P. Exertier

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 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.

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.

Fisrt application of the T2L2 ground to space time transfer : Characterisation of the DORIS USO.

The Time Transfer by Laser Link experiment is a joint CNES and OCA space mission that shall allow ground to ground time transfer with an expected stability of about 1 ps over 1,000 s and 10 ps over one day and an accuracy in the 100 ps range. The T2L2 instrument has been successfully launched with the Jason-2 space vehicle on the 20th of June 2008 and switched on 5 days later. It is been undergoing continuous operation since this date. The first days of operation have been devoted to functional tests, followed by a first evaluation of the performances of both the space instrument and the whole system. First results are very promising, preliminary data analysis shows a short term time stability of some tens of ps for an integration time of 1s, without any compensation of the behavior of the instrument. The next step, in progress, is improving of data processing, with the introduction of the relativistic compensation and of the numerical model of the instrument. That shall allow us to perform rapidly ground to space and ground to ground time transfer with the expected performances. Among science objectives of the mission, a first application of T2L2 shall be to precisely characterize the DORIS Ultra Stable Oscillator (USO) aboard Jason 2, independently from the DORIS system and with a better precision. By using an hydrogen maser as ground clock, relative stability of both T2L2 and DORIS USO shall allow to “read” the USO frequency for integration time of some tens of seconds. This paper will present the first restitution of the USO frequency thanks to the T2L2 ground to space time transfer.

A sub-ns comparison between GPS common view and T2L2

T2L2 (Time Transfer by Laser Link) permits the synchronization of remote ultra stable clocks over intercontinental distances. The principle is based upon laser telemetry technology with a network of laser stations on ground and dedicated space equipment designed to record arrival time of laser pulses at the satellite. T2L2 allows realization of some links between distant clocks with time stability of a few picoseconds and accuracy better than 100 ps. The instrumental metrology associated with such performance needs to be designed with utmost care. This requirement concerns all the instrumentation directly linked with the specific T2L2 equipment as well as the instrumentation doing the link between the laboratory reference and the T2L2 ground segment. Several campaigns were done to demonstrate both the ultimate time accuracy and time stability capabilities of T2L2. The paper is focused on the current high accuracy equipment that has been designed for the picosecond metrology and on some recent campaigns involving global calibrations of both laser stations and GNSS equipment. Results obtained during two months of comparisons between GPS in common view and T2L2 at three European laboratories show some differences below 300 ps with a standard deviation better than 500 ps. This is the first time that two different techniques of time transfer independently calibrated are in agreement at sub-ns level over continental distances.

Characterization of an ultra stable quartz oscillator thanks to Time Transfer by Laser Link (T2L2, Jason-2)

The T2L2 experiment (Time Transfer by Laser Link), on-board Jason-2, with an orbit at 1335 km, since June 2008 allows the clock synchronization between ground clock (generally H-maser) and space clock (quartz Ultra Stable Oscillator (USO) DORIS) with a stability of a few picoseconds over 100 seconds. In common view, when two laser stations see T2L2, the time transfer stability is less than 10 picosecondes over few seconds. In order to perform non-common view time transfer for synchronizing distant ground clocks, it is important to precisely characterize the on-board oscillator at least on 10,000 seconds (maximal flight time between two distant stations). The key is to study the space environment on the Jason-2 orbit, to separate deterministic and stochastic behaviors of the USO (shift and drift). We show that T2L2 is able to provide accurate frequencies, which are deduced from the ground to space time transfer over each laser station (few 10-13). Since 2008, these time transfers helped us to create an on-board frequency data base. The major contributors to these frequency variations on 10,000 seconds are temperature and space radiation especially due to the South Atlantic Anomaly (SAA) (in which Jason-2 pass through). Aging can be considered as a linear drift during 10,000 seconds and the effect of radiation like a very small shift over each SAA overflight. The effect of the temperature is drived by the on-board temperature measurement. A model is realized to represent these effects on USO with a RMS of few 10-13 over 10,000 seconds. Space phenomena are also playing an important role in long term. Actually, if we consider both accumulation dose received by radiation and aging, we can explain 99.9 % of the global frequency variation of the USO since the beginning of the T2L2 mission.