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The scientific community has recognized the need for a highly accurate terrestrial reference frame (TRF) for Earth Science applications. Current determination of the International Terrestrial Reference System is made by combining data from space geodetic techniques, namely Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), Doppler Orbitography and Radiopositioning Integrated by satellite (DORIS), Global Navigation Satellite Systems (GNSS), but also terrestrial measurements from local tie survey at co-location sites. For most of the sites, such local tie surveys are not performed on a regular basis. Thus, The PSInSAR (Persistent Scatterer Interferometric Synthetic Aperture Radar) technique allows for determining deformation maps over large areas with various spatial resolutions as function of the satellite missions. Due to the availability of freely available SAR data for a significant period of time at many sites, it is relevant to ask if such data could supplement local tie measurements for those sites where sufficiently repeated terrestrial surveys do not exist. This papers presents the work of the study group “SG 1.2.1: Relevance of PSInSAR analyses at ITRF co-location sites” of the international association of geodesy. Advantages and limitation of this technology will be addressed. A review of some published PSInSAR results in C- and X-band at some co-location sites will be presented. As high resolution X-band SAR images (TerraSAR-X/PAZ in “Staring SpotLight mode”) show great potential for mapping deformations at high resolution, an inventory of already available SAR images at ITRF co-location site has been established to motivate PSInSAR analyses on these data., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

International audience; Since 2015, in addition to the traditional green (532nm), infrared (1064nm) has been the preferred wavelength for lunar laser ranging at the Calern lunar laser ranging (LLR) site in France. Due to the better atmospheric transmission of IR with respect to Green, nearly 3 times the number of normal points have been obtained in IR than in Green [ C.Courde et al 2016 ]. In our study, in addition to the historical data obtained from various other LLR sites, we include the recent IR normal points obtained from Calern over the 1 year time span (2015-2016), constituting about 4.2% of data spread over 46 years of LLR. Near even distribution of data provided by IR on both the spatial and temporal domain, helps us to improve constraints on the internal structure of the Moon modeled within the planetary ephemeris : INPOP [ Fienga et al 2015 ]. IERS recommended models have been used in the data reduction software GINS (GRGS,CNES) [ V.Viswanathan et al 2015 ]. Constraints provided by GRAIL, on the Lunar gravitational potential and Love numbers have been taken into account in the least-square fit procedure. New estimates on the dynamical parameters of the lunar core will be presented.

Downlink measurement campaigns from the optical downlink terminal OSIRISv1 onboard the LEO satellite Flying Laptop were carried out with the French Observatoire de la Côte d’Azur and with two Optical Ground Stations of the German Aerospace Center. On/off keyed data at 39 Mb/s were modulated on the laser signal, and according telecom reception was performed by the ground stations. The pointing of the laser terminal was achieved by open-loop body pointing of the satellite orientation, with its star sensor as attitude control signal. We report here on the measurements and investigations of the downlink signal and the data transmission.

For the past fifty years, geodetic satellites have contributed to the measurement of numerous geophysical parameters. They thus enabled us to improve our knowledge on several Earth's physical parameters and behaviors, through their refined observation. In particular, the geocentric gravitational coefficient is a key contributor to Satellite Laser Ranging (SLR) validation of the stringent accuracy requirement on the radial positioning of altimeter satellites, defining sea level relative to the Earth’s center of mass. Indeed, SLR range biases estimated for the upcoming ITRF2020, derived from Medium Earth Orbit (MEO) Lageos and Etalon satellites, may not be applicable for Low Earth Orbit (LEO) altimeter satellites given the relatively large uncertainty of 2.0 ppb in the geocentric gravitational constant. The purpose of this paper is to reassess the value of the Earth’s GM from SLR measurements. This constant is defined by the product of the Earth's universal gravitational constant G and its mass M. The analysis is based on its simultaneous estimation with laser station and satellite biases, taking the most of eight well-chosen geodetic satellites. At first we focused on two specific spherical geodetic satellites, said to be without signature effects, Blits and Larets. We were able to revise estimates of the GM constant using CNES’ ZOOM precise orbit determination software. Additionally, by combining MEO and LEO satellites, a consistent value of GM has been reassessed, being GM = 398600.4419 ± 0.0002 km3/sec² (TT-compatible). This value is higher than the reference value estimated previously in 1992. Also, some problems related to the design and center-of-mass corrections of the targets used have been raised and would need to be further investigated.

In a joint effort, experts from measurement science and space-geodesy develop instrumentation and methods to further strengthen traceability to the SI definition of the metre for geodetic reference frames (GRF). GRFs are based on space-geodetic observations. Local-tie surveys at co-location sites play an important role for their computation. Novel tools are hence developed for reference point monitoring, but also for local tie vector determination and ground truth provision. This contribution reports on the instrumental approaches and achievements after 24 months project duration and discusses the remaining work in the project.

International audience; Intensity Interferometry offers an interesting path forward for astronomical interferometry at very long baselines and at short wavelengths, all leading to extremely high angular resolution. Single photon counting detectors (and associated correlator technology) has evolved to allow a sensitivity gain of two orders of magnitude since the seminal experiments of Hanbury-Brown and Twiss in the 1960s, but sensitivity remains the challenge to make this technique widely useful: extremely high angular resolution requires high surface brightness by definition. The simplest way to increase the sensitivity of an intensity interferometer is to obtain multiple simultaneous correlation measurements at different wavelengths since these are uncorrelated and improve the SNR as √ N , with N spectral channels. The first issue to address is the break-even point since the throughput of spectrographs can be low; for example a throughput of 50% requires at least 4 spectral channel to break even. The next issue we wish to address is one of reliability and ease of use as we intend to deploy these spectrographs at multiple locations. In this paper, we propose and compare three concepts, each with its advantages and drawbacks: a classical multimode fiber-fed spectrograph, a photonics lantern fed focal plane based concept and an integrated optics solution using photonics lanterns and an Arrayed Waveguide Grating Spectrograph. We hope to build a simple demonstrator of whichever concept we end up choosing for testing in the context of the I2C project.

International audience

POLAC (Paris Observatory Lunar Analysis Center) is an ILRS analysis center founded by J. Chapront, M. Chapront-Touzé and G. Francou in 1996. They developed in the 80's a semi-analytical solution of the lunar motion named ELP (Ephémérides lunaires Parisienne). Thus, the original purpose of POLAC was the adjustment of ELP to the lunar laser ranging observations (LLR) for improving the determination of fundamental astronomical parameters, such as the free modes of lunar physical librations, the tidal secular acceleration of the lunar longitude, or the transformation between celestial reference systems. Since the beginning, POLAC worked in close collaboration with the laser ranging station of Grasse (MéO) by providing a posteriori validation of their LLR normal points in order to avoid calibration and format issues. Since 2010 POLAC has evolved. Firstly, it additionally provides predictions for laser ranging observations - mainly for the Moon tracking but also, in an experimental mode, for two ways LRO (Lunar Reconnaissance Orbiter) tracking . The POLAC LLR predictions, originally dedicated and optimized for MéO station, are now the official ILRS prediction for LLR. Secondly, with the elaboration of a new lunar ephemeris called ELPN (Ephéméride lunaire Parisienne Numérique), POLAC also takes part to the long legacy of testing fundamental Physics with laser ranges to the Moon. Indeed, even if ELPN was built originally in the General Relativity (GR) framework, it allows for GR alternative theories of gravity as well. One of particular interest is the Standard Model Extension (SME) which parametrizes Lorentz symmetry violations, notably in the pure gravity sector and in the matter sector of the formalism. By fitting ELPN in the SME framework to the 50 years of collected data, we have been able to provide stringent and realistic estimates on possible Lorentz symmetry violations arising at the level of the weak and the strong Einstein equivalence principles.

International audience

International audience

International audience; In this poster we elaborate the use of raw navigation data (range and Doppler observations) from the Lunar Reconnaissance Orbiter (LRO) available on the Planetary Data System (PDS), in order to study the orbit of this probe using the orbit determination software (GINS) developed by the French space agency (CNES). The constraints that are derived from this process on combining with the high precision Lunar Laser Ranging (LLR) datasets which are spread over 40 years, facilitates an improved dynamical modeling of the Moon. In addition, the possible advantages that could be exploited by the LLR experiments when operated with lasers in the IR wavelength are analyzed.

International audience; 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.

International audience; Absolute distance measurement with accuracy below the micron scale is important in astronomical optical interferometry. We present here an absolute laser rangefinder which relies on the detection of a two mode interference signal. We exploit the specific signature of the signal to extract both the interferometric and synthetic phase measurements, leading to distance measurement with nanometric accuracy. A resolution of 100 pm has been achieved in 75 μs with a relatively simple laser source. Amplitude to phase coupling in the detection chains turns out to be the largest source of systematic errors. A specific detection scheme is implemented, using optical demodulation of the microwave optical signal, to reduce amplitude-to-phase related systematic errors to below the required level.

International audience; The Time Transfer by Laser Link (T2L2) project is designed for the comparison of ground clocks utilizing a space instrument able to time tag laser pulses reaching the satellite and a SLR station network. Compared to the microwave techniques, such as the GPS-Common View, T2L2 should improve the time transfer uncertainty by at least one order of magnitude. The accuracy of this optical technique gives the opportunity to compare todays most accurate frequency standards. This poster presents the results of the direct comparison between T2L2 & GPS-CV, along with the independent link calibrations.

International audience; 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 be- tween the two techniques remaining in average below 300 ps, with a standard-deviation below 500 ps mostly due to GPS CV.

International audience; When implementing a diluted telescope with large dimensions, one has to reach the equal path condition to the different segments of the primary mirror. In this work we suggest a way to implement a fast laser ranging method able to provide the error signal, using phase detection of the microwave modulation of a laser beam.

International audience; Some measurement schemes have been proposed and realized for the absolute measurement of long distances with an accuracy better than 10 nm. Published measurement setups use one or even two laser frequency combs. But significant engineering will be required to space qualify such a system. Simple methods, less technology-demanding would be valuable in the perspective of embedded instrumentation. We have designed and implemented a two-mode interference measurement scheme that allows sub-nanometer scale resolution in long distance measurement. The two-mode interference signal contains both - (sub-µm) interferometric information: the scale is the optical wavelength - (sub-15mm) modulation phase information: the scale is the "synthetic wavelength" corresponding to the frequency of the beat-note of the two modes. With the addition of a time-of-flight (ToF) measurement, the method allows to combine the three data (ToF, synthetic wavelength and interferometric) in a single, high-resolution, high accuracy length measurement, obtained every 50 ms. A measurement update rate of 100 µs is possible, but may rely on the availability of significantly higher data processing rates in the FPGA phase-meter. Implementation of this scheme has required that systematic errors on the phase and amplitude of the microwave optical signal be kept at a level well below 10^-4 cycle and 10^-4 respectively. One consequence of this requirement is the replacement of any parallel optics in the optical setup by wedged optics, so that multiple reflections do not interfere with the measurement and reference beams to better than 10^-8 in optical power. Systematic errors of electronic origin are more difficult to deal with because the amplitude-to-phase (AM-to-PM) couplings effects at 20 GHz appear to have, not only an instantaneous contribution, but also a transient contribution. This contribution, related to the heating of the photodiode junction under the dissipated Joule power, exceeds the limit of 10^-4 cycle by roughly two orders of magnitude. This thermal behaviour is not purely exponential with time and cannot be accurately corrected for. We will present the implementation of the setup, and the way we have suppressed, by 3 orders of magnitude, the AM-to-PM coupling effect by modifying the detection scheme of the 20 GHz beatnote. This last point is important, not only for the range meter presented, but also for in high accuracy and low phase noise microwave optical links.

Improving the angular resolution of telescopes implies an increase of their size. Spacecrafts in formation flying are more and more investigated for future space telescope projects. Operation of such systems requires absolute distance measurement between two spacecrafts with an accuracy of few tens of microns. The absolute distance measurement with high precision is also at the centre of fundamental physic experiments. My thesis work has consisted in the development of two rangefinders, T2M and Iliade which could be used for these projects. The difficulty is to combine a great measurement range with high sensitivity in a simple set-up for being eventually embedded instruments. Our goal is to measure a distance of one kilometre with an accuracy better than one micron. The difference between T2M and Iliade resides in their designs but also in their complexity and their resolution. The T2M rangefinder uses the phase difference of a beam modulated in amplitude and split in a measurement path and a reference path. The target resolution with this system is in the tens of nanometres. T2M is able to reach a high accuracy because the distance is obtained from frequency measurements. Cyclic errors linked to the use of phase comparison are suppressed using an optical switching system which exchanges the beams of the two paths with respect to the two detection chains. After having stabilized the frequency of the beatnote between the two laser, I have worked on the improvement of the resolution. I have also worked on the suppression of different errors which affect the accuracy of T2M. Objectives seem to be reached and the results are very encouraging for the future tests and developments. The Iliade rangefinder combines a time of flight measurement between optical pulses and a two-wavelength interferometric measurement. This sophisticated system compared to T2M, is expected to have a resolution below one nanometer. I have characterised the 20 GHz pulse source and shown that the 65 fs timing jitter is mainly limited by the phase noise of the initial two-mode beat-note. I have worked on a fibered Fabry Perot cavity for single-mode laser frequency pre-stabilization and demonstrated a scheme for obtaining a dispersive-shaped error signal without any modulation. I have studied a numerical method based on harmonic analysis in order to measure phase difference with an accuracy better than 10-5 radian. The Iliade rangefinder is presently under development.; Si les performances des instruments d'exploration de l'Univers continuent de s'améliorer, c'est souvent au prix d'une augmentation de leurs dimensions. Pour le développement de nouveaux télescopes spatiaux, le vol en formation de satellites est une solution technique de plus en plus envisagée. La mesure absolue des distances entre satellites avec une exactitude de l'ordre de quelques dizaines de microns est alors une nécessité pour l'exploitation des données obtenues avec de tels systèmes. Elle est aussi au cœur d'expériences de physique fondamentale. Mon travail de thèse a porté sur le développement de deux télémètres, T2M et Iliade pouvant répondre à ces besoins. La difficulté dans le développement de ces deux systèmes est de concilier un grand intervalle de mesure de distances avec une grande sensibilité tout en gardant une certaine simplicité et ainsi permettre que ces instruments puissent être embarqués. La performance visée est une mesure de distance de l'ordre du kilomètre exacte à mieux d'un micron. La différence entre T2M et Iliade réside dans leurs conceptions mais aussi dans leurs complexités et les performances visées en termes de résolution. Le télémètre T2M exploite la différence de phase d'une onde modulée en amplitude séparée entre une voie de mesure et une voie de référence. La résolution visée avec ce système est la dizaine de nanomètres. Ce système est capable d'atteindre une grande exactitude du fait que la mesure de distance est déduite d'une mesure de fréquence. Les problèmes d'erreurs cycliques inhérentes à l'utilisation de ce type de techniques sont éliminés par un système d'aiguillage optique permettant d'échanger les faisceaux propres aux deux voies de télémétrie en regard des deux voies de détection. Après la stabilisation du battement de fréquences des deux lasers, mon travail a porté sur l'amélioration de la résolution et sur l'élimination des diverses sources d'erreurs affectant l'exactitude. Les objectifs fixés semblent être atteints et les résultats obtenus sont très encourageants pour les futurs tests et développements à réaliser. Le télémètre Iliade combine une mesure de temps de vol d'impulsions laser et une mesure interférométrique à deux longueurs d'onde. Au prix d'une sophistication un peu plus importante que T2M, la résolution visée est inférieure au nanomètre. La caractérisation de la source d'impulsions d'Iliade présentant un taux de répétition de 20 GHz, a permis de montrer que la gigue temporelle de 65 fs à sa sortie est principalement due au bruit de phase du battement initial de deux lasers monomodes. J'ai travaillé sur une cavité Fabry Perot fibrée pour pré-stabiliser le rayonnement d'un laser monomode. J'ai démontré une technique permettant d'obtenir un signal d'erreur de type dispersif, sans recourir à une modulation. Enfin j'ai étudié une méthode numérique basée sur l'analyse harmonique permettant d'assurer une mesure de différence de phase avec une exactitude de 10-5 radian. Le télémètre Iliade est actuellement en cours de développement.

Improving the angular resolution of telescopes implies an increase of their size. Spacecrafts in formation flying are more and more investigated for future space telescope projects. Operation of such systems requires absolute distance measurement between two spacecrafts with an accuracy of few tens of microns. The absolute distance measurement with high precision is also at the centre of fundamental physic experiments. My thesis work has consisted in the development of two rangefinders, T2M and Iliade which could be used for these projects. The difficulty is to combine a great measurement range with high sensitivity in a simple set-up for being eventually embedded instruments. Our goal is to measure a distance of one kilometre with an accuracy better than one micron. The difference between T2M and Iliade resides in their designs but also in their complexity and their resolution. The T2M rangefinder uses the phase difference of a beam modulated in amplitude and split in a measurement path and a reference path. The target resolution with this system is in the tens of nanometres. T2M is able to reach a high accuracy because the distance is obtained from frequency measurements. Cyclic errors linked to the use of phase comparison are suppressed using an optical switching system which exchanges the beams of the two paths with respect to the two detection chains. After having stabilized the frequency of the beatnote between the two laser, I have worked on the improvement of the resolution. I have also worked on the suppression of different errors which affect the accuracy of T2M. Objectives seem to be reached and the results are very encouraging for the future tests and developments. The Iliade rangefinder combines a time of flight measurement between optical pulses and a two-wavelength interferometric measurement. This sophisticated system compared to T2M, is expected to have a resolution below one nanometer. I have characterised the 20 GHz pulse source and shown that the 65 fs timing jitter is mainly limited by the phase noise of the initial two-mode beat-note. I have worked on a fibered Fabry Perot cavity for single-mode laser frequency pre-stabilization and demonstrated a scheme for obtaining a dispersive-shaped error signal without any modulation. I have studied a numerical method based on harmonic analysis in order to measure phase difference with an accuracy better than 10-5 radian. The Iliade rangefinder is presently under development.; Si les performances des instruments d'exploration de l'Univers continuent de s'améliorer, c'est souvent au prix d'une augmentation de leurs dimensions. Pour le développement de nouveaux télescopes spatiaux, le vol en formation de satellites est une solution technique de plus en plus envisagée. La mesure absolue des distances entre satellites avec une exactitude de l'ordre de quelques dizaines de microns est alors une nécessité pour l'exploitation des données obtenues avec de tels systèmes. Elle est aussi au cœur d'expériences de physique fondamentale. Mon travail de thèse a porté sur le développement de deux télémètres, T2M et Iliade pouvant répondre à ces besoins. La difficulté dans le développement de ces deux systèmes est de concilier un grand intervalle de mesure de distances avec une grande sensibilité tout en gardant une certaine simplicité et ainsi permettre que ces instruments puissent être embarqués. La performance visée est une mesure de distance de l'ordre du kilomètre exacte à mieux d'un micron. La différence entre T2M et Iliade réside dans leurs conceptions mais aussi dans leurs complexités et les performances visées en termes de résolution. Le télémètre T2M exploite la différence de phase d'une onde modulée en amplitude séparée entre une voie de mesure et une voie de référence. La résolution visée avec ce système est la dizaine de nanomètres. Ce système est capable d'atteindre une grande exactitude du fait que la mesure de distance est déduite d'une mesure de fréquence. Les problèmes d'erreurs cycliques inhérentes à l'utilisation de ce type de techniques sont éliminés par un système d'aiguillage optique permettant d'échanger les faisceaux propres aux deux voies de télémétrie en regard des deux voies de détection. Après la stabilisation du battement de fréquences des deux lasers, mon travail a porté sur l'amélioration de la résolution et sur l'élimination des diverses sources d'erreurs affectant l'exactitude. Les objectifs fixés semblent être atteints et les résultats obtenus sont très encourageants pour les futurs tests et développements à réaliser. Le télémètre Iliade combine une mesure de temps de vol d'impulsions laser et une mesure interférométrique à deux longueurs d'onde. Au prix d'une sophistication un peu plus importante que T2M, la résolution visée est inférieure au nanomètre. La caractérisation de la source d'impulsions d'Iliade présentant un taux de répétition de 20 GHz, a permis de montrer que la gigue temporelle de 65 fs à sa sortie est principalement due au bruit de phase du battement initial de deux lasers monomodes. J'ai travaillé sur une cavité Fabry Perot fibrée pour pré-stabiliser le rayonnement d'un laser monomode. J'ai démontré une technique permettant d'obtenir un signal d'erreur de type dispersif, sans recourir à une modulation. Enfin j'ai étudié une méthode numérique basée sur l'analyse harmonique permettant d'assurer une mesure de différence de phase avec une exactitude de 10-5 radian. Le télémètre Iliade est actuellement en cours de développement.

21 pages; National audience; A laser ranging method is described that uses the phase of an amplitude-modulated laser beam: the frequency of the modulation is permanently adjusted to maintain a constant dephasing, and the distance is deduced from its measurement. We show that phase drifts and cyclic errors are suppressed using a fast polarization switch that interchanges the roles of the reference and measurement paths. Preliminary measurements are obtained using this novel design, showing a measurement stability better than 10 nanometres. The comparison of a displacement with an interferometric system shows an agreement of the order of one micron. An analysis of the source of the systematic errors suggests that an accuracy better than one micron can be expected with this very simple set-up.

Nous présentons deux télémètres basés sur un faisceau laser modulé en amplitude à haute fréquence (HF). L'information de distance est extraite de la phase du faisceau renvoyé par la cible. Dans la première mise en oeuvre proposée, l'erreur cyclique associée à une diaphonie optique entre la voie de référence et la voie de mesure est absente, compte tenu du choix qui consiste à travailler à différence de phase nulle. Dans la deuxième mise en oeuvre, elle est au contraire mise à profit pour fournir le signal interférométrique permettant la haute résolution. Les télémètres sont dédiés à la mesure absolue de distances comprises entre le mètre et le kilomètre, avec une précision submicronique, notamment pour les missions spatiales de vols en formation.