This book explores the significant challenges in sustaining and upgrading The Global Positioning System (GPS). The GPS provides positioning, navigation, and timing data to users world-wide, and has become essential to U.S. national security and a key tool in an expanding array of public service and commercial applications at home and abroad. GPS is integrated into nearly every facet of U.S. military operations, and the number of civil users is increasing. Other countries are now developing their own independent global navigation satellite systems that could offer capabilities that are comparable, if not superior to GPS. The U.S. government, which plans to invest more than $5.8 billion from 2009 through 2013 in the GPS space and ground control segments currently under development, provides GPS service free of charge. The Department of Defense (DoD) develops and operates GPS, and an interdepartmental committee manages the U.S. space-based positioning, navigation and timing infrastructure, which includes GPS. This book looks at the global economic and national security importance of GPS, the ongoing GPS modernisation effort and the international efforts to develop new systems.