Time Transfer by Laser Link - T2L2: Current status of the validation program

The Time Transfer by Laser Link (T2L2) experiment is a joint Observatoire de la Côte d\´Azur (OCA) and CNES space mission that will perform ground to ground time transfer. Using laser pulses instead of radio frequency signals, we are expecting a time stability of about 1 ps over 1,000 s and 10 ps over one day and a time accuracy in the 100 ps range. The T2L2 instrument is on board the Jason-2 space vehicle. Launched in June 2008, it has been working since this date. After a six months period devoted to the characterization and the calibration of the system, the mission has started its "operational" phase in January 2009. First ground to space time transfers have demonstrated noise levels of some tens of picoseconds and a preliminary time stability of a few picoseconds over integration times of some tens of second, clearly limited by the on-board clock. The 2009 T2L2 experimental program has contributed to the calibration / validation of T2L2 performances through two major experiments. The first one is a common clock time transfer between two co localized laser stations at the O CA : a fixed one MeO (for Optical Metrology) and the French Transportable Laser Station (FTLRS). The second experiment is the validation of remote clock comparisons using both laser and radio frequency techniques. This validation has been done using 3 regular laser stations (O CA France, Poland, Japan) and also the FTLRS station installed at Observatoire de Paris (SYRTE). The availability on those sites of GPS and TWSTFT stations allows a direct comparison with T2L2. In parallel, data processing has been improved. It now includes a compensation of the dependency of the time walk of the detector with the energy of the laser pulse and some initial filtering and interpolation process. Influence of the angle of incidence of the laser beam on the detector will also be introduced soon, together with the finalization of the filtering and interpolation strategy. The co location campaign too- place from April to September 2009, with the two laser stations MeO and FTLRS connected to the same cesium clock. The FTLRS station has then been moved into Paris for 4 weeks between October and November 2009. After reminding the principle and the objectives of the mission, this paper will present first results of these experiments.