once the proposal review process is complete, a disposition letter is sent to the principal investigator and co-investigators of each VLBA proposal. This letter contains comments from the cognizant Science Review Panel (SRP), a linear-rank score from the SRP, comments from the NRAO Technical Reviewer and, optionally, comments from the Time Allocation Committee (TAC). Guided by the linear-rank score and taking into account the time available, the TAC assigns a scheduling priority to each session in the proposal. If time is allocated at a scheduling priority of A, B, or C, the proposal is converted to a project which is eligible to compete for time in the dynamic queue.

The VLBA disposition letter is arranged in six portions:

Synopsis This portion lists the proposal id, title, type, authors, cognizant Science Review Panel (SRP), and time allocation summary. The latter tells the proposers whether or not time was allocated to the proposal. If time was allocated, its scheduling priority (A, B, or C) is given and the proposers are responsible for preparing schedules for that time.

Science Review This portion gives the linear-rank score from the SRP, along with comments from the SRP that summarize the proposal, give its strengths and weaknesses and, optionally, note any technical issues. The community-based SRP consists of a chair and five anonymous members. The linear-rank score is on a scale from 0 (a high-ranked proposal) to 10 (a low-ranked proposal).

Comments from the TAC The TAC consists of the chairs of the eight SRPs. Taking into account the time available as a function of Greenwich Sidereal Time (GST), the TAC assigns a scheduling priority to each session in each proposal. The TAC might comment on these assignments. The assigned scheduling priority depends on the linear-rank score of the proposal, the GST ranges involved in the session, the total time requested in the session, and the competition from better-ranked proposals requesting time at similar GST ranges.

Comments from the NRAO These describe the possible scheduling priorities, which are:

  • A = the observations will almost certainly be scheduled

  • B = the observations will be scheduled on a best effort basis

  • C = the observations will be scheduled as filler

  • N*= the observations will not be scheduled because they were explicitly rejected by the TAC

  • N = the observations will not be scheduled because they could not fit in the time available

  • H = not assigned because the proposal is being held for consideration at a future TAC meeting

Time Allocation For each session in the proposal, a table lists the session's name, time in hours, GST range, and scheduling priority. For a given observing semester, time approved at a scheduling priority of A, B, or C may be divided into multiple schedules as appropriate. Factors to consider in this division include the scheduling priorities, the observing frequencies, the tabulated GST ranges for the sessions, and the GST pressure plot at the start of the observing semester.

Further information for proposers and observers, including statistics and important GST pressure plots, is available from the most recent TAC report that is referenced in the disposition letter.

Comments from the NRAO Technical Reviewer Comments concerning the Technical Justification portion of the proposal. These comments are available to the SRP and the TAC.

Scheduling Constraints

Roughly 50% of the operational time on the VLBA is available for “open skies” observing. The other 50% is allocated for use by the US Naval Observatory (USNO). However, the amount of time dedicated to USNO observations varies from month to month.

The VLBA makes daily observations to measure the Earth orientation parameter UT1-UTC for the USNO. These observations are centered between 1730 and 1930 UT, are at most 2 hours long, and currently use only two antennas (usually MK and PT). The UT1-UTc observation may temporarily remove the necessary antennas from an ongoing science observation. The VLBA Schedulers always attempt to minimize the impact of these daily observations on science projects.

Interpreting the VLBA Pressure Plot

The figure below is an example of a VLBA pressure plot, which shows the number of VLBA observing sessions requested for each 1-hour bin in GST. The black line with squares is 52% of the wall-clock time available (after removing time for maintenance and sponsored time). The approximate Pie Town (PT) Local Sidereal Time (LST) is shown on the top horizontal axis.

The colors represent the scheduling priority assigned by the TAC, with the following meaning:

Green = pre-committed time (time allocated via partner observatories such as HST, Chandra, and Global VLBI; time already allocated from previous proposals requesting observations across multiple semesters)
Dark Blue = priority A projects (will almost certainly be observed)
Light Blue = priority B projects (will be scheduled on a best effort basis)
Yellow = priority C projects (will be scheduled as filler)
Red = priority N/N* projects (will not be scheduled)

It is obvious that some GST ranges are heavily over-subscribed, while others have relatively low pressure.

Priority and Schedule Duration

The duration of an individual schedule cannot exceed the time allocated to the project. Beyond this basic fact, the optimal duration of a schedule depends on the scheduling priority (A, B, or C) assigned to the project. Observers are encouraged to inspect the current month’s VLBA schedule, which is posted on the Schedsoc Home Page, for information about how much VLBA time is available in the near future.

Priority A: Dynamic scheduling enables priority A schedules to observe at the first available time. Schedules of any duration are fine. However, there may be other considerations in determining the best schedule duration. The nature of the observation may dictate specific times that are better for a project. For example, observations at high frequencies may require good weather conditions at certain stations.

Priority B: The amount of priority B time approved is designed to almost fit into the available hours per semester. However, if the science goals can be achieved with multiple short observations, it is often advantageous to submit schedules with durations of 6 hours or shorter. This allows these observations to be made in between longer, and possibly higher ranked, observations. Also, as noted above, keeping high frequency observations relatively short will make them easier to fit into times with good weather.

Priority C: These are filler projects. Short duration schedules (4 hours or shorter) are generally easier to slot in, but long schedules can be observed if there is sufficient open time on the telescope.

Tips for More Flexible Schedules

This section is most relevant for projects which have been assigned priority C, but they are also useful for priority B projects which have requested time during high-pressure GST ranges. Some of these tips may also be useful when preparing schedules for priority A projects, particularly those at high frequencies and tight weather constraints.

More flexibility will increase the chance that a particular schedule gets observed. However, if the conditions are too flexible or if the overhead becomes so large that the data cannot be calibrated or yields too little time or u-v coverage on the science target, the data set may not achieve the anticipated science goal. Always ensure that the data set can be calibrated. If there are options to consider, it may be better to choose the option that leans toward a more conservative calibration rather than the option that accrues more observing time on the science target. More conservative calibration will also ensure smoother data reduction.

Tips on how to increase the chance that a schedule will be observed:

  • Submit the schedule early. If the schedule is not available when a scheduling gap occurs, it cannot be selected for observation. Gaps can occur at any time, including at the planned start of the semester.

  • Submit a short schedule. Short gaps occur more often than long gaps. Make schedules short and repeat them as many times as desired to accumulate observing time.

  • Submit schedules with various durations. It is permissible to submit schedules that, in aggregate, exceed the total time allocated to a project. Use this to your advantage by submitting schedules with various durations. For example, if a scheduling gap of 4 hours is available then compete for it with an schedule of duration 4 hours instead of an schedule with a duration of 2 hours. That shorter schedule can be used to compete for a later scheduling gap of 2 hours. Also, as the pressure from priority A and B projects drops a few months into a semester, scheduling gaps can sometimes accommodate priority C schedules of relatively long duration.

  • Break up high and low frequency observations. There is more low frequency time than high frequency time, so it is usually harder to get observed if the weather limits are very low. If it does not matter if low and high frequencies are observed at the same time it is often advantageous to split observations into low and high frequency schedules.

  • Relax the weather limits for the schedule. For example, if the science target is strong enough for self-calibration then the project can be observed in less-than-optimal weather conditions.

  • Widen the possible start GST range(s) of the schedule. Especially at low frequencies, consider observing down to the elevation limit of the antennas.

  • Request fewer stations. If the science goal can be achieved with 6 or 7 stations rather than 8 or 9, that can help get a schedule observed when stations are down due to weather, maintenance, or other issues.

Schedule Life cycle

Project Creation

After the approval of a proposal, the observers are responsible for creating valid schedules to achieve their science goals. About one month prior to the start of an observing semester, NRAO will create the project associated with a successful proposal. The observers may then begin submitting schedules to be observed once the semester begins.

Schedule Creation and Submission

The creation of a schedule is the responsibility of the observer(s). All VLBA schedules must be submitted as SCHED keyin files. Observers should download and install the newest version of SCHED to avoid any issues that may arise from using a different version than the VLBA Data Analysts and Operators. Observers are encouraged to review the SCHED user manual before creating their schedules. NRAO staff are available to answer questions and help solve issues via the NRAO helpdesk.

Once a schedule is created, observers should submit the keyin via email to . The VLBA Data Analysts will verify that the keyin file can be used by SCHED to produce the necessary VLBA station control files. Verified keyin files will then be inserted into the queue for scheduling.

Schedule Observation

Schedules are observed according to priority, station availability, weather conditions, and other considerations. See the Dynamic Scheduling Guide or more information about how observations are scheduled.

Once a schedule has been observed, the observers will receive email notification and an operator’s log for that observation. Any issues with the observation should be noted in the log. If the observers are concerned about any log entries, they should contact the as soon as possible. If the observation is determined to be a failure, it will be re-observed.

Correlation and Data Archiving

The VLBA records observation data on disk packs at each station. The packs are kept at the station until they are full, and then they are shipped to Socorro, NM for correlation.

Once a project has been correlated, the observers will receive an email notifying them that the data are available in the NRAO Data Archive. Observers can then download their data (or request it be shipped on disks, if necessary) and begin the calibration process.

Re-Observation of a FAILED Projects

Occasionally, an observation will fail. This can be due to poor weather at several stations, mechanical failure at a station deemed necessary to the observation, extreme radio frequency interference, or several other possible factors. If a project is in the General Observing category, it will be re-observed when conditions allow. Those projects in the Shared Risk and Resident Shared Risk categories will not be re-observed if they fail.