Interactive Dynamic Mission Scheduling for ASCA
--by Alex Antunes & Fumiaki Nagase, ISAS
AbstractThe ASCA mission requires scheduling for each 6-month observation phase, further broken down into weekly schedules at a few minutes resolution. Two tools, SPIKE and NEEDLE, written in Lisp and C, use artificial intelligence (AI) techniques combined with a graphical user interface permit the fast creation and alteration of mission schedules. These programs consider viewing and satellite attitude constraints as well as observer-requested criteria and present an optimized set of solutions for review by the planner. Six-month schedules at one-day resolution are created for an oversubscribed set of targets by the SPIKE software, originally written for HST and presently being adapted for EUVE, XTE and AXAF. The NEEDLE code creates weekly schedules at one-minute resolution using in-house orbital routines and creates output for processing by the command generation software. Schedule creation on both the long- and short-term scale is rapid less than one day for long-term, and one hour for short-term.
ConstraintsTypical constraints involved include:
- Target Visibility
- Solar Angle (between Solar Paddles and Sun)
- Moon Contamination in FOV
- Angle to Bright Earth Limb
- SAA Passages
- Star Tracker limits with respect to the Sun, Moon, and Bright Earth
- Contact Pass Availability and Telemetry Requirements
- Time Critical and Phase Critical Observation Windows
- Telemetry Limits, KSC Contact Passes and DSN Downlink
These multiple levels of constraint are handled by the SPIKE long-term scheduler. The constraints are combined via a function called Suitability, representing the overall preference of an observation for any given time. The automatic scheduling strategies for SPIKE work directly with this Suitability Function to create efficient schedules. As the process involves both rule-based and neural network approaches, multiple runs should be done, and a final solution chosen by the operator. Editing and customizing the final schedule is simple due to the graphical user interface (GUI).
TOO and Benchmarks(Just How Fast Are We?)
A complete long-term schedule of 6-7 months, including 258 targets with 16 time-critical targets (as was the case for the first AO), takes approximately six hours to prepare detailed orbit files, and one day to schedule. Computer time to make a schedule is on the order of minutes, but time must be taken by the planning staff to choose and adjust the details of the schedules created.
To create a typical one-week schedule (each target is generally one or one-half day in duration) takes approximately one hour, again dominated by human evaluation time, with computer calculation time taking less than five minutes.
Targets of OpportunityASCA is able to schedule a TOO (Target of Opportunity), such as SN1993J, very quickly. Schedule alteration time is approximately one hour, and subsequent command load preparation requires less than one day.
DP10DP10 is a set of routines that include a graphic front end and PostScript output. It is used to plot the conditions of a single target for 15 orbits (just over one day), including visibility, bright earth, SAA, contact pass, DSN downlink and altitude information, geomagnetic rigidity, and star tracker information. The "touban" (Duty Scientist) uses DP10 plots as a reference when adjustments to the plan are required.
Summary of Planning and SchedulingPlanning begins with a list of accepted proposals, generally oversubscribing the amount of time available by 25%. This list is loaded from the Observation Data Base (ODB) into SPIKE and run until a viable schedule is found. The schedule then is reviewed and adjustments are made as needed. The resulting Long Term Schedule (LTS) is loaded into the ODB and also made publically available. Each week, a Short Term Schedule (STS) is made using NEEDLE (which gets its input from SPIKE), and this information is loaded into the ODB and also sent directly to the attitude and command planning team. When TOO or schedule variations occur, the LTS also is updated, while keeping time-critical targets as they are, and reloaded into the ODB.
During the PV phase of the half year after the launch of ASCA in February of 1993, this scheduling system (SPIKE-NEEDLE) has worked satisfactorily. The system has shown good flexibility when faced with unexpected events such as transfer to safe hold mode, inserting of TOO observations, and the cancellation of observations due to large typhoons damaging the Kagoshima Space Center. We expect similar success with the first GO phase, already in progress.
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