GPS Clock Synchronization

Electronic clocks control critical functions in many applications. However, clocks are often designed for low cost rather than for keeping accurate time.

GPS Satellite Icon

Even fairly accurate computer clocks are likely to vary due to manufacturing defects, changes in temperature, electric and magnetic interference, the age of the quartz crystal, or even system load. Additionally, even the smallest errors in keeping time can significantly add up over a long period. Consider two clocks that are synchronized at the beginning of the year, but one consistently takes an extra 0.04 milliseconds to increment itself by a second. By the end of a year, the two clocks will differ in time by more than 20 minutes. If a fairly decent clock is off by just 10 parts per million, it will gain or lose almost a second a day. Most network architects consider the drift of a server clock to be 50 times worse (200 parts per million).

Atomic Clocks

Clocks locked to atomic standards are much more stable timekeepers. Rubidium, Cesium and Hydrogen Maser clocks are very accurate. Of the three, Rubidium clocks often provide the best combination of cost, size and overall performance and are often a requirement for high reliability master clock systems. However, atomic clocks themselves do not guarantee traceability and synchronization with other clocks. That where GPS comes in.

Synchronization to GPS

GPS satellites (and now other global navigation systems commonly refered to as GNSS) include three or four atomic clocks that are monitored and controlled to be highly synchronized and traceable to national and international standards (known as UTC). So for time synchronization, the GPS signal is received, processed by a local master clock, time server, or primary reference, and passed on to "slaves" and other devices, systems, or networks so their "local clocks" are likewise synchronized to UTC. Typical accuracies range from better than 1 microsecond to a few milliseconds depending on the synchronization protocol. It is the process of synchronization to GPS that can provide atomic clock accuracy without the need for a local atomic clock. Still, local atomic clocks are sometimes desired as a long-term back-up solution to loss-of-GPS, either in the case or a weather-related outage, GPS interference, or other scenarios.

In any case, GPS clock synchronization eliminates the need for manual clock setting (an error-prone process) to establish traceability to national and international standards so various events can be correlated even when they are time-stamped by different clocks. The benefits are numerous and include: legally validated time stamps, regulatory compliance, secure networking, and operational efficiency.

Rackmount Appliances, Plug-in Cards, OEM Boards & Modules for GPS and GNSS Clock Synchronization

Spectracom integrates GPS receivers into various timing platforms with internal oscillators and the ability to synchronize to other references if GPS is not available. Then these devices generate precision time and frequency signals as needed by all devices. Today, most Spectracom products are compatible with other GNSS systems such as GLONASS, with a roadmap to enable Beidou, Galileo and regional systems as they come on line. 

Testing time transfer via GPS

Spectracom's GPS simulator can be used to test the integration of GPS receivers into synchronization systems. They offer a variety of timing functions and calibrated performance to better than 1 nanosecond between a physical 1PPS signal and the generated GPS on-time point in the navigation message.