Apr 14, 2007
As space systems age or operate beyond their planned life spans, the U.S. Air Force Space Command (AFSPC) needs to understand how budgeting for the maintenance and sustainment of the ground segments under its purview affects the performance of their associated space systems. Current metrics for monitoring the performance of the systems' ground segments need to be reexamined so that the links between budgetary investments in maintenance and sustainment and overall space system operational performance can be quantified. The RAND Corporation's Project AIR FORCE (PAF) developed a framework for analysis that predicts how well a space system will perform its operational mission, given various levels of maintenance resources allocated. The framework includes these key features:
This top-down approach places the components of the systems' ground segments in context and indicates which components are most problematic to the overall system and therefore deserving of the highest level of attention in failure and repair data-collection and analysis. This framework is in contrast to models that analyze the failure rates of each of the many components in the ground segments, along with the time required to restore function. The latter approach alone does not capture the full behavior of the system or predict the full cost of ensuring consistent and accurate service to users.
To illustrate this framework, PAF focused on the Air Force's Global Positioning System (GPS), a satellite-based system that provides accurate location and timing data for military and civilian users. For GPS, the appropriate measure of operational performance is how well the system provides location and timing data. Specifically, the analysis explored the metric of how much variance there is over time in the location and time estimates the system provides to users. PAF examined the effect of the reliability of one subsystem of the GPS ground segment, the ground antennas, on that performance metric. The antennas send updates from the ground monitoring stations and Master Control Station to the system's satellites and are thus critical for maintaining the accuracy of the system. PAF determined predictive curves for how the mean time between critical failures and mean time to restore function of the GPS ground antennas quantitatively determine the accuracy of a user's GPS location estimate.
Next steps would be to expand the work to the remaining subsystems of the GPS ground segment and to link the PAF pilot framework with ongoing component-level work at AFSPC. The result should be a complete, predictive model of GPS that reveals how dollars allocated in the budget affect overall performance of the system.