GRGS > Research > Projects > T2L2

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T2L2 (Time Transfer by Laser Link): The T2L2 space instrument was planned for a flight aboard the satellite Jason2 in 2005. This system is based on an onboard instrument, combined with a clock (Ultra-Stable Oscillator, DORIS), and a network of laser ground stations equipped with clocks and laser ranging systems.

The OCA has adapted the techniques of laser ranging to time transfer and developed laser links dedicated to the synchronization of distant clocks. The Centre National d'Etudes Spatiales (CNES) and the OCA developed the T2L2 (Time Transfer by Laser Link) with the aim of conducting time transfers with picosecond stability. The project is onboard the altimetry satellite Jason2 (launched in June, 2008). See the site of the Scientific Mission Center of the OCA.


The laser ground stations emit pulses towards the satellite and record the departure times of the laser pulses in their local time. Onboard the satellite, the T2L2 instrument records the arrival time of photons in the reference time onboard. Like conventional laser ranging, a retro-reflector sends back a portion of the photons to ground stations that register their returns in local time. After downloading in batch mode the times recorded on board, and combining data from ground stations and space, it is then possible to calculate the gap between the ground clocks and onboard clocks.
A ground-space time transfer thus obtained can be implemented at the same time by several laser stations which all share visibility of the satellite; the association of two ground-space time transfers then allows us to establish a ground-ground time transfer between the two stations.

Principe du Transfert de Temps en vue commune

Principe du Transfert de Temps en vue commune


The T2L2 instrument, originally slated to fly on the Mir mission in 1999, then with the ACES mission, was accepted by the Centre National d'Etudes Spatiales (CNES) in July 2005 to be flown as a passenger on the altimetry satellite Jason2.Note: industrial study of the T2L2 instrument began when it was planned for MIR 99. This study was then taken up again for the ACES mission. Although T2L2 was removed from the ACES project in 1999, the study continued until the end of instrument phase B.

Today, adding T2L2 to the Jason2 mission creates synergy with the characterization of the frequency reference of the DORIS positioning system and orbit restitutions by one-way laser ranging. T2L2 should provide independent verification of DORIS’ oscillator (USO), which was missing on Jason1, given that the USO was more sensitive than expected to radiation.

Jason-2 represents an excellent opportunity as its high altitude allows for time transfer with very long integration times in common view mode (two stations can "see" the satellite on the same pass) for most of the continental links.

The T2L2 mission presents an opportunity to enrich the results of the ACES mission for 2013 and beyond. The scientific communities involved in time /frequency and theoretical physics should thus have a continuity of links with very high stability to compare with clocks at 10-16 or 10-17.