The following describes worst case performance load calculations for each Commanding function performed during a contact. Figure C-8 displays a summary of commanding loads for each Commanding subfunction.
Execute Command Plan - It is assumed that the worst case commanding load is 28.8 Kbps, and that a command word contains 30 bits. This translates into 960 words/second, which is a negligible processing load.
Constraint Check - It is also assumed that the worst case command loading would be a block command with each command step in the block having one separate command constraint check. In this unlikely, but worst-case, scenario, each command step in the block would require evaluation of one constraint check at the maximum commanding rate. Since the 28.8 Kbps rate translates into 960 commands/sec (see above), a maximum constraint check rate of 960 checks/sec is needed. It was determined from analysis that the maximum number of rules for a command constraint is approximately four. Based on these assumptions, worst-case constraint checking would require (960*4) 3840 rules/second. Based on benchmarks of leading real-time rule-based expert system shells , it was determined that such systems can support ~270 rules/sec/SPECint92, resulting in a total constraint check load of 14.22 SPECint92.
Transmit - This function is included in Execute Command Plan and is a negligible load.
Verification - It is also assumed that commanding functional verification will be performed using rule bases. Estimates for the number of rules per command evaluation are based on prototype systems developed at The Aerospace Corporation that perform functional verification using expert systems: ISTAR for IUS and SAGE for DMSP. Based on analysis of these systems, it is estimated that the maximum number of rules used for functional verification would be about ten. Since we assume ten verification rules/command and 960 commands/sec, we must support 9600 verification rules/sec. This equates to a verification load of 35.6 SPECint92.
Display - The display load is included in Telemetry as a combination of telemetry and commanding loads for a contact.