National Physical Laboratory

Time and Frequency Comparison and Dissemination Time & Frequency Comparison & Dissemination

Methods for comparing widely separated clocks and frequency standards are vital for the development of improved standards, generation of the international time scale and provision of services to UK industry.

The NPL time scale UTC(NPL) is compared regularly with other national time scales using two independent techniques. One method is based on the reception of timing signals from global navigation satellite systems (GNSS) such as GPS. The second method uses communication satellites to exchange precisely timed signals between national timing centres, and is known as two-way satellite time and frequency transfer (TWSTFT). It is the most accurate means currently available for comparing the time and frequency differences between remote clocks.

The development of optical frequency standards promises a substantial improvement in the performance of atomic clocks, and novel time and frequency transfer techniques are needed to allow full use to be made of their capabilities. We are researching methods for transmitting timing signals and highly stable optical and microwave frequencies over optical fibre networks. We are also exploring the possibility of locating a microwave link ground terminal at NPL for the ACES (Atomic Clock Ensemble in Space) mission, which would allow the optical standards at NPL to be compared to others around the world with much greater precision.

As the UK's centre of precise time and frequency measurement, NPL offers a range of services that provide access to a reliable and trusted source of time: the Time from NPL.

Time products & services

  • The MSF signal transmitted from Anthorn (in Cumbria) provides a signal strength in excess of 100 microvolts per metre at a distance of 1000 km. This level should be sufficient to allow the time and date code to be received without difficulty.
  • 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 MSF 60 kHz standard-frequency and time signal, broadcast by Babcock on behalf of NPL, is occasionally taken off-air to allow maintenance work on the masts and antennas at Anthorn Radio Station to be carried out in safety.
  • The MSF radio signal is a dedicated time broadcast that provides an accurate and reliable source of UK civil time, based on the NPL time scale UTC(NPL). It is available 24 hours a day across the whole of the UK and beyond.
  • The NPL Monthly Bulletins for time and frequency cover the three standard-frequency and time radio signals that are most widely used in the UK: the MSF 60 kHz standard-frequency and time signal, the 198 kHz long-wave transmission from Droitwich radio station and the GPS navigation satellite signals.
  • If the computer is to tell the time accurately it must set its internal clock regularly against a trusted external source of time, which must be both accurate and reliable. NPL offers two services that provide access to just such a source.


Please note that the information will not be divulged to third parties, or used without your permission