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INTRODUCTION 17 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.
INTRODUCTION 18 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.