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INTRODUCTION 16 security. Some of the improvements could be made immediately, and others could be incorporated on some of the Block IIR spacecraft that are currently being built and included in the specification requirements for the next generation Block IIF spacecraft. REPORT ORGANIZATION In making recommendations, the NRC committee considered the requirements of various civilian and military users. These requirements are discussed in detail in Chapter 2. Recommendations that enhance the basic GPS for all users and recommendations that enhance the basic GPS for specific user groups are presented in Chapter 3. Chapter 4 examines possible enhancements that have particular merit for future incorporation, but which require further study. Data and analyses supporting the NRC committee's recommendations and a more detailed technical overview of GPS and its augmentations and enhancements are compiled in the appendices. Specifically, Appendix C provides a detailed technical and programmatic overview of GPS. GPS PROGRAM OVERVIEW The incomparable navigation, positioning, and timing system that is known today as the GPS, is a combination of several satellite navigation systems and concepts developed by or for the U.S. DOD. In 1973, the best characteristics of each of these programs were combined under the auspices of a Joint Program Office (JPO) located at the U.S. Air Force Space and Missile Organization in El Segundo, California. From its inception, the NAVSTAR Global Positioning System was designed to meet the radionavigation requirements of all the military services, and those of civilian users as well.2 Responsibility for the day-to-day management of the GPS program and operation of the system continues to rest with the DOD and is carried out primarily by the Air Force.3 DOD policy for the GPS program is set by the Under Secretary of Defense for Acquisition and Technology, with the help of the DOD Positioning/Navigation Executive Committee. This committee receives input from all the DOD commands, departments, and agencies and coordinates with the DOT Positioning/Navigation Executive Committee and the Assistant Secretary for Transportation Policy. Together, the two secretaries mentioned above make-up the GPS Executive Board. 2 The GPS system is officially known as the NAVSTAR Global Positioning System; however, the NAVSTAR name is rarely used. For the remainder of this report the system will simply be referred to as GPS. 3 As with all other federally funded navigation systems, the ultimate decision-making authority over GPS operations, in peacetime and in wartime, is the National Command Authority (NCA), consisting of the President or the Secretary of Defense with the approval of the President.