T2L2: Five years in space

The Time Transfer by Laser Link (T2L2) experiment aims to synchronize remote ultra stable clocks over very long distances using the Satellite Laser Ranging (SLR) technique. T2L2 was launched in July 2008, on board the Jason2 satellite; from 5–6 stations ranging T2L2 during the first months of the mission, around 22 stations of the worldwide SLR network are now participating in the tracking. In addition to the permanent data acquisition and processing (accessible from our website https://t2l2.oca.eu/), several field experiments have been conducted to alternatively demonstrate the ultimate time transfer capability of T2L2, in terms of stability, exactness, comparison with the GPS and Two-Way microwave techniques. This paper synthetizes the best performances that T2L2 allows us to achieve, as a result of recent improvements made in the data reduction. The time stability of the T2L2 ground to space time transfer is established at 6–8 ps at 75 seconds, for SLR systems equipped with an H-maser as the reference clock. The ground to ground time transfer stability between 2 SLR stations (in Common View) is estimated at 11 ps rms (average) over one passage and better than 50 ps over several days. We present also the advantages and drawbacks of this unique time transfer technique based on an optical link.