National Physical Laboratory

GPS Common-View Time and Frequency Transfer Service

GPS Common View
Image courtesy of iStockphoto

NPL operates a time and frequency transfer service that provides its customers with greater accuracy and data integrity than other commercially available time and frequency traceability routes. The service is based on the GPS common-view method, and it builds on infrastructure already in place for maintaining the global time standard, Coordinated Universal Time (UTC).

The Key Benefits of the Service

The service will be essential for those needing the highest levels of time and frequency accuracy, whether to provide top-level calibration capabilities or to develop and validate time and frequency products such as hydrogen masers, caesium clocks, rubidium oscillators, GPS disciplined oscillators and other GPS timing devices. The key benefits are:

In-Situ Measurements: The GPS common-view receiver brings the UK national time standard into your laboratory, keeping your time and frequency standards in place and productive.

Wide-Coverage: GPS common-view allows NPL to reach other national laboratories in North America and Europe, so coverage across the whole of the UK and beyond is well inside the technical limits of the service.

Integrity: Your time and frequency standard is monitored relative to the UK national time standard and analysed by NPL staff. The service provides you with independent integrity checks and can be used to support your quality systems.

Time Accuracy: Traceability to UTC time-of-day at a level suitable for characterising GPS timing signals:

  • 10 ns uncertainty relative to UTC(NPL) (1 sigma)
  • 20 ns uncertainty relative to UTC (1 sigma)

Frequency Accuracy: Traceability for fractional frequency offset measurements at a level suitable for characterising GPS timing signals:

  • 5x10-14 relative to UTC(NPL) (1 sigma, averaged over 1 day)
  • 5x10-14 relative to UTC (1 sigma, averaged over 1 day)

Note that this service is the only practical option for those who need to reliably quantify the in-situ performance of their GPS timing equipment.

Traceability: The service provides explicit traceability to national standards and to the international time scale UTC.

Accreditation: The NPL GPS common-view service is UKAS accredited.

GPS Common-View Receiver Results

The figure above shows results from the GPS common-view service being used to monitor
the performance of a GPS disciplined oscillator operating at a UK customer's site over
a 10-day period. Each point is the mean value obtained from all satellites in view at that epoch


The Service

The common-view service provides a complete on-site clock monitoring service, comparing a local measurement system to the UK national time standard with the results being processed and checked by NPL time experts. For frequency comparisons, the 5 MHz or 10 MHz signal from the local standard is connected to the GPS common-view receiver. For time comparisons, the local 1 pulse-per-second (1PPS) reference signal is also connected to the receiver.

The service has two distinct operational stages:

  1. Start-Up: A suitable GPS common-view receiver (see comments below on receivers) is tested and calibrated at NPL before shipping to the customer's site; the user installs the receiver and provides data to NPL for us to test and validate its operation remotely.
  2. Routine Service: Daily data files are collected and transferred to NPL automatically; NPL processes the data files weekly and issues a weekly report by e-mail, providing a mean value and plot of the time offset between UTC(NPL) and the user's local time reference and appropriate time domain statistics (ADEV, MDEV and TDEV).

If the user requires time traceability, a re-calibration of their GPS receiver is recommended every 3-5 years. Either the user's receiver is returned to NPL for calibration, or NPL sends a second calibrated receiver that can be operated in parallel with the user’s in-situ receiver to allow cross-calibration.

An example of common-view service data for a GPSDO is shown in the figure.

The GPS Common-View Receiver

The TimeTrace GPS common-view receiver manufactured by Time & Frequency Solutions Ltd is recommended for the service. However, the service can be operated using any GPS timing receiver that performs measurements against external frequency or pulse inputs and generates output files in the international standard common-view format.

Contact

Customer Service tel: +44 20 8943 6796
E-mail: timefreq_enquiries@npl.co.uk

Last Updated: 5 Nov 2014
Created: 8 Jun 2009

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