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

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

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.

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.