T2L2: 6 Years of Sub Nanosecond Time and Frequency Metrology

The Time Transfer by Laser Link (T2L2) experiment aims at achieving ground to ground time transfer between remote clocks using time tagging of laser pulses. The principle is derived from laser telemetry technology with a dedicated space equipment designed to record arrival epoch of laser pulses at the satellite. The use of laser pulses, instead of microwave signals, allows realizing some time transfer with time stability of a few picoseconds and accuracy better than 150 ps. The mission is mainly dedicated to the metrology of time and frequency, nevertheless the objectives are multiple. First technological, T2L2 will demonstrate the feasibility and the performance of the time transfer by laser link. Then scientific, the expected performances of T2L2 will allow ultimate comparisons with microwave system or to perform some fundamental physics tests. Initially planned for duration between 2 and 3 years, the mission has just passed through 6 years in operation. These 6 years of missions, punctuated by several specific campaigns of measurements, allowed filling the major part of the assigned objectives. Through the story of these 6 years of missions, this paper will redraw the main results, from the first estimations of stability of the ground to space time transfer to the absolute direct comparison with GPS Common-View, consistent to 240 ps or below over continental baselines.