Emphasizing conflict resolution versus conflict avoidance during schedule generation

Abstract

Abstract Classical planning systems attempt to solve a planning problem by avoiding possible conflicts before the actions are put on a timeline. This is computationally very expensive and the search for all possible future conflicts may be prohibitive. A conflict resolution approach can check for immediate conflicts and try conflict resolution strategies as each activity is put on a timeline without regard for possible future conflicts. A more practical approach is to use a combination of conflict avoidance and conflict resolution based upon heuristics which limit the amount of search required when either is used. Because humans are not good at solving problems which require complex lookahead, this combined approach, with emphasis on conflict resolution, is what human schedulers actually use when they develop schedules. A system which simulates this human approach to scheduling has been developed at NASA's Goddard Space Flight Center for scheduling satellite activities. This system, which includes the Planning And Resource Reasoning (PARR) shell, allows expert schedulers to specify conflict resolution strategies as well as conflict avoidance strategies to be used during the scheduling process. PARR has been used since May 1987 to schedule the Tracking and Data Relay Satellite System services for the Earth Radiation Budget Satellite. PARR will also be used to schedule platform resources on the Explorer Platform, scheduled for launch in early 1992 . This paper describes the advantages of using a combined conflict avoidance and resolution approach in a satellite scheduling system.