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Résumé :
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Aging systems and systems operating longer than their anticipated life span, sometimes because of program slips in follow-on systems, have intensi ed the need for understanding how maintenance and sustainment affect the performance of space systems. In this monograph, we develop a pilot framework for analyzing these and related questions in the ground segment of the Global Positioning System and recommend steps for implementing this framework. In doing so, we address the issue of modeling approach and how to define appropriate metrics of performance. We develop the guidelines for metrics and analytic methods as generally as possible so that they will be useful for other space systems. Much of the spirit of the current metrics used to monitor the maintenance of the ground segments of space systems follows that of metrics used for aircraft. But, space systems have some attributes that differ significantly from those of aircraft systems, and these attributes suggest that the metrics for maintenance and sustainment for space systems be reconsidered. From a modeling perspective, the central difference is that space systems are highly integrated systems in near constant operation, not fleets of aircraft, any one of which can perform the specified mission. This difference leads to three challenges for the analyst. First, the logical metric used in the aircraft realm the fraction of the fleet that can perform the stated mission is not applicable in the space realm. Space command systems function as an integrated whole, and the whole must meet operational mission goals at all times. What is needed for space systems is either a measure or measures that reflect the overall system performance, even when the system is operating nominally. The metric should also be sensitive to sustainment perturbations.
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