J.-M. Torre

Time Transfer by Laser Link — T2L2: Current status and future experiments

T2L2 (Time Transfer by Laser Link), developed by both CNES and OCA permits the synchronization of remote ultra stable clocks over intercontinental distances. The principle is derived from laser telemetry technology with dedicated space equipment deigned to record arrival time of laser pulses at the satellite. Using laser pulses instead of radio frequency signals, T2L2 permits to realize some links between distant clocks with a time stability of a few picoseconds and accuracy better than 100 ps. The T2L2 space instrument is in operation onboard the satellite Jason 2 since June 2008. Several campaigns were done to demonstrate both the ultimate time accuracy and time stability capabilities. It includes some experiments implemented in co-location to directly compare T2L2 time transfer residuals with the direct link between stations, and some ground to ground time transfer between ultra stable clocks. Important works have been done, between OCA and OP, to accurately compare T2L2 with microwave time transfer GPS and TWSTFT. These comparisons are based on laser station calibrations with a dedicated T2L2 calibration station designed to accurately set the optical reference of the laser station within the PPS reference of the microwave systems. Other experiments are also planned in the future: 3D localization with the lunar space vehicle LRO, T2L2 coverage extension over the Pacific Ocean (Tahiti), DORIS comparison and a third international campaign.

Time transfer by laser link: a complete analysis of the uncertainty budget

The Time Transfer by Laser Experiment (T2L2) on the Jason 2 satellite is a mission allowing remote clocks synchronization at the picosecond level. It is based on laser ranging technologies, with a laser station network on the ground and a dedicated instrument on board the satellite. It was launched in June 2008 and has been working continuously since then. T2L2 performances are very promising for time and frequency metrology and also for fundamental physics. The scientific objectives of the whole experiment rely on a rigorous uncertainty budget. This is governed by the characteristics of the space instrument and the laser stations network, the post treatment done on the ground, and also the process used to calibrate the laser stations. The uncertainty budget demonstrates that T2L2 is able to perform common-view time transfers between remote sites with an expanded uncertainty better than 140 ps (coverage factor = 2).

Time transfer by laser link T2L2 first results

The optical time transfer project T2L2 has been successfully launched from California in 2008 on the Jason-2 satellite. T2L2 permits the synchronization at the pico-second level of remote ultra stable clocks and the determination of their performances over intercontinental distances. The principle is derived from laser telemetry technology with a space equipment designed to record arrival times of laser pulses at the satellite. Since the launch, several thousands of passes have been acquired by the laser ranging community.

Time Transfer by Laser Link: The T2L2 experiment on Jason 2

The new generation of optical time transfer (T2L2 - Time Transfer by Laser Link) under development at OCA and CNES will allow the synchronization 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 the clocks that are to be synchronized. T2L2 is the follow-on mission to LASSO (LAser Synchronization from Stationary Orbit) with performances improved by two orders of magnitude. Expected T2L2 performances are in the 100 ps range for accuracy, with an ultimate stability better than 1ps over 1,000s and than 10ps over one day. After a short overview of the instrumental heritage and the historic course of the project up to todays acceptation on Jason 2, we will report on a ground experiment conducted by OCA permitting to envision a performance improvement of at least one order of magnitude as compared to the best time transfer techniques available. Then preliminary performance budgets for the T2L2 on Jason 2 mission will be given, based on measurements conducted by OCA and expected performances of the space instrument. We will finish with the status of the space instrument development and a summary of recently conducted measurements of the electronics breadboards performances

T2L2: Five years in space

The Time Transfer by Laser Link (T2L2) experiment aims to synchronize remote ultra stable clocks over very long distances using the Satellite Laser Ranging (SLR) technique. T2L2 was launched in July 2008, on board the Jason2 satellite; from 5-6 stations ranging T2L2 during the first months of the mission, around 22 stations of the worldwide SLR network are now participating in the tracking. In addition to the permanent data acquisition and processing (accessible from our website https://t2l2.oca.eu/), several field experiments have been conducted to alternatively demonstrate the ultimate time transfer capability of T2L2, in terms of stability, exactness, comparison with the GPS and Two-Way microwave techniques. This paper synthetizes the best performances that T2L2 allows us to achieve, as a result of recent improvements made in the data reduction. The time stability of the T2L2 ground to space time transfer is established at 6-8 ps at 75 seconds, for SLR systems equipped with an H-maser as the reference clock. The ground to ground time transfer stability between 2 SLR stations (in Common View) is estimated at 11 ps rms (average) over one passage and better than 50 ps over several days. We present also the advantages and drawbacks of this unique time transfer technique based on an optical link.

Sub-ns time transfer consistency: a direct comparison between GPS CV and T2L2

This paper presents a direct comparison between two satellite time transfer techniques: common-view (CV) of satellites from the global positioning system (GPS) constellation, and time transfer by laser link (T2L2) through the low orbiting satellite Jason-2. We describe briefly both techniques, together with two independent relative calibration campaigns of the links involving four European laboratories. Between the same remote time scale reference points, the mean values of the calibrated differences between GPS CV and T2L2 are below 240 ps, with standard deviations below 500 ps, mostly due to GPS CV. Almost all sample deviations from 0 ns are within the combined uncertainty estimates. Despite the relatively small number of common points obtained, due to the fact that T2L2 is weather dependent, these results provide an unprecedented sub-ns consistency between two independently calibrated microwave and optical satellite time transfer techniques.

A direct comparison between two independently calibrated time transfer techniques: T2L2 and GPS Common-Views

We present a direct comparison between two satellite time transfer techniques on independently calibrated links: Time Transfer by Laser Link (T2L2) and Common-Views (CV) of satellites from the Global Positioning System (GPS) constellation. The GPS CV and T2L2 links between three European laboratories where independently calibrated against the same reference point of the local timescales. For all the links the mean values of the differences between GPS CV and T2L2 are equal or below 240 ps, with standard deviations below 500 ps, mostly due to GPS CV. Almost all deviations from 0 ns are within the combined uncertainty estimates. Despite the weak number of common points obtained, due to the fact that T2L2 is weather dependent, these results are providing an unprecedented sub-ns consistency between two independently calibrated microwave and optical satellite time transfer techniques.

A direct comparison between two independently calibrated time transfer techniques: T2L2 and GPS common-views

This paper presents a direct comparison of two time transfer techniques on independently calibrated links: Time Transfer by Laser Link (T2L2) and Global Positioning System (GPS) satellite common-views (CV). The two different techniques are described, along with the independent link calibrations. The results achieved during one measurement campaign involving three European laboratories are presented, the differences between the two techniques remaining in average below 300 ps, with a standard-deviation below 500 ps mostly due to GPS CV.