Objective:The objective of the activity is to develop and prototype a free-space optical Frequency and Timing transfer solution over a coherent optical link optimised for the use case of linking critical infrastructure and metrology applications.Targeted Improvements:The two parameters to be improved over existing RF-based techniques are time and frequency transfer accuracy (factor 5 to 10 improvement), resistance to interference and robustness of the link.Description:Starting from the two sets of User Requirements, this paper study shall first analyse and trade-off the orbit scenarios (LEO, MEO, GEO/IGSO) and signal characteristics (wavelengths, modulation, coding ) in terms of performance, availability, operation, cost This shall include visibility analysis, link budget, noise and stability budgets, accommodation constraints considering the contribution of the end terminals and the propagation effects (fading, turbulences ). In a second step, a baseline scenario for the two targeted users requirements shall be selected as a result of the trade-off analysis. The requirements for the ground and on-board terminals shall be defined and a high-level design shallbe proposed. A review of available hardware (in particular under development e.g., for telecom) shall be performed to identify the required steps for further development. Critical items for both hardware development and link performance (e.g., noise, effect of turbulence) shall be identified as well as associated de-risking activities. A development and validation plan (including possible in-orbit validation) shall be established for both types of users. The final task will focus on the definition and implementation of testbed for the validation of the end-to-end performances that shall include the breadboarding of the most critical element targeting one type of user s requirements. The testbed shall allow field test and performances verification. The possibility to validate the synchronisation performances in a critical infrastructure environment shall be assessed. Problem to be solved: A wide range of marketsegments increasingly depends on global synchronisation of critical infrastructures with increased levels of performance, robustness against interferences and, in some case, regulatory compliance. This includes commercial applications like future telecom networks(e.g., 5G/6G), power grids, trading and institutional applications like security, metrology and science infrastructure with cutting edge performance and robustness. Current techniques for global synchronisation are based on GNSS or other type or RF signals that are limited in performance and that are increasingly subject to interferences (including intentional) and outages. The proposed optical link will address these two limitations for commercial and institutional applications.