In a first part, we will briefly recall the basic principles of atomic clocks and discuss the main trends in the domain. We will then present an overview of the current developments that may lead to a new generation of space atomic clocks, in particular for satellite positioning and navigation systems. Some technological challenges, such as the availability of suitable laser sources and other opto-electronics components will also be discussed.

1. INTRODUCTION
High stability and accuracy time scales are enabling tools for cutting-edge technological and scientific applications. Extremely demanding applications, like GNSS operation and interoperability, as well as fundamental physics tests, exploit the state-of-the-art time scale performances and can take advantage by future improvements.
Atomic fountains, first developed in National Metrology Institutes as Primary Frequency Standards, are now able to run reliably for extended periods, and can be used as clocks providing long-term stability performances to time scales. Some important laboratories are now using one or more fountains in their time scales systems, with simple algorithms and reducing the need of large clock ensembles.
Rb fountains have some convenient properties with respect to Cs for long period reliable and stable operation, such as a reduced density shift. Laser cooling is accessible to frequency-doubled Er:doped fiber systems, a laser technology which has become very reliable in the last few years.

2. DESIGN AND REALIZATION
In the period 2014-2016, two Rb fountains have been realized within a collaboration among Vremya-CH (private company, Russia), Muquans (private company, France) and SYRTE laboratory (public sector, France) for VNIIFRI, the National Metrogy Institute of the Russian Federation responsible for the generation of UTC(SU) time scale (involved in the GLONASS GNSS program). The two fountains feature a compact footprint, robust set-up, and they are designed for long period, uninterrupted operation. They do not require on-site personnel for ordinary maintenance and they are completely remote operated. The tight realization schedule and the fabrication of pre-assembled parts by three different partners has required a semi-industrial engineering approach.
The two fountains shares the same design, except for the atomic source for the optical molasses, which is hot vapour for one (Ver. A) and 2D-MOT cooled beam for the other one (Ver. B).

3. STABILITY PERFORMANCES AND RELIABILITY
The two Rb fountains started their operations in 2016 and their stability and reliability performances grew as far as the operation parameters were optimized and some critical components replaced with updated versions. As the fountains use a high performance H-maser (VCH-1003M opt.L, produced by Vremya-CH) as a local oscillator (LO), their stability optimization relies an accurate trade-off between quantum noise and noise aliased from the LO.
In 2018, they reached a remarkable level of performance for a fountain driven by an H-maser as a LO: the stability at τ =1 s is σy(τ)=1.15x10^-13 for Ver. A and σy(τ)=7.5x10^-14 for Ver. B. This values allow the two fountains to match or even surpass the stability requirements of the project (σy(τ)= 4x10^-16 at τ =1 day and σy(τ)= 1x10^-16 at τ =16 days) and qualify them among the most stable microwave clocks for medium/long integration times. Since several months, the two fountains are simultaneously running at their best performance level and they participate to the free-running time scale TA(SU) on a regular basis.

Topics: M01, M03

We present our developments towards a compact and high-performance vapour-cell atomic clock with a frequency stability of better than 1x10^-14 at one day timescales, for applications such as on-board atomic clocks in satellite navigation systems. Our clock employs a time-domain Ramsey double-resonance (DR) scheme [1] applied to a thermal rubidium vapour held in a custom-made vapour cell. This approach allows ensuring a GNSS-grade clock performance from a highly compact instrument. The clock setup mainly comprises of a compact magnetron-type microwave cavity resonator [2] holding the vapour cell, and an in-house-made stabilized laser head with pulsed optical output [3]. Previously we reported on the expected performance advantages of this clock approach [4] compared to the simpler continuous-wave DR scheme and on the achieved short-term stability [1].

Here we report on the experimental evaluation of the long-term stability of our Ramsey DR vapour-cell clock setup, focussing on the analysis of various instability contributions at the level of 1x10^-14 or below, at long-term timescales around one day. Under conditions of free atmosphere on ground, a dominant contribution to the clock instability arises from the barometric effect, i.e. the sensitivity of the clock frequency to environmental pressure variations mediated by the vapour cell [5]. We quantified the barometric effect and reduced its impact by two orders of magnitude.

When operating the clock under stable external pressure, the measured clock instability reaches <2x10^-14 at one day. The main instability contribution arises from intensity light-shift, due to fluctuations in light intensity at the clock’s vapour cell. Other effects, namely the cell and stem temperatures effect, second-order Zeeman shift, frequency light-shift, and microwave power-shift effects, give instability contributions of <4x10^-15.

At the conference, we will present the detailed instability budget for the clock’s long-term stability. Possible directions for further improvements will be discussed, in view of the realization of a future compact and high-performance Ramsey DR vapour-cell clock for space applications.


This work was supported by the Swiss National Science Foundation, grant 156621. It also received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no 820393, European Quantum Flagship project macQsimal. We acknowledge previous support from the European Metrology Research Programme (EMRP project IND55-Mclocks), the European Space Agency, and the Swiss Space Office.


References

[1] S. Kang, M. Gharavipour, C. Affolderbach, F. Gruet, G. Mileti, Journal of Applied Physics 117, 104510 (2015).

[2] C. Stefanucci, T. Bandi, F. Merli, M. Pellaton, C. Affolderbach, G. Mileti, A. K. Skrivervik, Review of Scientific Instruments 83, 104706, (2012).

[3] S. Kang, M. Gharavipour, F. Gruet, C. Affolderbach, G. Mileti, proceedings of the joint IFCS-EFTF Conference, Denver, USA, April 12-16, 2015, pp. 800-803.

[4] C. Affolderbach, M. Gharavipour, S. Kang, F. Gruet, G. Mileti, Towards a highly compact Rb atomic clock with improved stability for space applications, 5th International Colloquium on Scientific and Fundamental Aspects of the Galileo Programme, 2015.

[5] W. Moreno, M. Pellaton, C. Affolderbach, G. Mileti, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 65, 1500 – 1503 (2018).

A novel concept of the ONCLE (ONe CLock Ensemble) for a robust time and frequency reference system was firstly proposed and has been developed and verified by Spectratime since 2009. With minimum three input clocks, the system is able to generate an output signal with improved robustness and performances, by advanced features in real-time: 1) clock ensemble based on the weighted averaging, 2) clock fault detection and correction based on a cascade of low-pass recursive filters and associated logic, and 3) steering loop to keep all clocks in phase and frequency. Aiming at a simple and reliable solution, the algorithms can be implemented possibly by a FPGA for space application or by a very simple industrial micro-controller chip for ground application.
In the first generation of the Galileo onboard timing system, vulnerabilities and risks have been identified. In the frame of the European GNSS Evolution Programme (EGEP)*, the feasibility of hardware and algorithm approaches to realize the ONCLE concept was demonstrated, and the algorithm performances have been verified. During 2011-2014, Spectratime developed an Elegant BreadBoard (EBB) of the Robust On-board Frequency Reference Subsystem (FRS) with the support of GMV. An Engineering Model (EM) of the On-board Clock Ensemble CMCU (CMCU+) has been developed in the cooperation of Airbus DS and Spectratime during 2015-2019. In this phase, apart from the hardware subsystem contributions, Spectratime has been in charge of algorithms design, development and verification. High-level performance verification tests of CMCU+ EM with various scenarios have been conducted, and improved availability, robustness and stability on CMCU+ output have been demonstrated successfully.
The results and experiences gathered for on-board timing system have provided the substantial background at Spectratime to propose the ONCLE solution for the time system on ground, targeting for the next generation of the Precise Timing Facility (PTF) of the Galileo ground segment. The limitations of the current PTF are that the short-term stability of the Galileo System Time (GST) relies on one master Active Hydrogen Maser (AHM), and the real-time handling of feared events on the master clock (unless clock signal loss) is not allowed which detriments the continuity of the GST. The proposed solution will ensure a fully continuous and performance improved GST. The simulation results with four high-performance AHMs have demonstrated the capabilities to provide a robust timescale generation. Moreover without the need of the human intervention this integrated solution is expected to simplify the operation and thereby the operation cost for ground segments.
* The work of FRS and CMCU+ reported in this paper has been supported under contracts of the ESA in the frame of the EGEP. The views presented in the paper represent solely the opinion of the authors and should be considered as R&D results not necessarily impacting the present Galileo system design.