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C-2. Commanding Assumptions

Several assumptions were made relative to the implementation of the Commanding function. It was assumed that a rule-based system will be used to perform command constraint checking, and command functional verification. This assumption is based on the desire and success of rule-based approaches in several leading SV commanding COTS products. For example, Intelligent Mission Toolkit (IMT), Spacecraft Command Language (SCL), and RTWorks each provide a rule-based approach for constraint checking, functional verification, and anomaly diagnosis.

Estimates were made on the number of rules needed to perform command constraint checking and command functional verification. For command constraint checks, estimates of the number of rules needed per constraint check were generated based on analysis of existing implementations, such as constraint rules for the MISTY spacecraft, developed using IMT at the Air Force Center For Research (CERES).

It was assumed that the worst case commanding load would be a block command with each command step in the block requiring a separate constraint check in real time. In the absolute worst case, each command step would require evaluation of a constraint check in real time. This condition is highly improbable, but represents the worst case.

Estimates for the number of rules per command for functional verification were based on two prototype systems developed at The Aerospace Corporation. The systems used for this evaluation were the Intelligent System for Telemetry Analysis in Real Time (ISTAR), developed for the Inertial Upper Stage (IUS) program, and the Satellite Ground Experiment (SAGE), developed for the Defense Meteorological Satellite Program (DMSP). Based on analysis of these systems, it was estimated that the maximum number of rules used for functional verification would be approximately ten per command.

It was assumed that the maximum commanding rate would be 28.8 Kbps, and that the command word length would be 30 bits. This translates to 960 commands per second. At the low end, we assume a typical commanding rate of 1 Kbps with a command word length of 30 bits, resulting in 33.3 commands per second.