MISSIONS AND SERVICES FOR METROLOGY AND TO THE SOCIETY

The time and frequency teams of the three observatories in Besançon, Côte d’Azur and Paris work in coordination at the national level in this metrology field. One of their missions is to establish national time and frequency references, to make them available to all users, to facilitate their access and use and, through studies, to improve these references. The missions and services provided, on the one hand, fulfill the needs of the astronomy and the astrophysics communities and, on the other hand, fulfill the requirements of this community towards the country, in terms of scientific, legal, civil metrology, etc. The INSU (Institut National des Sciences de l’Univers, one of the CNRS Institutes), the Laboratoire National de Métrologie et d’Essais (LNE) which steers national metrology in all fields, the universities and observatories supervising the laboratories, the laboratories own resources, are the main sources of funding supporting these missions.

At the national level, the three laboratories collaborate on the generation and on the permanent availability of national time and frequency references for scientific users, legal metrology, etc. For applications requiring the highest performance, they form the basis of a national network made up of about a dozen organizations including CNES, DGA, Orange (formerly France Telecom), and research laboratories. For other applications, the references are made available to users by multiple methods (ALS162, GPS and GALILEO links, internet, SCPTime, etc.). In addition, the three laboratories provide support for specific scientific projects.

At the international level, they actively participate in programs and collaborations that allow to produce and to connect international references: TWSTFT and GPS remote clock comparisons, provision of clock data to the Bureau International des Poids et Mesures (BIPM) to contribute to the computation of the Coordinated Universal Time (UTC). They also participate in major infrastructure projects requiring time and frequency references, such as the European satellite navigation system Galileo.

All these activities are strongly operational. Since these time references are broadcast, they are visible in real time to many users. It is therefore necessary to maintain the ensemble of clocks and the instrumentation operated by the observatories at the best level and to have redundancies in order to exclude any interruption in timescales. Maintaining services at the best level is only possible if it is supported by dynamic research.

(Note: the time intervals handled on a daily basis in the observatories are generally very short: for information, 1 nanosecond -ns- is one billionth of a second, 1 picosecond -ps- is one thousandth of a nanosecond).

At Observatoire de Paris, the SYRTE laboratory hosts the Références Nationales de Temps (RNT) service. It is responsible for producing and disseminating two timescales: UTC(OP) which is the real-time representation for France of UTC (Coordinated Universal Time) computed each month by the BIPM. It is carried out using a hydrogen maser whose output signal is frequency-steered thanks to the calibrations of the laboratory’s primary standard, the atomic fountains. UTC(OP) is the Legal Time basis disseminated in France, and is one of the best timescales in the world, remaining within a few nanoseconds close to UTC since several years, which approaches the uncertainty of the time transfer techniques. The primary link of UTC(OP) to UTC is ensured by the  TWSTFT OP-PTB link. Its combined uncertainty, as calculated and published by the BIPM, is less than 2 ns, which constitutes one of the lowest  uncertainties of the TAI network; the French Atomic Time -TA(F) is a timescale calculated each month from data from about thirty high-performance cesium clocks located in about ten French laboratories. It is also frequency-steered thanks to the SYRTE atomic fountains. The TA(F) constitutes a common reference with an accuracy around 10^-15 in normalized relative frequency provided to the contributing laboratories. The collected data are also sent to the BIPM as the French contribution to the computation of the free atomic timescale EAL,  the first step in the UTC calculation, which places France among the top 10 contributing countries. Maintaining the TA(F) timescale contributes to the improvement of the links between French laboratories of the scientific, industrial and military domains, promoting good practices in time and frequency metrology. To carry out these activities, the RNT service operates continuously about a hundred instruments, including in particular 3 commercial cesium thermal beam clocks, 5 hydrogen masers, time interval counters and frequency comparators, frequency offset generators and time transfer systems  by GNSS (GPS and  GALILEO) and by TWSTFT  (Two-Way Satellite Time and Frequency Transfer), allowing to perform comparisons between time references on a daily basis, at national and international levels. In parallel, many scientific experiments are developed in the laboratory (different techniques of satellite time and frequency transfer, GPS PPP, broadband TWSTFT, TWCP, TWSTFT/SDR, T2L2, contributions to the European satellite navigation system Galileo, future space mission ACES, new optical fibers time and frequency transfers under development, etc.). In addition, the RNT service ensures the connection to the two other observatories and to the laboratories contributing to TA(F), to other laboratories via the LTFB (at the Observatoire de Besançon), and to users via the EGNOS system. Finally, the SYRTE broadcasts its references to the public via SCPTime, NTP and ALS162 (formerly called France inter grandes ondes).

At Observatoire de la Côte d’Azur, GEOAZUR operates a strong theme of time and space measurement based on laser ranging instruments, clocks and both microwave and optical time transfer systems. The OCA’s time and frequency laboratory has become a time transfer reference station equipped with modern and adapted instrumental means. While maintaining a fundamental activity around the national and local references realization, the OCA’s time and frequency group has given new horizons including projects on high performance time transfers, on fundamental physics projects and on metrology of ranging techniques. The T2L2 Laser Link Time Transfer experiment, developed at CNES and GEOAZUR, allows to synchronize space and terrestrial clocks at the picosecond level. T2L2 is in the field of time metrology at the base of many applications such as the dissemination of timescales, navigation, telecommunications and fundamental physics experiments. In the continuity of the T2L2 project, a new laser synchronized on the local time scale of the observatory has been implemented to integrate the ESA ACES (Atomic Clock Ensemble in Space) project via the ELT (European Laser Timing) optical link. In addition, several metrology instruments (event timer and signal generator) have been developed within the framework of laser ranging and time transfer in collaboration with the industry.

The latest developments (2019-2020) include collaborations with INPHYNI and Lagrange/OCA laboratories on intensity correlation experiments. This project aims at synchronizing the MeO and C2PU telescopes with a 10 ps accuracy, ​​which allows intensity correlations measurements with a baseline of more than 100 m. This corresponds to an angular resolution of stars similar to the best amplitude interference interferometers such as the Very Large Telescope Interferometer (VLTI) or the Center for High Angular Resolution Astronomy (CHARA). Since the end of 2019, we have been participating in DORIS by testing the new 4G beacon and we are sharing with the SLR station the same maser-based timescale. This first step initiates spatial geodesy research through the creation of a co-located and synchronized multi-techniques Geodesic Observatory.

At Observatoire de Besançon, the LTFB relies on the common infrastructure of the two laboratories UTINAM and FEMTO-ST. It operates and maintains 3 high-performance commercial cesium clocks, 3 hydrogen masers, GPS, GALILEO and 2-way satellite links (TWSTFT) which ensure the traceability of all its references to the national references drawn up by SYRTE at Observatoire de Paris. A copy of the UTC(OP) national timescale is generated from one of the masers and the time difference measurements between this maser and UTC(OP) provided by the GNSS and TWSTFT; over the year 2020, this replica was maintained within 5 ns close to UTC(OP).

The LTFB’s metrological mission is to disseminate national references through calibration services under ISO/CEI 17025-2017 accreditation. The calibrated devices range from the simple quartz chronograph used in regulated industrial processes which require traceability to national references of the instruments used, to atomic clocks which can be calibrated either on site in the observatory’s premises or remotely by GNSS links (GPS/GALILEO).

These remote calibrations are based on equipment (SYREF receivers) developed within the laboratory itself. These compact GNSS receivers allow the dissemination of the national frequency reference through a connection service operated since the 1990s. Since 2003, 3 versions of the receiver have been developed to adapt to the evolution and (fast) obsolescence of the components and thanks to the feedback of the users on the successive versions. In total, around 25 receivers are deployed at customers’ premises, at SYRTE, at Observatoire de Besançon, and at the site of the ALS162 signal transmitter. Alongside the ultimate performance of atomic standards, Observatoire de Besançon remains a direct heir to the watchmaking tradition associated with the city of Besançon and the Franche-Comté region: its creation in 1885 was motivated to support local and national watchmaker industry. It therefore continues to deliver mechanical chronometers service reports and certifications (ISO 3159) to watch manufacturers willing to associate the excellence image of Observatoire de Besançon with their production.