4.6. Scheduling System

The GMT Operation Modes, as given by the Science Requirements [GMTO10], will consist of Queue, Investigator Directed, and Interrupt Observing. The SWCS will provide tools to support the planning and execution of these modes, described below. The requirements that flow down to the SWCS are shown here:

Table 4.4 SWCS Scheduling System Requirements

Title	Statement
Queue Mode	Support queue scheduling.
Program Execution Planning	Provide observatory staff software for advanced planning of observing programs.
Observation Status Tracking	Provide tools for advanced planning of science programs, and capabilities to monitor the status of observing programs.

The observing modes determine how science programs are scheduled for long-term implementation (night to night, month to month), as well as how the targets are prioritized during a night:

• Investigator Directed – In this mode, the science investigators are present on-site or at a remote observing location to conduct observations. The duration of the program is based on the number of nights used rather than the completeness of the program.

  For Investigator Directed programs, the observatory staff puts together a long-term schedule taking into account preferences expressed by the PIs in their observing proposals. The staff also tries to optimize scheduling based on parameters that can be known long in advance, such as proposal ranking, moon phase, instrument availability, and the largest window of opportunity based upon the RA and DEC ranges of the targets. Once the long- term schedule is set, it generally remains fixed throughout an observing semester. In this mode, the Scheduling System does not perform scheduling automatically. Rather, it provides observatory staff advice, via tools to aggregate program information and statistics, sort programs according to certain criteria (see below), and produce visualizations to facilitate scheduling. During runtime, the observers (PI or Co-PI) have full discretion over how to prioritize target scheduling. If desired, observers can also use queue scheduling tools to advise them on other sequences of observing, factoring in variables that they may wish to consider.

• Queue – In queue mode, observatory staff members conduct the science observations in place of the program PIs and Co-PIs. All queue programs enter into a central pool to be rank-ordered for scheduling. The scheduling is usually based on individual observations rather than entire programs at once, unless the PI requires that the observations be grouped. In contrast to Investigator Directed mode, long term queue scheduling can change throughout an entire observing cycle because the order of observations responds to runtime changes in order to optimize the use of existing observing conditions. The scheduling of the targets is based on a number of parameters, including science ranking, seeing condition, sky brightness, percentage of completion, and instrument availability, etc. (see below). The importance of each parameter is partly based on how closely a runtime condition matches the desired condition, e.g., the closer the seeing matches the specified value, the higher the priority. However, observations often have to account for programmatic variables, e.g., overhead cost, cable wrap, program completeness, decreasing or increasing window of opportunity, etc., requiring one to optimize on factors besides the best observing conditions. Therefore each parameter also has a weight attached so that the importance of one parameter may be adjusted relative to other parameters. The effect of changing parameter weights is to instantaneously rearrange the observing schedule to prioritize targets whose observing requirements best match both demand conditions and extenuating circumstances. Several graphical tools are being designed to aid in visualization and reweighting of the observing criteria.

• Interrupt – The Interrupt Mode is used to observe targets of opportunity, which may require observers to react quickly to acquire a target that may not already exist in the normal observing schedule database. Interrupt Mode is usually invoked and granted by an observer on a per-target basis, rather than for multiple targets or to usurp a block of time, after which the telescope returns to the original program in progress. Because of the unpredictability of the Interrupt Mode, no long range scheduling is possible. The primary effect on short-term schedules is to delay the execution of targets in Queue and Investigator Directed observing.

In short, the purpose of the Scheduling System is to help optimize the scheduling of the observations so as to:

• Maximize the amount of time available to use for on-source integrations.
• Properly match target selection with observing conditions and instruments during queue mode.
• Ensure as many science programs in the queue are successfully executed as possible.
• Ensure proper balance of observing times between partner institutions.
• Minimize the amount of overhead times used for slewing between distant targets, required instrument switches, etc.

4.6.1. Ranking Criteria for Observing Program and Runtime Target Selection

In the early phases of telescope commissioning and operations, the Scheduling System of GMT for all the operational modes will be an advisory system rather than an intelligent scheduler like ones used with HST, VLT, or Gemini. An intelligent scheduler typically uses an optimization algorithm (e.g., simulated annealing, metropolis, genetic) to find a plausible “global-maximum” solution according to some merit function, by comparing the required observing criteria (proposal ranking, overhead times, seeing, sky brightness, etc.) with some existing/known conditions. Each criterion has a weight, a range or dispersion, or a hard limit, according to the importance of meeting that criterion.

For long term scheduling, known conditions are parameters such as moon phases, observing window and airmass. During runtime, however, optimizing a schedule involves responding to conditions during the night. A truly intelligent algorithm therefore has to be able to respond to on- the-fly scheduling demands (due to changing conditions), balancing them with longer-term criteria. While such an algorithm has been used successfully in several observatories, the largest benefit of doing so comes after gaining considerable knowledge about the operational “personality” of the system so as to provide rigorous assignment of weights.

In contrast, an advisory system merely informs the operator about target and program scheduling, while permitting the operator to adjust the weighting scheme to respond to run-time observing conditions, thus changing the schedule on-the-fly. For instance, it may sometimes be more efficient to select a target closest to the current cluster of observations from different programs, while sacrificing image quality because doing so still meets the science requirements. The software may not easily capture such an intention especially if the software does not accurately model certain parameters, e.g., the overhead times and other contingencies, and they turn out to be overriding considerations. However, doing so is possible when an operator has a larger and more nuanced control over the telescope operation.

The most dynamic approach is to allow for both possibilities, with the advisory system eventually providing weights to inform the intelligent scheduler, while also permitting advisory capability during runtime. During the early phases of the operations, a system will be implemented that only advises the operator about target and program scheduling, allowing him/her to adjust the weights of observing parameters, via a visualization interface, accordingly. As the operational concept evolves, and with better understanding of the system, it would be feasible to seamlessly enrich the Scheduling System by adding an automated scheduler as a component in the OPS while retaining the advisory capability.

The following criteria will be used to sort the priority of the observing programs and targets to schedule, both for long term and for runtime:

• Science program ranking – Sort observations by program ranking, which usually comes from reviews by the telescope Time Allocation Committee.
• Priority rating – Priority rating can be assigned to targets of opportunity, director’s discretion, high priority targets, time critical observations, or based on instrument availability, etc. Priority rating is generally under the discretion of observatory staff and director, who may consider unforeseen factors that, may override normal scheduling.
• Percentage completion – For large programs, weigh the observing schedule to favor programs based on percentage of completion, e.g., number of targets or fields observed relative to the total.
• S/N achieved versus required – Queue observations sometimes involve making multiple passes over the same target, and over non-contiguous times, in order to achieve the required S/N. This criterion assigns weights according to S/N achieved versus required.
• Institutional time share – Allow the scheduling system to enforce fair share of observing time among all involved institutions.
• Airmass – Observations may be restricted according to airmass criteria.
• Hour angle restrictions – The hour angle of observation may be restricted due to wind direction, cloud coverage, or cable wrapping considerations.
• West-East arrangement – Higher or lower weights may be given to targets that are rising or setting.
• Meridian distance – Give more weight to targets that are closest to transiting the meridian, so as to be observed at the lowest airmass.
• Integration time duration – Sort targets based on the duration of the integration time. The integration time is for the smallest useful unit of observation that is defined by the observer.
• Total time of observation – Sort observations based on the total time, including integration, and overheads. Overheads include additional slew time, calibration requirements, AO requirements, etc.
• Seeing requirement – Limit and sort observations based on the seeing required by the science program. For some science programs that use AO, it may be more useful to place a requirement on the Strehl. During Phase I proposal preparation, the PSF simulator module, in conjunction with the observation planning tool, will estimate the required seeing to achieve the required Strehl, taking into account the location and brightnesses of the natural and laser guide stars, relative to the science target.
• Sky brightness, IR emissivity – Restrict targets based on sky brightness criteria.
• Instrument availability – Limit observations based on instrument requirements.
• Photometric vs. non-photometric conditions – Limit observations based on requirements of photometric or non-photometric conditions, which is a binary decision.
• Proximity of target to current position – Sort observations according to proximity to current telescope position so as to minimize slew time and the settling time of the active optics.
• Program coherence – Select targets according to how important it is for the targets to be observed closely in time vs. distributed over the entire semester.

4.6.2. Subsystem Description

The core Scheduling System components will depend on the following components:

• Observing Program Server – The GMT Observing Program Server facilitates the development of the observing proposals (Phase I and Phase II), validation, submission and acceptance of the proposals into the observatory database. It also manages the distribution of proposals to the TAC for review, collects comments from the panel, and releases them to the science PIs and Co-PIs. In Phase II, astronomers submit detailed proposals with information on targets, guide stars, instrument configuration, observing visibility window, etc., and then forward the information to the observatory database. For proposals that are granted time by the TAC, the manager helps scientists to fully define the observation requirements: configuring instruments, selecting guide stars, positioning of science apertures, creating observing blocks and sequences, etc. Upon validation, all the information will be forwarded to the GMT data archive for scheduling and implementation.
• Schedule Ranking Server – Given a set of criteria and weights, such as moon phase, wind direction, observing window, airmass, seeing, etc., the schedule ranking server performs rank-ordering of observing programs. The importance of each criterion is given by numerical weights. Medium-term and long-term programs usually have fixed weights whereas short-term observations typically have weights that are adjusted on-the-fly depending on changing observing conditions. The ranking informs a human scheduler who may choose to accept the ranking, modify it, or ignore it when implementing the actual schedule short term, medium term, or long term. Usually schedules for classical observing and engineering are fixed once assigned. In contrast, the window of queue observing period may be fixed by observability, but the runtime schedule is highly dynamic and may be changed depending on variable conditions like weather, wind, instrument readiness, target-of-opportunity, interrupts, etc.
• Long Term Server – The long term scheduling server manages schedules on time scales longer than a single proposal cycle. Multi-cycle and some director discretion times often fall into the regime of long term scheduling.
• Medium Term Server – The medium term scheduling server manages schedules on time scales longer than a single week, but shorter than a proposal cycle. Factors that this server may consider are moon phases, weather forecast (to the extent possible), engineering time, queue observing blocks, long term program blocks, multi-cycle blocks, and director discretion. Single cycle programs work, though not exclusively, in this regime.
• Short Term Server – The short term scheduling server manages schedules of observations on-the-fly to lengths of a week. Queue scheduling falls under this regime. However, observers of Investigator Directed observing programs may also use the same server to receive advice for runtime decisions.
• Calendar Panel – A calendar is used to schedule and visualize medium and long term observations. An example is shown in Figure 4.12. The display allows managers to visualize according to categories of program ID, instrument request, observers, or institutions. Selecting a date would bring up a night’s observation schedule and target information as known at that time.
• Nightly Schedule Panel – Several versions of nightly schedule panels will likely be made available, depending on the desired information to sort on or visualize. An example of a night schedule panel is shown in Figure 4.13, Figure 4.14 and Figure 4.15. In these depictions the interest is to arrange targets according to their airmass constraints. The schedule can be rearranged on the fly interactively by dragging and dropping the bands. Gaps in between the bands will take into account estimated overhead time for slewing from one target to another, while the bands include overhead times estimated for an observing configuration. Users can “drill down” to finer grain details (programs -> observing blocks -> individual exposure times, etc.) by selecting or zooming in on the visualization.

Fig. 4.12 Long-Term Calendar Mockup

Fig. 4.13 Target Observation Order. Global scheduling of the nightly observations is shown. The greyed-out region represents the past. Selecting a target band brings up details for that observing block. Gaps in between observations include estimated overheads required to change from one observation to the next. View can be zoomed in, out, or panned.

Fig. 4.14 Target Observation Order Showing Airmass Tracks. The schedule can be rearranged by moving the thick bands relative to one another. Additional tracks may be selected and added to the viewer.

Fig. 4.15 Magnified View of a Target Track. One can select target tracks to magnify and show the individual observing blocks. Observing blocks may be selected and modified via an editor spawned from this view, or they may be deleted from the observing sequence.