GPS Disciplined Oscillator Traceability

In 1997, our UK Service Center participated in an NPL-industry collaborative investigation into the use of frequency standards that are disciplined using the US DoD Global Positioning System. As part of the project objectives, NPL published an official report of their findings. Along with additional evidence from our own investigation, it resulted in UKAS accreditation being granted to HP in early 1999 for use of a HP58503A GPS-disciplined oscillator to maintain formal traceability of the laboratory's HP5071A (cesium beam) Primary Frequency Standard.

But why was GPS-DO Traceability in Question ?

Whilst GPS has become an accepted tool for ultra-precise time comparison using the common view technique (requiring simultaneous measurement of signals from specific satellites), its use as a reference for automatically correcting the frequency of an oscillator has not yet been universally recognized by national metrology bodies. Since such equipment is now available from a variety of manufacturers resulting in widespread industrial application, the matter of traceable accuracy has become significant.

Why GPS-DO and not Common View ?

• Equipment cost -- GPS-DOs are relatively inexpensive and widely available -- Timing receivers are scarce/costly to buy
• Used as a stand-alone frequency standard
• Timing receiver needs an independent lab frequency standard (usually cesium beam)
• Ease of operation -- "Plug and play" versus "lab guru"

But why Common View at all ?

• Multiple and complex uncertainty components are associated with GPS use -- some can be eliminated by synchronous comparison of satellites visible to all parties
• Provides utmost accuracy when comparing two remotely located "clocks"

Methodology

We used two techniques  to compare the frequency produced by the GPS-DO with our laboratory reference:-

• Continuous phase comparison recorded to a chart, requiring manual interpretation to produce numerical data for further analysis
• Periodic time interval measurement, controlled and data-logged by computer and ideal for NPL evaluation.

NPL's Official Report

Davis, J.A. and Furlong, J.M., Report on the Study to Determine the Suitability of GPS Disciplined Oscillators as Time and Frequency Standards Traceable to the UK National Time Scale UTC(NPL).

• NPL