Pierre Exertier

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).

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.

DORIS-class oscillator under radiations: The Jason family of satellites

The Jason family of satellites, which were launched in 2001, 2008 and 2016, embarked an ultra-stable-oscillator (USO) as the on-board frequency reference (short-term stability of a few 10−13 at 10-100 seconds) for the Doppler Orbitography and Radiopositioning Integrated on Satellite (DORIS) tracking system. All three satellites shared the same orbit (circular, 66°, 1335 km) and platform (PROTEUS) which is 3-axes stabilized. And all three oscillators have been perturbed by repetitive radiation exposures above the so-called South Atlantic Area, which implied deleterious frequency variations of various levels roughly from 10−12 to 10−11. The primarily objective of this work is to compare the frequency response of these USOs to radiations. On the assumption that all environmental parameters (proton flux from the SAA, position, orientation of the platform, etc.) are identical, we demonstrate that the oscillators present the same behavior with a pseudo-periodic frequency variation at 59 days whilst enhancing a different sensitivity. From all the perturbation sources, a dedicated exposure to radiations is identified due to a very particular situation — platform and attitude law, orbit, satellite anisotropy.

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.