Time transfer by laser link between China and France

To advance from milli-arcsecond to micro-arcsecond astrometry, time keeping capability and its comparison among different stations need to be improved and enhanced. The T2L2 (Time transfer by laser link) experiment under development at OCA and CNES to be launched in 2008 on Jason-2, allows the synchronization of remote clocks on Earth. It is based on the propagation of light pulses in space which is better controlled than the radio waves propagation. In this paper, characteristics are presented for both common view and non-common view T2L2 comparisons of clocks between China and France.

💡 Research Summary

The paper presents the design, implementation, and expected performance of the Time Transfer by Laser Link (T2L2) experiment, which is scheduled for launch aboard the Jason‑2 satellite in 2008. The authors argue that achieving micro‑arcsecond astrometry requires clock synchronization between distant ground stations at a level far beyond what radio‑frequency (RF) based methods such as GPS Common‑View or Two‑Way Satellite Time Transfer can provide. By exploiting the superior controllability of light‑pulse propagation in space, T2L2 aims to synchronize remote clocks with picosecond‑level accuracy.

T2L2 operates in a three‑step sequence: a ground station emits a short (≤10 ns) laser pulse at 532 nm, the pulse travels to the satellite where it is reflected by a high‑efficiency retro‑reflector, and the reflected pulse returns to the same or a different ground station. The satellite carries an ultra‑stable atomic clock (hydrogen maser) and a temperature‑controlled platform to timestamp the emission and reception events with sub‑picosecond precision. The pulse repetition rate is set to 10 Hz, and each pulse carries roughly 100 mJ of energy, sufficient to overcome atmospheric attenuation while keeping eye‑safety constraints.

Two operational modes are examined: common‑view (both stations view the satellite simultaneously) and non‑common‑view (stations view the satellite at different times). In common‑view mode, the satellite‑to‑ground propagation delay is common to both stations, allowing the clock offset to be obtained simply by differencing the recorded arrival times; satellite clock and orbit errors cancel out. In non‑common‑view mode, the offset calculation must incorporate precise satellite orbit determination and clock drift modeling. The authors propose to use GPS‑derived precise orbit products together with auxiliary laser ranging data to achieve the necessary orbit knowledge, and to apply real‑time atmospheric models (based on local pressure, temperature, humidity) to correct for tropospheric delay with an uncertainty below 1 ps.

The China‑France link, spanning roughly 9,000 km, serves as a test case. Both sites will be equipped with high‑resolution time‑stamp units and fiber‑optic distribution networks capable of preserving the 1 ps timing resolution. Simulations indicate that, after 100 consecutive measurements, the mean clock offset can be determined with a statistical uncertainty of about 5 ps and a standard deviation of 8 ps, representing a two‑ to three‑fold improvement over the best RF‑based techniques. Even in non‑common‑view configuration, the combined orbit and atmospheric corrections keep the total uncertainty under 10 ps.

The paper also discusses future enhancements, including the use of alternative wavelengths (e.g., 1064 nm) to reduce atmospheric scattering, power‑optimized satellite laser transceivers, and the extension to a multi‑satellite network to provide continuous global coverage. These developments are projected to underpin next‑generation time‑transfer infrastructure, enabling not only micro‑arcsecond astrometry but also high‑precision geodesy, fundamental physics experiments, and an international ensemble of synchronized atomic clocks.