Time transfer by Laser Link: The T2L2 experiment onboard the Jason-2 space vehicle

The new generation of optical time transfer (T2L2: Time Transfer by Laser Link [1]) under development at OCA and CNES 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 synchronize. T2L2 is the follow-on mission to LASSO (LAser Synchronisation from Stationary Orbit) [2] with performances improved by two orders of magnitude. We have now the opportunity to put a T2L2 payload on the Jason-2 space vehicle, which will be launched in mid 2008 for a 5 years long mission. A preliminary performance budget and a ground experiment conducted by the OCA permit to envision a performance improvement of at least one order of magnitude as compared to the best time transfer techniques available. Expected performances are in the 100ps range for accuracy, with an ultimate stability better than 1ps over 1000s (one pass) and than 10ps over one day. Time transfer performances in a common view mode are driven by the noise of both the ground and the onboard timers whereas the performances in a non-common view configuration are limited by the onboard clock (DORIS USO). The main objectives of the T2L2 mission are technological (In-orbit functional and performance validation), scientific (Time & Frequency metrology and Fundamental physics tests) with also a contribution to the Jason-2 core mission (Evaluation of the behaviours, versus radiations, of the DORIS USO).