5-12 Acutime 2000 Synchronization Kit User Guide
System Operation 5
5.4 GPS 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 comb ines 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.