National Academies Press: OpenBook

The Global Positioning System: A Shared National Asset (1995)

Chapter: GPS Signal Characteristics and Operational Concepts

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Suggested Citation:"GPS Signal Characteristics and Operational Concepts." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 155
Suggested Citation:"GPS Signal Characteristics and Operational Concepts." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 156
Suggested Citation:"GPS Signal Characteristics and Operational Concepts." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 157

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APPENDIX C 155 computer software converts it into information that can be usefully displayed for a user, such as position coordinates, or input to another type of user equipment, such as an inertial navigation system.21 Although the functions of a current GPS receiver are the same as those present in user equipment tested in the 1970s, they have little else in common. The size and cost of user equipment has decreased dramatically, while capabilities and the size of the commercial market continue to increase. In 1993, the total value of the GPS user equipment market was estimated to be $420 million, with over 100 companies marketing GPS receivers.22 U.S. manufacturers maintain a competitive advantage over their Japanese counterparts, who are currently the principal competitors. However, the advantage could easily be lost. Larger U.S. companies, like Trimble Navigation, Ltd., invest as much as $25 million per year in GPS research to maintain their technological advantage. At the present time, U.S. domestic sales per unit represent less than 50 percent of the worldwide GPS market, and 45 percent of U.S. industry sales are to overseas markets.23 GPS Signal Characteristics and Operational Concepts The GPS relies on the principle of "pseudoranging" to provide accurate positioning to its users. Each satellite in orbit continuously transmits a radio signal with a unique code, called a pseudorandom noise (PRN) code, that includes data about the satellite's position and the exact time that the coded transmission was initiated, as kept by the satellites' onboard atomic clocks. A pseudorange measurement is created by measuring the distance between a user's receiver and a satellite by subtracting the time the signal was sent by the satellite from the time it is received by the user.24 Once three ranges (or distances) from three known positions are measured, a position in all three dimensions can be determined. In the case of GPS, however, a fourth satellite is generally needed in order to eliminate a common bias in the pseudoranges to all satellites caused by a lack of synchronization between the satellite and receiver clocks. Once this clock bias is eliminated by the presence of a fourth signal, a highly accurate three-dimensional position can be determined. Figure C-5 below further illustrates the GPS pseudoranging concept. 21 The coordinate reference system utilized by most GPS receivers is the World Geodetic System 1984 (WGS 84). WGS 84 is the fourth global geocentric coordinate system developed by the DOD since 1960. 22 These estimates have been provided by the U.S. GPS Industry Council. 23 Response from Trimble Navigation Limited, Sunnyvale, California, 13 September 1994. 24 This measurement is also affected by signal delay caused by the Earth's atmosphere, as will be discussed later in this appendix.

APPENDIX C 156 Figure C-5 Pseudorange concept. (Courtesy of the Aerospace Corporation) Instead of transmitting one PRN code on one radio signal as described above, each satellite actually transmits two distinct spread spectrum signals that contain two different PRN codes, called the Coarse Acquisition (C/A) code and the Precision (P) code. The C/Acode is broadcast on the L-band carrier signal known as L1, which is centered at 1575.42 MHz. The P-code is broadcast on the L1 carrier in phase quadrature with the C/A carrier

APPENDIX C 157 and on a second carrier frequency designated as L2, that is centered at 1227.60 MHz. Figure C-6 illustrates the characteristics of both the L1 and the L2 signals. Figure C-6 Characteristics of the L1 and the L2 signals. (Courtesy of the Aerospace Corporation) The L1 C/A-code provides free positioning and timing information to civilian users all over the world, and is known as the Standard Positioning Service (SPS). The timing information on the C/A-code is also used by some receivers to aid the acquisition of the more accurate P-code. The P-code is normally encrypted using National Security Agency cryptographic techniques, and decryption capability is available only to the military and other authorized users as determined by the DOD. When encrypted, the P-code is normally referred to as the Y-code. The encryption process utilized, known as Anti-Spoofing (A-S), denies unauthorized access to the Y-code, and also significantly improves a receiver's ability to resist locking onto mimicked GPS signals, which could potentially provide incorrect

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The Global Positioning System (GPS) is a satellite-based navigation system that was originally designed for the U.S. military. However, the number of civilian GPS users now exceeds the military users, and many commercial markets have emerged. This book identifies technical improvements that would enhance military, civilian, and commercial use of the GPS. Several technical improvements are recommended that could be made to enhance the overall system performance.

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