Distributed Expert Systems for Ground and Space Applications

The workstation, minicomputer, and microcomputer marketplaces have been revolutionized in the past decade by systems that are both open and distributed. As a leader in this revolution, the Naval Research Laboratory’s (NRL) Naval Center for Space Technology (NCST), has been employing reusable software components to build a series of test beds, test and checkout systems for satellite assembly line operations, and distributed control of satellite tracking stations. The Navy has taken this one step further by unifying ground and space operations with the development of the Spacecraft Command Language (SCL). SCL is a hybrid software environment borrowing from expert system technology, fifth generation language development, and multitasking operating system environments. SCL was developed by the Navy to be the controlling software for their Advanced Systems Controller (ASC). The ASC is a MIL-STD-1750A based Telemetry, Tracking, and Control (TT&C) controller for a new generation of Navy spacecraft having the capability of autonomous operation for up to 180 days. Today’s spacecraft are becoming increasingly more complex, with added sensors, higher data rates, and more capable standalone and distributed processors. The SCL system allows on-board processing of data, which has traditionally been considered to be in the realm of the ground segment. The distribution of processing to the space segment allows the spacecraft controller to analyze data points onboard and make decisions based on knowledge stored in the SCL scripts and rules. In addition, the spacecraft bus and payload systems are commonly developed independently, each having their own processors/controllers. Using a common distributed control language results in significant savings in total software development. The space-based SCL system can support distributed environments using a hierarchical scheme allowing subsystem controllers to communicate with a central controller. A distributed approach is also used with the ground segment. Data points downlinked from the spacecraft are routed to workstations that analyze and view spacecraft and artificial telemetry points in real time. The workstation’s knowledge base is used to analyze the telemetry and adjust the spacecraft’s high level tasking to maintain the mission profile. To unify the space and ground segments, the NCST has chosen the SCL system as the standard for use on-board the spacecraft as well as in the ground stations. The SCL system will run on a central ground station computer as well as on individual workstations used for subsystem monitoring and control. The SCL Real-Time Executive (RTE) on-board the spacecraft will be monitoring health and welfare, processing telemetry, scheduling mission tasking, and managing payload configuration changes. Connectivity between multiple SCL nodes is not limited to exchanges of database items. A