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Maintaining the national time scale

We maintain the national time scale and the primary standards for frequency, contribute to global timekeeping, and disseminate accurate time and frequency to users across the UK.

The modern world relies on precise timing, based on highly stable atomic clocks. Complex laboratory systems measure the second with the highest achievable accuracy, while other clocks, able to run without interruption over many years, provide the essential continuity needed for keeping time.

At NPL, a group of commercial atomic clocks act as the continuous 'flywheel' for the national time scale, UTC(NPL). The duration of the time scale unit interval is fine-tuned by two caesium fountain primary frequency standards, capable of determining a deviation in UTC(NPL) of one part in 1015 (or 100 picoseconds) over one day. Together, these clocks provide the reference time scale for the UK and contribute to the generation of the global reference time system, Coordinated Universal Time (UTC). By applying very fine adjustments to the clock frequencies, UTC(NPL) is kept within a few nanoseconds of UTC.

As the UK's centre for precise time and frequency measurement, NPL disseminates reliable and trusted time to industrial, business and public users across the UK through a range of services. The MSF radio time signal and NTP-based internet time service are widely used to synchronise clocks and time-stamping equipment, while the NPLTime® service provides an accurate, managed time service over optical fibre links to the finance sector.

National time scale

Timekeeping today is coordinated globally, with around 70 national timing institutes contributing to the generation of the international reference time scale, Coordinated Universal Time (UTC). All precise timekeeping and frequency measurement worldwide is based on UTC. However, UTC is computed monthly and is not available directly; it is instead accessed through the time scales maintained by the contributing institutes. The NPL time scale is termed UTC(NPL), and provides the reference for precise timekeeping in the UK.

UTC(NPL) is based on continuously-running commercial atomic clocks of two complementary types: active hydrogen masers and caesium clocks. Together, these clocks form a highly resilient system that ensures uninterrupted operation of the time scale. Measurements of the clocks are also supplied to the International Bureau of Weights and Measures (BIPM) for use in the calculation of UTC. 

We have developed considerable experience in time scale construction and operation over many years, and have designed the time scale facilities for the Square Kilometre Array (SKA), a radio telescope that will be formed from thousands of antennas located in South Africa and Australia.

The UTC(NPL) time scale is integrated into the international timing system by time transfer links that continuously measure its time offsets from other national time scales to high accuracy. Two complementary methods are used routinely:

  • Two-way satellite time and frequency transfer (TWSTFT) is based on the simultaneous exchange of time signals between two laboratories via a communications satellite
  • GNSS time transfer utilises the precisely-timed signals from navigation satellites as a common reference that can be measured simultaneously by two laboratories, allowing the difference between the laboratories' time scales to be determined

These methods are accurate to a few nanoseconds, but we are also researching new techniques to compare time scales over optical fibre links with much greater precision.

Time and frequency dissemination

To be useful, a national time scale such as UTC(NPL) must be made readily available to users across the country. Accurate and reliable methods for disseminating time from NPL are essential for meeting the needs of UK industry, business and government agencies. 

At the highest levels of accuracy, the satellite-based techniques used to compare UTC(NPL) with other national time scales can be used. However, two-way satellite time and frequency transfer (TWSTFT) is complex and expensive, and can only support a small number of users. Common-view of GPS signals is more straightforward but is dependent on good reception of the weak satellite signals.

To avoid these limitations, we have developed the NPLTime® service to provide time resiliently over optical fibre links with an accuracy better than one microsecond, in partnership with distributor companies. The service is initially aimed primarily at the finance sector, and is managed and certified by NPL to enable customers to meet new regulations on timestamping of electronic trades. In addition, we are investigating novel methods for time dissemination over fibre that should give improved accuracy, and potentially operate over much longer distances.

We also disseminate time across the UK by two well-established methods: radio and internet. The MSF standard-frequency and time service is broadcast on 60 kHz from Anthorn radio station in Cumbria and can be received throughout the UK and beyond. The signal carries a time and date code that can set inexpensive radio-controlled clocks to the correct UK time. The internet time service utilises the Network Time Protocol (NTP) to synchronise a computer’s internal clock to one of two time servers located at NPL. Both services are available continuously and free of charge, and both provide access to the time from NPL with an accuracy better than 10 milliseconds.

Caesium fountain primary frequency standards

The first caesium atomic clock was brought into operation at NPL in 1955, leading 12 years later to the adoption of the resonance frequency of caesium atoms as the definition of the second in the International System of Units (SI).  Since then the accuracy of the best caesium primary standards has been improved by a factor of a million. 

The latest generation of caesium clock is the caesium fountain, and NPL is one of the leading national measurement institutes that operate these primary standards for the realisation of the SI second. A caesium fountain uses laser light to form a cloud of slowly-moving caesium atoms. The cloud of atoms is then pushed upwards, again by the laser beams, and flies freely under gravity through a region where the atomic resonance is probed by microwaves.

We have built several caesium fountain standards gaining considerable experience in their design. Two of these fountains are currently in operation – NPL-CsF2 and NPL-CsF3 – serving as frequency reference for the UTC(NPL) time scale, enabling regular corrections to be applied. We also contribute data to the international time scale, helping to keep the duration of the UTC second as close as possible to the SI definition.

Our unique approach allows state-of-the-art stability and accuracy to be achieved with a relatively simple design of the fountain physics package. We have also improved the reliability and robustness of the system, leading to long periods of operation without unintentional stoppages. Such maturity of the design has allowed us to supply complete fountain systems to other national institutes.

Find out more about NPL's commercial Caesium fountain

Find out more about NPLTime®

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