C. Courde

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).

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.

Lunar laser ranging in infrared at the Grasse laser station

For many years, lunar laser ranging (LLR) observations using a green wavelength have suffered an inhomogeneity problem both temporally and spatially. This paper reports on the implementation of a new infrared detection at the Grasse LLR station and describes how infrared telemetry improves this situation. Our first results show that infrared detection permits us to densify the observations and allows measurements during the new and the full Moon periods. The link budget improvement leads to homogeneous telemetric measurements on each lunar retro-reflector. Finally, a surprising result is obtained on the Lunokhod 2 array which attains the same efficiency as Lunokhod 1 with an infrared laser link, although those two targets exhibit a differential efficiency of six with a green laser 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.

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.

The next generation of satellite laser ranging systems

Satellite laser ranging (SLR) stations in the International Laser Ranging Service (ILRS) global tracking network come in different shapes and sizes and were built by different institutions at different times using different technologies. In addition, those stations that have upgraded their systems and equipment are often operating a complementary mix of old and new. Such variety reduces the risk of systematic errors across all ILRS stations, and an operational advantage at one station can inform the direction and choices at another station. This paper describes the evolution of the ILRS network and the emergence of a new generation of SLR station, operating at kHz repetition rates, firing ultra-short laser pulses that are timestamped by epoch timers accurate to a few picoseconds. It discusses current trends, such as increased automation, higher repetition rate SLR and the challenges of eliminating systematic biases, and highlights possibilities in new technology. In addition to meeting the growing demand for laser tracking support from an increasing number of SLR targets, including a variety of Global Navigation Satellite Systems satellites, ILRS stations are striving to: meet the millimetre range accuracy science goals of the Global Geodetic Observing System; make laser range measurements to space debris objects in the absence of high optical cross-sectional retro-reflectors; further advances in deep space laser ranging and laser communications; and demonstrate accurate laser time transfer between continents.

Satellite and lunar laser ranging in infrared

We report on the implementation of a new infrared detection at the Grasse lunar laser ranging station and describe how infrared telemetry improves the situation. We present our first results on the lunar reflectors and show that infrared detection permits us to densify the observations and allows measurements during the new and the full moon periods. We also present the benefit obtained on the ranging of Global Navigation Satellite System (GNSS) satellites and on RadioAstron which have a very elliptic orbit.