Impact on the metrology and scientific communities The most direct impact of this project will be on the future realisation and dissemination of the SI unit of time, through the Consultative Committee for Time and Frequency (CCTF) and its working groups. Our coordinated programme of optical clock comparisons will allow the international consistency and long-term reliability of the clocks to be assessed, and is expected to make significant contributions to improved recommended frequency values for secondary representations of the second. It will build confidence in the new generation of clocks and help to identify the most promising candidates for a future redefinition of the SI second. Improvements to the robustness and automation of optical clocks will enable them to be operated regularly as secondary representations of the second, being used to steer local UTC(k) timescales and broadening the base of high accuracy clocks available for TAI steering. This will ultimately result in international timescales with improved stability. Our work will also have spin-off benefits to fundamental physics. For example, comparisons between optical clocks with verified uncertainties can be used to set limits on possible present-day variation in fundamental physical constants, to test the predictions of special and general relativity and to set experimental constraints on dark matter detection. Impact on industrial and other user communities Our work to improve the robustness of optical clocks and to automate their operation will be applied within the ROCIT project to improve the unattended uptimes of laboratory systems. However, the techniques and hardware we develop will also benefit research groups and industrial organisations aiming to develop compact, portable and turn-key optical clocks for applications beyond the laboratory. Several project partners already have strong links with European and national programmes that aim to develop such clocks for applications including geodetic height measurements, synchronisation of telecommunications and future satellite navigation systems. These links will be targeted as a route for uptake and exploitation of the project results. Longer-term economic, social and environmental impacts Through its contributions to improving European and global time and frequency infrastructure, this project will bring economic benefits to end users across a wide range of sectors. In the field of geodesy, networks of optical atomic clocks can be used to measure the Earth’s gravity potential with high temporal and spatial resolution, via the gravitational redshift of their operating frequencies. In this way it will be possible to bring national height systems into alignment across Europe, helping to prevent potentially costly mistakes being made in engineering projects. In geodynamic and climate research, it will be possible for scientists to track seasonal and long-term trends in ice sheet masses and overall ocean mass changes. Such data provides critical input into models used to study and forecast the effects of climate change. Precise time and frequency standards and measurement lie at the core of many technologies upon which society increasingly relies. Perhaps the most notable example is global navigation satellite systems (GNSS), but other systems such as electric power grids, mobile telecommunication networks and the internet all depend on time and frequency standards. The developments realised within this project will enable time and frequency to be disseminated with unprecedented stability and accuracy, not only to end users of the timescales maintained by European laboratories, but also globally through their impact on the international timescales TAI and UTC. As a result, the project is expected to lead to widespread impact on innovation, science and daily life.