A sub-ns comparison between GPS common view and T2L2

T2L2 (Time Transfer by Laser Link) permits the synchronization of remote ultra stable clocks over intercontinental distances. The principle is based upon laser telemetry technology with a network of laser stations on ground and dedicated space equipment designed to record arrival time of laser pulses at the satellite. T2L2 allows realization of some links between distant clocks with time stability of a few picoseconds and accuracy better than 100 ps. The instrumental metrology associated with such performance needs to be designed with utmost care. This requirement concerns all the instrumentation directly linked with the specific T2L2 equipment as well as the instrumentation doing the link between the laboratory reference and the T2L2 ground segment. Several campaigns were done to demonstrate both the ultimate time accuracy and time stability capabilities of T2L2. The paper is focused on the current high accuracy equipment that has been designed for the picosecond metrology and on some recent campaigns involving global calibrations of both laser stations and GNSS equipment. Results obtained during two months of comparisons between GPS in common view and T2L2 at three European laboratories show some differences below 300 ps with a standard deviation better than 500 ps. This is the first time that two different techniques of time transfer independently calibrated are in agreement at sub-ns level over continental distances.