The T2L2 ground experiment time transfer in the picosecond range over a few kilometres

The new generation of optical time transfer (T2L2: Time Transfer by Laser Link [1]), under development at CNES (Centre National d'Etudes Spatiales) and OCA (Observatoire de la Cote d'Azur), shall allow the synchronisation of remote ultra-stable clocks and the determination of their performances over intercontinental distances. The principle is based on the propagation of light pulses between clocks to be synchronized. T2L2 is the follow-on mission to LASSO (LAser Synchronisation from Stationary Orbit) [2, 3] with performances in the picosecond range for both stability and accuracy. T2L2 is based on a space instrumentation that includes an event timer linked to the space clock, and some laser ranging stations linked to ground clocks. In order to validate the concept and to evaluate precisely the error budget, we realized two global experiments with some prototypes of both the ground and space instrumentations. These prototypes are based on electronic event timers and photo-detection devices that we developed in the framework of T2L2 project. In 1998, we performed a first ground experiment involving two OCA's laser stations and a first prototype of the space instrumentation 2.5 km apart. Results were promising but with some limitations due to the use of a bad quartz oscillator as the space clock. A second experiment was decided in 2004 with the OCA Lunar laser ranging station and a more finalized space segment installed closed to the station. The experiment was based on a free space propagation across 10 kilometres thanks to a large corner cube placed 2.5 km away. In order to measure the noise of the link without the noise of the clocks, a third event timer was able to measure the direct phase between signal clocks of both the space and ground prototypes.