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--> 1 Introduction The Task In the National Defense Authorization Act for fiscal year 1994, the congressional committees that authorize the activities of the Department of Defense (DOD) requested that a joint study on the Global Positioning System (GPS) be conducted by the National Academy of Sciences and the National Academy of Public Administration (NAPA).1 The National Academy of Sciences was asked to recommend technical improvements and augmentations that could enhance military, civilian, and commercial use of the system. NAPA was asked to address GPS management and funding issues, including commercialization, governance, and international participation. Specifically, the National Academy of Sciences was asked to address the following three technical questions: (1) Based on presentations by the DOD and the intelligence community on threats, countermeasures, and safeguards, what are the implications of such security-related safeguards and countermeasures for the various classes of civilian GPS users and for future management of GPS? In addition, are the Selective Availability and Anti-Spoofing capabilities of the GPS system meeting their intended purpose? (2) What augmentations and technical improvements to the GPS itself are feasible and could enhance military, civilian, and commercial use of the system? (3) In order to preserve and promote U.S. industry leadership in this field, how can communication, navigation, and computing technology be integrated to support and enhance the utility of GPS in all transportation sectors, in 1 The National Academy of Sciences carries out its studies through the National Research Council (NRC), the operating arm of the National Academy of Sciences and the National Academy of Engineering, using a committee of experts and a small staff. NAPA utilizes its own staff members and consultants to conduct its studies, which are reviewed throughout the process by an oversight panel of distinguished individuals.
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--> scientific and engineering applications beyond transportation, and in other civilian applications identified by the study in the context of national security considerations? NAPA was asked to address the following four questions related to future GPS management and funding: (1) How should the GPS program be structured and managed to maximize its dual utility for civilian and military purposes? (2) How should the GPS program/infrastructure be funded to assure consistent, sustainable, and reliable services to civilian and military users around the world? In consideration of its worldwide user community, are there equitable cost-recovery mechanisms that may be implemented to make the GPS program partially or fully self-supporting without compromising U.S. security or international competitive interests? (3) Is commercialization or privatization of all or parts of the GPS consistent with U.S. security, safety, and economic interests? (4) Is international participation in the management, operation, and financing of GPS consistent with U.S. security and economic interests? Joint Study Approach Both the National Academy of Sciences and NAPA are chartered by Congress and conduct studies for the government on issues of national and international importance. National Academy of Sciences studies, which are carried out under the auspices of the National Research Council (NRC), are generally focused on scientific and engineering issues, and NAPA studies are generally focused on management issues. Because Congress was interested in a GPS study that covered both technical and management issues, a joint study was requested in the 1994 Defense Authorization Act. Because each academy operates differently, the NRC and NAPA portions of the study followed different schedules with different report-writing procedures. Nevertheless, the NRC and NAPA staffs worked closely together throughout the study, drafting joint outlines, exchanging information, attending both the NRC committee and NAPA panel meetings, and meeting frequently to work out details of the joint report. The NRC's peer-review process applied only to the portions of the joint report authored by the NRC committee. The NRC technical portion of the study began in June 1994 and entered peer review in February 1995. The NAPA management and funding portion began in August 1994 and entered review in March 1995. In May 1995, both reports were combined to form a single document, and the final joint report was delivered to Congress in May 1995.
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--> National Research Council Study Approach In mid-1994, the NRC formed the Committee on the Future of the Global Positioning System, hereafter referred to as the NRC committee, under the auspices of the Commission on Engineering and Technical Systems. (A membership list is included in the front of this report.) The NRC committee met June 23 through June 25, 1994; July 28 through July 30; August 16 through August 18; September 29 through October 1; October 19 through October 21; November 18; December 15; January 13, 1995; February 11; April 8; and April 17. During these meetings the committee heard over 70 briefings from government officials, industry representatives, commercial interest groups, and technical experts on GPS issues. A complete list of participants is given in Appendix A. Appendix B contains brief biographies for the committee members. In addition, several committee members visited on-site locations to gather additional information and further clarify important issues. At the first meeting, the NRC committee heard presentations from U.S. Air Force and U.S. Department of Transportation (DOT) representatives. The committee also familiarized itself with the history, management, operation, and technical components of GPS. The second committee meeting focused on the technical requirements of the various civilian and military users. Presentations were made by representatives in the aviation, maritime, transportation, agriculture, surveying and mapping, and scientific communities. Information also was provided by representatives of the precise timing and telecommunications communities. At the third meeting, the committee considered the GPS requirements of the U.S. military services and heard detailed presentations on the GPS space segment, ground control segment, and the user equipment segment. Presentations related to the Russian Global Orbiting Navigation Satellite System (GLONASS) and the U.S. GPS industry also were made at this meeting. The fourth meeting then focused on (1) the threats and vulnerabilities both to and from the use of GPS and (2) GPS jamming, spoofing, and interference issues. In October and November 1994, and again in January, February, and April 1995, the committee met to discuss its findings and recommendations. In addition, several committee members met on numerous occasions to work on the draft report. On April 17, the NRC committee and the NAPA panel held a joint meeting to finalize the combined report. Major Issues And Considerations Although the number of civilian users now exceeds the military users, GPS is a dual-use system that was originally designed to provide our forces with a military advantage. From the onset of this study, the NRC committee agreed that national security was the most critical issue in considering any recommendation in this report and that, without exception, the U.S. military advantage should be maintained. During the course of the study, the NRC committee examined various technologies and augmentations applicable to GPS. The NRC committee determined that several improvements could be made to the system that would enhance its use for civilian, commercial, and military users without compromising national
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--> 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.
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--> The official source of planning and policy information for each radionavigation service provided by the U.S. government, including GPS, is the Federal Radionavigation Plan.4 The plan is jointly developed by the DOD and the DOT, and is updated biennially. The Federal Radionavigation Plan represents an attempt to provide users with the optimal mix of federally provided radionavigation systems and reflects both the DOD's responsibility for national security and the DOTs responsibility for public safety and transportation economy. GPS Technical Overview The GPS constellation comprises 24 Earth-orbiting satellites, which transmit radio signals that consist of the satellite's position and the time it transmitted the signal. These signals can be received on Earth with a relatively inexpensive device that costs around $400 or so. The distance between a satellite and a receiver can be computed by subtracting the time that the signal left the satellite from the time that it arrives at the receiver. If the distance to four or more satellites is measured, then a three-dimensional position on Earth can be determined. GPS positioning capability is provided at no cost to civilian and commercial users worldwide at an accuracy level of 100 meters (2 drms).5 This accuracy level is known as the standard positioning service (SPS). The U.S. military and its allies, and a select number of other authorized users, receive a specified accuracy level of 16 meters (SEP), known as the precise positioning service (PPS).6 The full accuracy capability of GPS is denied to users of the SPS through a process known as Selective Availability, or SA. SA is the purposeful degradation in GPS navigation accuracy that is accomplished by intentionally varying the precise time of the clocks on board the satellites, which introduces errors into the GPS signal, and by providing incorrect orbital positioning data in the GPS navigation message. SA is normally set to a level that will provide 100-meter (2 drms) positioning accuracy to users of the SPS, as defined in the Federal Radionavigation Plan. PPS receivers with the appropriate encryption keys can eliminate the effects of SA. In practice, there are several additional sources of error other than SA that can affect the accuracy of a GPS-derived position. These include unintentional clock and ephemeris 4 U.S. Department of Transportation and U.S. Department of Defense, 1992 Federal Radionavigation Plan, DOT-VNTSC-RSPA-92-2/DOD 4650.5 (Springfield, Virginia: National Technical Information Service, January 1993). 5 SPS accuracy is normally represented using a horizontal 2 drms measurement, or twice the root mean square radial distance error. Normally, 2 drms can be represented graphically as a circle about the true position containing approximately 95 percent of the position determinations. The definition of 2 drms and other positioning accuracy definitions are discussed in greater detail in Appendix D. 6 SEP, or spherical error probable, represents an accuracy that is achievable 50 percent of the time in all three dimensions (latitude, longitude, and altitude). PPS accuracy is normally represented in this manner. The 2 drms PPS specified accuracy value is 21 meters SEP, as shown in Figure C-7 in Appendix C.
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--> errors, errors due to atmospheric delays, multipath errors, errors due to receiver noise, and errors due to poor satellite geometry. Each of these error sources is discussed in Appendix C. Even before the implementation of SA in 1990, many potential GPS users envisioned a need to improve the accuracy of the system, as well as some of its other specified characteristics. These other operational characteristics include integrity, availability, continuity of service, and resistance to radio frequency (RF) interference. These important concepts are defined and discussed in Appendix C. Many techniques and technical systems designed to improve the capabilities of the basic GPS have been proposed, are under development, or are already in operational use. These techniques range from the use of GPS in a differential mode, to software and hardware improvements for GPS user equipment, to the integration of GPS user equipment with other navigation/positioning systems. Examples of each of these major areas of GPS enhancement are discussed in Appendix C.
Representative terms from entire chapter: