The Time group at NPL carries out research in a range of areas aimed at improving atomic time scales and time and frequency transfer.

The UK's national time scale is based on a group of 6 atomic clocks located at NPL. The more stable clocks are four active hydrogen masers, and these are supplemented by two caesium clocks which are portable and less sensitive to their environment.

We have considerable expertise in the analysis of data from clocks and time transfer systems. Noise processes in both clocks and time transfer links are investigated and modelled using Kalman filter-based methods. These can provide an optimal combination of the clocks in an ensemble, to generate a composite time scale that is more stable than any of the clocks contributing to it. Algorithms to combine results from different time transfer systems are also being developed. The techniques developed at NPL are being applied to two key components of the system time scale for Galileo, the European satellite navigation system.

The atomic clocks at NPL are compared with those at other national timing facilities using two time and frequency transfer methods. One method is based on the reception of timing signals from global navigation satellite systems (GNSS) such as GPS. We are developing improved analysis methods for GNSS timing data, and investigating improved designs for future GNSS receivers optimised for time transfer.

The second method uses geostationary satellites to exchange precisely timed signals between national timing centres, a method known as two-way satellite time and frequency transfer (TWSTFT). It is the most accurate method currently available for comparing the time and frequency differences between remote clocks, and is the primary method for contributing clock data to the world time scale UTC, or Coordinated Universal Time. NPL has two satellite earth stations, and is participating in international studies aimed at developing improvements to the technique.