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GPS APPLICATIONS AND REQUIREMENTS 52 GPS TIMING AND TELECOMMUNICATIONS APPLICATIONS Because the pseudoranging method used by GPS to establish three-dimensional position locations requires a highly accurate time standard, the system is ideally suited for applications that require precision timing and precise time transfer. GPS pseudorange measurements are based on the transit time of a signal from the GPS satellite to the user. Thus, if the locations of both the satellite and the observer are known, the difference in the user-clock offset from that of the satellite can be readily determined. Furthermore, if the satellite clock is referenced to a standard such as Universal Coordinated Time (UTC), as is the case with GPS, the observer can then determine user-clock offset from UTC.40 Current and Future Applications and Requirements The time-transfer community was one of the first to realize benefits from GPS, since a full satellite constellation is not required for most time-transfer methods. In fact, the most accurate method of time transfer to date, known as GPS common-view, relies on the ability of two users on the globe to observe the same GPS satellite simultaneously, despite a large geographic separation. GPS common-view is currently used by the 55 international timing centers that are charged with the task of maintaining International Atomic Time (TAI) and UTC throughout the world.41 A chain of common-view observations also is used to link the widely separated sites that are part of the National Aeronautics and Space Administation's (NASA) Deep Space Network.42 Other time-transfer methods that utilize a single GPS satellite, as well as methods that require observations from multiple satellites, are used for a number of scientific research activities that require precise time synchronization of equipment located in different laboratories.43 40 UTC is often referred to as Greenwich Mean Time because it refers to the time of day in Greenwich, England (U.K.). 41 The official international timing center in the United States is the National Institute of Standards and Technology (NIST) Metrology Laboratory in Boulder, Colorado. This facility, along with 53 others, keep time relative to the master facility at the Bureau International des Poids et Measures (BIPM) in France. 42 The Deep Space Network (DSN) consists of three tracking stations located near Barstow, California; Canberra, Australia; and Madrid, Spain. These stations receive telemetry data from deep space missions such as Galileo, and send commands that control spacecraft navigation and operation. The three tracking stations are monitored by the DSN's control center at the NASA Jet Propulsion Laboratory in Pasadena, California. 43 Single satellite time-transfer methods in addition to common-view include GPS direct and clock flyover. Methods using multiple satellites include Enhanced GPS, GPS used as Very Long Baseline Interferometry (VLBI), and Geodetic Positioning Time Transfer. For more information on these methods see: David Allen, Jack Kusters, and Robin Giffard, ''Civil GPS Timing Applications," in Proceedings of ION GPS-94: 7th International Technical Meeting of the Satellite Division of the Institute of Navigation (Salt Lake City, Utah, September 1994), pp. 25-32.
GPS APPLICATIONS AND REQUIREMENTS 53 GPS is also increasingly utilized by many telecommunications companies to synchronize their land-based digital telecommunications networks.44 Most often, these users compare a reference clock directly to GPS time by viewing one or more satellites, rather than transferring time from one reference clock to another. AT&T, in particular, now uses GPS to maintain time synchronization throughout its long distance telephone system,45 and an international digital telecommunications system that uses a GPS-based timing system began operating in Moscow in 1991.46 As synchronous fiber optic networks such as SONETs increase in size and complexity, GPS time synchronization may replace the more common practice of using land lines to disseminate timing information from a small number of land-based clocks.47 The "Stratum n" performance level hierarchy, developed by the American National Standards Institute (ANSI) T1 Committee on Network Synchronization Methods and Interfaces, specifies the requirement for synchronization. At the present, the one to four Stratum performance levels (with one being the most stringent) could be satisfied by the long-term frequency stability available from the GPS standard positioning service.48 The ANSI T1 requirements are listed in Table 2-9. Precise GPS timing also has the potential to significantly improve mobile cellular communications. Currently most cellular telephone networks are subject to transmission degradation as a call is transferred from one cell's channel to another, but if all of a network's cells used the same channel, this problem would be eliminated. This can be accomplished by providing each cell with a unique code rather than a unique frequency using a technique known as Code Division Multiple Access (CDMA).49 Major CDMA manufacturers have recognized GPS as an effective way to provide the precise time synchronization required by their systems.50 Timing accuracies similar to those required for digital networks are sufficient for this application. 44 Information presented in this section on the use of GPS by the telecommunications industry, unless annotated otherwise, is based on the following report: Eric A. Bobinsky, GPS and Global Telecommunications: A Summary Briefing Prepared for the National Research Council Committee on the Future of the Global Positioning System (Washington, D.C., 29 July 1994). 45 E. Krochmalny, "GPS Synchronizes the Lines," GPS World, May 1992, p. 39. 46 M. J. Toolin, "GPS in a Russian Telecommunications Network," GPS World, June 1992, pp. 28-34. 47 SONETs, or Synchronized Optical NETworks, were originally proposed by Bellcore, and are now becoming the worldwide standard format for optical transmissions. The term "synchronous" highlights the fact that a SONET is aligned in time with respect to a common timing source. 48 There are currently no ANSI T1 "Stratum n" requirements for absolute timing accuracy. The absolute timing accuracy specification for the GPS SPS is 340 nanoseconds relative to UTC. 49 Code Division Multiple Access is the same technique that allows a GPS receiver to distinguish one satellite from another despite the fact that they all use the same frequency. 50 U. H. Werner, "Improving Mobile Communications with GPS," GPS World, May 1993, pp. 40-43.
GPS APPLICATIONS AND REQUIREMENTS 54 Cellular signals are also subject to the local conditions in each cell that may vary from cell to cell, such as weather or landform geometry. By putting GPS positioning capability in the mobile receiver and by transmitting the position information to the mobile control and operations center of the mobile system, the network control operations could determine user location and travel direction. With this information available, the network controller can provide optimal hand over as well as real-time dynamic performance optimization for each location. A typical communications cell ranges from a few tens of meters to over a hundred square kilometers, so a positioning accuracy of a few hundred meters will suffice. When dealing with small, oddly shaped cells, however, or when trying to map signal and propagation characteristics within a complex area such as an "urban canyon," accuracy on the order of a few meters in three dimensions may be required. These general values for positioning accuracy have not yet been defined as requirements, and therefore are not included in Table 2-9. In the future, many information services may require "time-of-day" information to a much higher degree of accuracy than is typical of today's services. Examples include universal personal communications services and broadband integrated services digital networks which may require a high degree of time-of-day precision in order to interface with several different types of communications systems to transmit tremendous amounts of digitally packeted information.51 Timing accuracies of 100 to 300 nanoseconds relative to UTC will likely be required for these services. Table 2-9 Timing and Telecommunications Requirementsa Application Accuracyb Reliabilityc Time Frequency Common-View Time Transfer NASA Deep Space Network 1 ns 1 x 10-15 Not specified BIPM for TAI and UTC 1 ns 1 x 10-14 Not specified International Timing Centers 0.1-1 ns 1 x 10-14 Not specified NIST Global Time Service 10 ns 1 x 10-14 Not specified Time Power Industry 10 ns Not available High Synchronization ANSI T1 Stratum 1 Not specified 1 x 10-11 High Time-of-Day Services 100-300 ms Not specified High a. Source of requirements for common-view time transfer and power industry time synchronization: David Allen, Jack Kusters, and Robin Giffard, "Civil GPS Timing Applications," p. 28. Source of time-of-day requirement: Eric A. Bobinsky, "GPS and Global Telecommunications." ANSI Stratum 1 requirements provided by Mr. Bruce M. Penrod of True Time, Santa Rosa, CA. 51 Iridium, Orbcomm, Globalstar and other proposed low-Earth orbit (LEO) satellite communications systems are all examples of UPC services. Broadband integrated services digital networks, are digital telephone lines capable of transmitting data, voice, graphics, and video information at a rate much faster than modems.