National Academies Press: OpenBook

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

Chapter: Time Synchronization

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Suggested Citation:"Time Synchronization." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
Page 55

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GPS APPLICATIONS AND REQUIREMENTS 55 b. The timing accuracies listed include both time relative to UTC in nanoseconds (ns) or milliseconds (ms), and long term frequency stability measured over one day, except for the ANSI Stratum 1 long term frequency stability, which is measured over any time interval greater than 1000 seconds. c. For commercial time synchronization applications, "reliability" corresponds to overall system reliability as explained in this section, not the continuity of service requirement applicable to GPS aviation applications. Challenges to Full GPS Utilization Time Transfer For GPS time-transfer applications, the challenge of mitigating the effects of SA's clock dithering in order to improve accuracy appears to have been met. Methods to filter SA-induced noise have allowed time transfers to occur using C/A-code receivers, which achieve accuracies of better than 1 nanosecond relative to UTC, and long-term frequency stabilities of better than 1 x 10-14.52 Laboratories responsible for the world's primary time standards, such as NISTs metrology laboratory in Boulder, Colorado, are hoping to conduct time transfers with this type of accuracy on a routine basis. These accuracies are required in order to maintain standards that are two orders of magnitude better than the timing accuracies required by industry. If errors from SA are removed and ionospheric errors are minimized by using dual-frequency receivers, clock and ephemeris errors become dominant. Improvements to the GPS space and ground control segments will be required in order to reduce these errors.53 Time Synchronization For the telecommunications industry, requirements such as integrity and availability fall under the general category of overall system reliability. Communications and information service providers will not rely on any technical system that does not guarantee them the ability to satisfy the needs of their customers on a continuous 24 hour-a-day basis. Many potential users of GPS in the telecommunications industry feel that GPS, as currently configured, cannot provide this level of reliability. As with many other GPS applications, the absence of SPS integrity monitoring is unacceptable to many in the telecommunications industry. These potential users have expressed a desire to have GPS performance monitoring data available to them in real time in order to feel comfortable with its reliability. 52 David Allan, Jack Kusters, and Robin Giffard, "Civil GPS Timing Applications," pp. 26-27. 53 Candidate improvements are discussed in Chapter 4.

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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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