Applications of Time Transfer Using NAVSTAR GPS

The Navstar Global Positioning System (GPS) allows extremely accurate and global determination of time, as well as position and velocity, and it does it economically. Without even using the precise GPS codes and dual frequency measurements, the transfer of time with GPS can be accomplished to better than 100 nanoseconds. In fact, with a simultaneous common view approach, relative timing can be accomplished to better than 10 nanoseconds. Although a widespread need for this type of accuracy may not be apparent, one would be surprised at the number of applications for that accuracy surface once that its availability has been established. Probably more important than the accuracy capability is the ease by which time transfer can be accomplished with GPS, both technically and logistically. GPS time transfer systems are more portable than precise clocks primarily because batteries are not required to keep the clock running. Time with GPS can be reinitialized at any time. Economically, because of the technology involved, the current cost of precise clocks (Cesium) is on a steady rise, while GPS time transfer systems will become less expensive as time passes, enhancing the advantage over the use of precise clocks. Currently, GPS is not an operational system. For time transfer it is, but not continuously, available (only available on the order of 16 to 20 hours a day). However, as the number of satellites increases, the time transfer will become continuous long before GPS is declared fully operational. It is that time that GPS time transfer systems could replace many of the precise clocks in existence. This paper will discuss how GPS time transfer satisfies the time keeping requirements of various timing applications, enhances the time keeping capabilities of those applications and reduces the expense of time keeping in those applications. The performance of GPS time transfer in its different modes of usage will also be discussed.