Plan Execution Interchange Language (plexil) for Executable Plans and Command Sequences

Space mission operations require flexible, efficient and reliable plan execution. In typical operations command sequences (which are a simple subset of general executable plans) are generated on the ground, either manually or with assistance from automated planning, and sent to the spacecraft. For more advanced operations more expressive executable plans may be used; the plans might also be generated automatically on board the spacecraft. In all these cases, the executable plans are received by a software system that executes the plan. This software system, often called an executive, must ensure that the execution of the commands and response of the fault protection system conforms to pre-planned behaviour. This paper presents a language, called PLEXIL, that is designed specifically for flexible and reliable command execution. It is designed to be portable, lightweight, predictable, and verifiable, and at the same time it is more expressive than command sequencing languages currently used on space missions.