Outlines criteria for analyzing how sustainment investments affect the operational performance of space systems, focusing on the Global Positioning System; describes a pilot model for such analysis; and examines how model results might be used in policy analysis. Outlines criteria for analyzing how sustainment investments affect the operational performance of space systems, focusing on the Global Positioning System. Outlines criteria for analyzing how sustainment investments affect the operational performance of space systems, focusing on the Global Positioning System.