Atomic clocks constantly need to be compared, whether the object is to synchronise them or to evaluate their performance. This operation is known as time transfer and may be performed between remote land-based clocks or between land-based and space-borne instruments. Basically, it involves the propagation of electromagnetic waves transmitted by satellite. With the constant improvement in the stability of clocks, systems for comparing them should be increasingly accurate.

Together with the Observatoire de la Côte d’Azur, CNES has developed a Time Transfer by Laser Link (T2L2) experiment. This system differs from those currently in use in that it depends on optical signals. The researchers behind T2L2 hope to increase the accuracy of these measurements by one or two orders of magnitude in comparison with systems such as GPS, which use microwaves.

T2L2 uses very short laser pulses, propagated between the clocks to be compared. A ground-based laser station equipped with a very accurate clock transmits a series of pulses. Three date-stamps are generated for each pulse:

• departure from the ground-station,
• arrival onboard the satellite,
• return to the ground station

The measurements are then processed on the ground in order to reconstitute the triplets of data for each pulse and to establish any time-shift between the different land-based clocks.  There are many immediate applications for atomic clock synchronization, including banking operations or geolocation systems.

The T2L2 instrument is a part of the Jason-2 satellite’s technology payload and is used to evaluate its efficiency and that of the Doris ultra-stable oscillator, also carried by Jason 2. It is expected to play a role in several science missions, mainly concerning time/frequency metrology and fundamental physics.

The concepts behind the T2L2 experiment and the onboard instrument arise from work performed by the Gemini department of the Observatoire de la Côte d’Azur. Three manufacturers developed the instrument in the record time of 20 months : Erems, Seso and the BTS-Coriolis Group with CNES as internal supervising contractor. It was integrated into the Jason 2 satellite which was launched on 20 June 2008.

_{Last updated: December 2008}