National Physical Laboratory


As the UK’s National Measurement Institute, NPL has a vital role in maintaining the UK’s time scale. The NPL time scale, called UTC(NPL), is used as the basis for time all over the UK, as well as providing one of the contributions to the international time scale, Coordinated Universal Time (UTC).

NPL’s clock system

In order to keep time as accurately as possible, NPL has seven continuously-operating atomic clocks. Four hydrogen masers measure the frequency of the radiation produced when hydrogen atoms change their energy state, while three caesium clocks measure the frequency of an energy shift in a caesium atom’s electrons. NPL also operates a caesium fountain primary frequency standard , which measures the radiation frequency of the energy changes in a caesium atom much more accurately but does not operate continuously. 

The hydrogen masers are more stable than the caesium clocks, so they form the basis of the time scale. One of the hydrogen maser clocks, called the reference maser, is used as the UTC(NPL) clock, and this is the ‘master clock’ that the UK’s time is based on. To make sure that it is keeping accurate time, the maser is regularly checked against the caesium fountain standard, which measures the SI second as accurately as possible.

The other masers are used as backup clocks, in case the primary maser fails. All of the clocks are constantly compared to the primary UTC(NPL) clock to check for any differences, allowing a faulty clock to be identified immediately. The UTC(NPL) time is also compared to the UTC(k) time scales in other measurement institutes worldwide, via time transfer links

Providing a primary frequency standard

NPL’s caesium fountain, NPL-CsF2, is currently the most accurate clock in the world, able to measure the SI second more precisely than any other clock. It is used both as the reference for UTC(NPL) and as one of the contributors to the international frequency standard, which is calculated at the BIPM.

A UK-wide time service

The time measured by NPL’s time scale, UTC(NPL), is distributed through three main methods. The first, and most widely used, is the MSF time service. This is a radio signal, broadcast at 60kHz, based on a caesium clock at Anthorn radio station from where the signal is transmitted. This clock, in turn, is monitored through a time transfer link to NPL as well as comparison of the MSF signal received at NPL with NPL’s clocks, to make sure that it is accurate. A large number of radio-controlled clocks receive the MSF signal all over the UK and beyond.

NPL also offers a dial-up Telephone Time Service (TTS) and an Internet Time Service (ITS) using the Network Time Protocol (NTP), for keeping computers synchronised with NPL’s clocks.

Last Updated: 28 Oct 2011
Created: 26 Oct 2011


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