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INTRODUCTION 15 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