Optical clocks have reached a proven accuracy of the order of 10−18 in relative frequency, surpassing the present Cs fountains by two orders of magnitude and driving the Consultative Committee for Time and Frequency (CCTF) to initiate work towards a redefinition of the second. However, comparing these clocks with a similar uncertainty is only possible using optical fibres and is presently limited to a few links at distances not exceeding 1000 km. Radio techniques have world-wide coverage and are currently regularly used in UTC with GNSS-based techniques and Two-way time transfer using geostationary satellites, but they have limited accuracy