Trimble Outdoors Part Number 45005-00-ENG, AcutimeTM2000 Synchronization Kit manual GPS Timing

System Operation 5

5.4GPS Timing

In many timing applications, such as time/frequency standards, site synchronization systems, wireless voice and data networks, and event measurement systems, GPS receivers are used to steer a local reference oscillator. The steering algorithm combines the short-term stability of the oscillator with the long-term stability of the GPS PPS. An accurate GPS PPS allows the use of cost-effective crystal oscillators, which have poorer stability than expensive, high-quality oscillators, such as atomic cells.

The GPS constellation consists of at least 24 orbiting satellites. Unlike most telecommunications satellites, GPS satellites are not geostationary, so satellites in view are constantly changing. Each GPS satellite contains four highly-stable atomic clocks, which are continuously monitored and corrected by the GPS control segment. Consequently, the GPS constellation can be considered a set of 24 orbiting "clocks" with worldwide 24-hour coverage.

A Trimble GPS receiver uses the signals from these GPS "clocks" to correct its own internal clock, which is not as stable or accurate as the GPS atomic clocks. A GPS receiver like the Acutime 2000 outputs a highly accurate timing pulse (PPS) generated by its internal clock, which is constantly corrected using the GPS clocks. In the case of the Acutime 2000, this timing pulse is synchronized to UTC within 50 nanoseconds (one sigma) after survey is complete.

In addition to serving as highly-accurate stand-alone time sources, GPS receivers are used to synchronize distant clocks in communication or data networks. This is possible because all GPS satellites are corrected to a common master clock. Therefore, the relative clock error is the same, regardless of which satellites are used. For synchronization applications requiring a common clock, GPS is the ideal solution.

5-12 Acutime 2000 Synchronization Kit User Guide