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Over the decades, testing gravitational red-shift (GRS) based on microwave links has made great process, including the GPA experiment, the planned Atomic Clock Ensemble in Space mission, and the China Space Station (CSS). Until now, the formulations of microwave links are almost all based on the time comparison. However, there are advantages of using frequency comparison instead of time comparison to test GRS. Here we develop a tri-frequency combination method based on the measurements of the frequency shifts of three independent microwave links between a space station and a ground station. Aiming at the frequency links’ accuracy of $$3\times 10^{-16}$$ 3 × 10 - 16 , we should consider various effects, including the Doppler effect, second-order Doppler effect, atmospheric frequency shift, tidal effects, refraction caused by the atmosphere, and Shapiro effect, with accuracy levels of tens of centimeters. The CSS will complete construction in 2022, and the formulation proposed in this study will enable us to test GRS at an accuracy level of at least $$2\times 10^{-6}$$ 2 × 10 - 6 , which is one order higher than the present accuracy level of $$7\times 10^{-5}$$ 7 × 10 - 5 .

According to general relativity theory, a precise clock runs at different rates at positions with different geopotentials. Atomic Clock Ensemble in Space (ACES) is a mission using high-performance clocks and links to test fundamental laws of physics in space. The ACES microwave link (MWL) will make the ACES clock signal available to ground laboratories equipped with atomic clocks. The ACES-MWL will allow space-to-ground and ground-to-ground comparisons of atomic frequency standards. This study aims to apply the tri-frequency combination (TFC) method to determine the geopotential difference between the ACES and a first order triangulation station in Egypt. The TFC uses the uplink of carrier frequency 13.475 GHz (Ku band) and downlinks of carrier frequencies 14.70333 GHz (Ku band) and 2248 MHz (S-band) to transfer time and frequency. Here we present a simulation experiment. In this experiment, we use the international space station (ISS) orbit data, ionosphere and troposphere models, regional gravitational potential and geoid for Africa, solid Earth tide model, and simulated clock data by a conventionally accepted stochastic noises model. The scientific object requires stabilities of atomic clocks at least 3 × 10 −16 /day, so we must consider various effects, including the Doppler effect, second-order Doppler effect, atmospheric frequency shift, tidal effects, refraction caused by the atmosphere, and Shapiro effect, with accuracy levels of decimetres. This study is supported by the National Natural Science Foundations of China (NSFC) under Grants 42030105, 41721003, 41804012, 41631072, 41874023, Space Station Project (2020)228, and the Natural Science Foundation of Hubei Province of China under Grant 2019CFB611.

The Atomic Clock Ensemble in Space is a fundamental physics mission of the European Space Agency to be launched in August 2021. It relies on a high-performance clock onboard the International Space Station (ISS), a network of high-performance clocks on ground, a dedicated 2-way microwave link (MWL) enabling space-to-ground and ground-to-ground clock comparisons, as well as an optical link (ELT). PHARAO/SHM (Projet d’Horloge Atomique par Refroidissement d’Atomes en Orbite/Space Hydrogen Maser), the clock onboard the ISS, has a relative frequency accuracy at the $${10}^{-16}$$ level, a relative frequency stability (Allan deviation) equal to $${10}^{-13}/\sqrt{\tau }$$ ( $$\tau$$ being the integration time in seconds) and a time deviation of 12 picoseconds after one day of integration. The MWL is designed to reach a time deviation below 7 ps after one day of integration. While space-to-ground clock comparisons will enable precise tests of the gravitational redshift, tests of deviations from General Relativity at the $${10}^{-6}$$ level, and tests of local Lorentz invariance at the $${10}^{-10}$$ level, ground-to-ground clock comparisons will enable a search of the time variation of fundamental constants with uncertainty at the $${10}^{-17}$$ level after one year. In this contribution, we review the mission setup with a particular emphasis on the MWL, discuss the simulation and data analysis software developed to investigate mission performance, focusing on its primary scientific objective: the test of the gravitational redshift.

There will be better atomic clock system and micro-wave time comparison link in the space station, like Chinese Space Station and European ACES(Atomic Clock Ensemble in Space) project, than those in the GNSS(Global GNSS satellite System) satellites. Therefore, space station CV(Common View) will realize more accurate time comparison than GNSS CV in theory. But due to the orbit characteristic of the space station, there are some limitations if traditional GNSS CV method is applied to the space station. In order to solve these problems, the GNSS CV method is optimized and the method that is appropriate for the space station is proposed. First, the differences between GNSS and space station CV are studied, and the influences of orbit error on these two CV methods are analyzed in detail. Then traditional CV method is optimized to be fit for the space station. Finally, the performance of the optimized method is validated by simulated experiments. The simulation results show that the space station time comparison accuracy of several tens of picoseconds can be obtained by the optimized method. Furthermore, the problem of CV blind area is solved by the optimized method effectively.

Atomic Clock Ensemble in Space (ACES) is a mission designed to test Einstein’s theory of General Relativity from the International Space Station (ISS). A primary frequency standard based on laser cooled caesium atoms (PHARAO) and an active H-maser (SHM) generate a clock signal that is distributed to a network of clocks on the ground to perform space-to-ground comparison. With a fractional frequency stability of 1 × 10−16 after 10 days of integration time and an accuracy of 1 – 2 × 10−16, ACES will provide an absolute measurement of the gravitational redshift, it will search for time variations of fundamental constant, and perform Standard Model Extension (SME) tests. The ACES payload is currently completing its qualification tests before flying. The mission status, the latest test results, and the ACES performance for testing General Relativity are discussed.

The paper discusses the problematic of data timing in the framework of the space astrometry mission Gaia. For various reasons related both to astrometry and to the measuring principles of the Gaia instrument it is mandatory to assign a highly stable and accurate time tag for each observational point. For this purpose Gaia has a Rb clock on board. That on-board clock is a free-running oscillator and must be regularly synchronized with TCB which is used as the underlying relativistic coordinate time scale in the whole Gaia data processing. To monitor the reading of the on-board clock with respect to TCB (or any other timescale) a one-way clock synchronization scheme is implemented. This scheme takes into account all known theoretical effects (e.g., relativity, tropospheric delay, etc.) and allows one both to monitor the health of the on-board clock and to create a clock model at the accuracy of better than 1 microsecond.

We investigate the performance of the upcoming ACES (Atomic Clock Ensemble in Space) space mission in terms of its primary scientific objective, the test of the gravitational redshift. Whilst the ultimate performance of that test is determined by the systematic uncertainty of the on-board clock at 2-3 ppm, we determine whether, and under which conditions, that limit can be reached in the presence of colored realistic noise, data gaps and orbit determination uncertainties. To do so we have developed several methods and software tools to simulate and analyse ACES data. Using those we find that the target uncertainty of 2-3 ppm can be reached after only a few measurement sessions of 10-20 days each, with a relatively modest requirement on orbit determination of around 300 m., 31 pages, submitted to CQG

International audience; The Atomic Clocks Ensemble in Space (ACES/PHARAO mission, ESA & CNES) will be installed on board the International Space Station (ISS) next year. A crucial part of this experiment is its two-way microwave link (MWL), which will compare the timescale generated on board with those provided by several ground stations disseminated on the Earth. A dedicated data analysis center is being implemented at SYRTE—Observatoire de Paris, where our team currently develops theoretical modelling, numerical simulations and the data analysis software itself. In this paper, we present some key aspects of the MWL measurement method and the associated algorithms for simulations and data analysis. We show the results of tests using simulated data with fully realistic effects such as fundamental measurement noise, Doppler, atmospheric delays, or cycle ambiguities. We demonstrate satisfactory performance of the software with respect to the specifications of the ACES mission. The main scientific product of our analysis is the clock desynchronisation between ground and space clocks, i.e. the difference of proper times between the space clocks and ground clocks at participating institutes. While in flight, this measurement will allow for tests of general relativity and Lorentz invariance at unprecedented levels, e.g. measurement of the gravitational redshift at the level. As a specific example, we use real ISS orbit data with estimated errors at the 10 m level to study the effect of such errors on the clock desynchronisation obtained from MWL data. We demonstrate that the resulting effects are totally negligible.

“Atomic Clock Ensemble in Space” (ACES) is a mission in fundamental physics that will operate a new generation of atomic clocks in the microgravity environment of the International Space Station (ISS). The ACES clock signal will combine the medium term frequency stability of a space hydrogen maser (SHM) and the long term stability and accuracy of a frequency standard based on cold cesium atoms (PHARAO). Fractional frequency stability and accuracy of few parts in 1016 will be achieved. The on-board time base distributed on Earth via a microwave link (MWL) will be used to test fundamental laws of physics (Einstein’s theories of Special and General Relativity, Standard Model Extension, string theories…) and to develop applications in time and frequency metrology, universal time scales, global positioning and navigation, geodesy and gravimetry. After a general overview on the mission concept and its scientific objectives, the present status of ACES instruments and sub-systems will be discussed.

PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite), which has been developed by CNES, is the first primary frequency standard specially designed for operation in space. PHARAO is the main instrument of the ESA mission ACES (Atomic Clock Ensemble in Space). ACES payload will be installed on-board the International Space Station (ISS) to perform fundamental physics experiments. All the sub-systems of the Flight Model (FM) have now passed the qualification process and the whole FM of the cold cesium clock, PHARAO, is being assembled and will undergo extensive tests. The expected performances in space are frequency accuracy less than 3.10-16 (with a final goal at 10-16) and frequency stability of 10-13 τ-1/2. In this paper, we focus on the laser source performances and the main results on the cold atom manipulation.

This paper presents the distribution system providing the UTC(OP) time and frequency reference signals to the microwave time transfer ground terminal (MWL-GT) of the Atomic Clock Ensemble in Space mission (ACES) at SYRTE. UTC(OP) is generated in the first floor of the building B. The MWL-GT will be installed on the top of the building A inside the campus of Observatoire de Paris, which offers the clearest field of view of the ISS passes. An ensemble of 6 optical fibers and coaxial cables installed between the two locations allows to transmit and monitor the reference signals and to calibrate the delays.

The I-SOC (Space Optical Clock on ISS) mission [1] is an ESA mission whose main goal is testing the Einstein Equivalence Principle and performing relativistic geodesy from space. It will be based on a strontium lattice clock on the ISS, which will be compared with ground clocks using advanced frequency link technologies, optical and microwave. The space clock will have 1×10−17 fractional inaccuracy, and ground clocks intercomparisons will be possible at the 10−18 level.

[1] The Atomic Clock Ensemble in Space (ACES) will be a future ESA experiment which utilizes ultra-stable clocks on-board the International Space Station (ISS). This mission is expected to perform tests of fundamental physics (relativity, possible drift of fundamental constants with time) and at the same time allows to compare the ACES time reference with respect to ground stations by using a novel microwave link concept. However, uncorrected dispersive troposphere delays pose the risk of degrading the performance of this microwave link over longer integration periods. Thus, a semi-empirical correction model has been developed which is only based on input from meteorologic sensors at the ground stations. The proposed model has been tested with simulated ISS overflights at different potential ACES ground station sites, and it could be demonstrated that this model is capable to remove biases and elevation dependent features caused by the dispersive troposphere delay difference between the uplink and downlink. The model performs well at all sites by reducing the impact on all reasonable averaging time scales by at least 1 order of magnitude. Similar studies like this might be of importance for other time and frequency transfer instruments or future space geodetic instruments.

There are many different atomic frequency standard technologies but only few meet the demanding performance, reliability, size, mass, and power constraints required for space operation. The Jet Propulsion Laboratory is developing a linear ion-trap-based mercury ion clock, referred to as DSAC (Deep-Space Atomic Clock) under NASA’s Technology Demonstration Mission program. This clock is expected to provide a new capability with broad application to space-based navigation and science. A one-year flight demonstration is planned as a hosted payload following an early 2017 launch. This first-generation mercury ion clock for space demonstration has a volume, mass, and power of 17 L, 16 kg, and 47 W, respectively, with further reductions planned for follow-on applications. Clock performance with a signal-to-noise ratio (SNR)*Q limited stability of $1.5{\rm E}-13/\tau^{1/2}$ has been observed and a fractional frequency stability of 2E–15 at one day measured (no drift removed). Such a space-based stability enables autonomous timekeeping of $\Delta t with a technology capable of even higher stability, if desired. To date, the demonstration clock has been successfully subjected to mechanical vibration testing at the $14\,{\rm g}_{\rm rms}$ level, thermal-vacuum operation over a range of $42^\circ{\rm C}$ , and electromagnetic susceptibility tests.

Ultra-precise optical clocks in space will allow new studies in fundamental physics and astronomy. Within an European Space Agency (ESA) program, the Space Optical Clocks (SOC) project aims to install and to operate an optical lattice clock on the International Space Station (ISS) towards the end of this decade. It would be a natural follow-on to the ACES mission, improving its performance by at least one order of magnitude. The payload is planned to include an optical lattice clock, as well as a frequency comb, a microwave link, and an optical link for comparisons of the ISS clock with ground clocks located in several countries and continents. Within the EU-FP7-SPACE-2010-1 project no. 263500, during the years 2011-2015 a compact, modular and robust strontium lattice optical clock demonstrator has been developed. Goal performance is a fractional frequency instability below 1x10^{-15}, tau^{-1/2} and a fractional inaccuracy below 5x10^{-17}. Here we describe the current status of the apparatus' development, including the laser subsystems. Robust preparation of cold {88}^Sr atoms in a second stage magneto-optical trap (MOT) is achieved., Comment: 27 Pages, 15 figures, Comptes Rendus Physique 2015

Systems Engineering has successfully supported the space industry since its inception with methodologies and techniques to handle complex projects. However, the conventional design approach, ‘Document-Based Systems Engineering’ (DBSE), is more and more reaching its limits. This research evaluates the benefits and the cost associated with the paradigm of ‘Model Based Systems Engineering’ (MBSE) instead of DBSE by applying the Systems Modelling Language (SysML) in the frame of the Atomic Clock Ensemble in Space (ACES) project. ACES is indeed deemed to be a suitable project for this study as it includes both a very complex ground segment as well as a challenging space segment on board the International Space Station. Following an introduction to SysML and the ACES mission, we first develop the metrics on which the cost-benefit analysis is based on. Then, we explain the methodology and models used to perform the analysis which targets to characterise DBSE versus MBSE based on the ACES ground segment development. Then, we use the Analytical Hierarchical Process to determine weighted criteria where attention is also given to the transitional process between DBSE and MBSE. After a critical reflection upon the analysis methodology and its results, we focus on lessons-learned from the use of SysML for the implementation of MBSE in space projects. We have identified five key areas of lessons-learned for using MBSE with SysML itself as well as main deficiencies for Enterprise Architect, the tool used to implement SysML. We conclude with seven suggested improvements which are considered valuable to help improving the performance and acceptance of MBSE for the development of (space) projects in the future.

We present a new type of measurement and the first results of determination of the optical to electrical delay of a photon counting detector. This type of measurement has not been reported for photon counting. The absolute value of the time interval between the time of arrival of the signal photon onto the detector input aperture and the time when the electrical output signal exceeds the pre-defined level must be determined. The optical to electrical delay value is required for ground-to-space laser time transfer with picosecond accuracy. The laser time transfer link is under construction for the European Space Agency for its application in the experiment Atomic Clock Ensemble in Space. We have developed the measurement technique and have measured the detection delay of the solid state photon counter. The experiment is described along with the first results.

The development of techniques for the comparison of distant clocks and for the distribution of stable and accurate time scales has important applications in metrology and fundamental physics research. Additionally, the rapid progress of frequency standards in the optical domain is presently demanding additional efforts for improving the performances of existing time and frequency transfer links. Present clock comparison systems in the microwave domain are based on GPS and two-way satellite time and frequency transfer (TWSTFT). European Laser Timing (ELT) is an optical link presently under study in the frame of the ESA mission Atomic Clock Ensemble in Space (ACES). The on-board hardware for ELT consists of a corner cube retro-reflector (CCR), a single-photon avalanche diode (SPAD), and an event timer board connected to the ACES time scale. Light pulses fired toward ACES by a laser ranging station will be detected by the SPAD diode and time tagged in the ACES time scale. At the same time, the CCR will re-direct the laser pulse toward the ground station providing precise ranging information. We have carried out a ground-based feasibility study at the Geodetic Observatory Wettzell. By using ordinary satellites with laser reflectors and providing a second independent detection port and laser pulse timing unit with an independent time scale, it is possible to evaluate many aspects of the proposed time transfer link before the ACES launch.

In this article we give an overview of the applications of ultrastable clocks in space. We focus on the case of the ESA space mission ACES, which is scheduled for flight onboard the international space station in 2013. With a laser cooled cesium clock, PHARAO, a space hydrogen maser, SHM, and a precise time and frequency transfer system, MWL, several precision tests in fundamental physics can be performed such as a measurement of Einstein's gravitational frequency shift with 2 ppm sensitivity and a search for time variations of the fundamental physical constants at 10 -17 /year. We present the advancement of the various mission instruments and present briefly applications in geodesy and Global Navigation Satellite Systems (GNSS).

The Human Spaceflight, Microgravity, and Exploration (HME) Directorate of the European Space Agency is strongly involved in fundamental physics research. One of the major activities in this field is represented by the ACES (Atomic Clock Ensemble in Space) mission. ACES will demonstrate the high performances of a new generation of atomic clocks in the microgravity environment of the International Space Station (ISS). Following ACES, a vigorous research program has been recently approved to develop a second generation of atomic quantum sensors for space applications: atomic clocks in the optical domain, aiming at fractional frequency stability and accuracy in the low 10-18 regime; inertial sensors based on matter-wave interferometry for the detection of tiny accelerations and rotations; a facility to study degenerate Bose gases in space. Tests of quantum physics on large distance scales represent another important issue addressed in the HME program. A quantum communication optical terminal has been proposed to perform a test of Bell's inequalities on pairs of entangled photons emitted by a source located on the ISS and detected by two ground stations. In this paper, present activities and future plans will be described and discussed.

Laser time transfer is and attractive technique to transfer time ground to space with picosecond precision and systematic errors on the level of tens of picoseconds. Recently the European Laser Timing experiment is under construction in the frame of the European Space Agency mission Atomic Clock Ensemble in Space. The objective of this laser time transfer is synchronization of the ground based clocks and the clock on board the International Space Station with picosecond precision and the accuracy better than 50 picoseconds We are reporting on a progress in identification and calibration of the biases associated to both ground and space segment. To characterize the delays of a ground segment the Calibration Device has been developed and tested. It enables to calibrate the ground based laser systems for their systematic timing biases. As a result the ground to ground time transfer in an ELT experiment should be accomplished with systematic errors not higher than 25 picoseconds. To determine the delays of a space segment the new calibration procedure has been designed and tested as well. The calibration test results will be presented.

In navigation satellites constellation comprised of multi-satellite, introducing an atomic clock ensemble in space is beneficial for improvement of its measurement accuracy and ability of autonomous operation. This paper summarizes the methods adopted currently at home and abroad towards atomic clock ensemble management in ground or in space and then proposes an initial design scheme of ACES management on a single satellite or among different satellite clocks within constellation which is based on the designing conception of atomic clock ensemble in navigation satellite constellation. Considering generation and characterize measurement of clock ensemble signal, anomaly detection and processing of atomic clock, this paper gives analysis and recommendation as for the key technologies and factors affecting ACES management, which will provide technical basis in designing a continuous, reliable, stable, accurate ACES running system for BEIDOU global navigation satellite system project.

We develop a high-precision model for relativistic observables of the Atomic Clock Ensemble in Space (ACES) experiment on the International Space Station (ISS). We develop all relativistic coordinate transformations that are needed to describe the motion of ACES in Earth orbit and to compute observable quantities. We analyze the accuracy of the required model as it applies to the proper-to-coordinate time transformations, light time equation, and spacecraft equations of motion. We consider various sources of nongravitational noise and their effects on ACES. We estimate the accuracy of orbit reconstruction that is needed to satisfy the ACES science objectives. Based on our analysis, we derive models for the relativistic observables of ACES, which also account for the contribution of atmospheric drag on the clock rate. We include the Earth's oblateness coefficient $J_2$ and the effects of major nongravitational forces on the orbit of the ISS. We demonstrate that the ACES reference frame is pseudo-inertial at the level of accuracy required by the experiment. We construct a Doppler-canceled science observable representing the gravitational redshift. We derive accuracy requirements for ISS navigation. The improved model is accurate up to $, 24 pages, 2 figures, 2 tables, revtex4

International audience; In this paper we describe the cold atom clock PHARAO, designed for microgravity operation. All elements of the PHARAO engineering model have been manufactured and delivered to CNES, the French space agency. We present the clock design, its main characteristics, and initial science operation. PHARAO is one of the main components of the Atomic Clock Ensemble in Space payload that is scheduled to fly on board the International Space Station in 2010.

The Primary Atomic Reference Clock in Space (PARCS) mission is designed to perform certain tests of relativity theory, to study the performance of individual GPS space-vehicle clocks, to study the dynamics of atom motion in microgravity, to advance the state-of-the art for space clocks, and to serve as a pathfinder for precision instruments based on laser cooling of atoms. After a brief overview of the project, this paper discusses the specific objectives of PARCS, describes the key subsystems, and discusses the systematic frequency shifts that limit the accuracy of the clock. � 2005 Published by Elsevier Ltd on behalf of COSPAR.

With the start of the Space Station era a new group of fundamental physics experiments is being developed to take advantage of the unique conditions available. Recently a microwave cavity clock experiment was selected to perform experiments in relativity. The project has four main goals: an improved test of Local Position Invariance, improved Kennedy–Thorndyke and Michelson–Morley type experiments, and an enhancement of the performance of atomic clocks being developed elsewhere for use in space. We describe the background and status of the project, which involves the development of superconducting microwave oscillators with high frequency stability. On the International Space Station unwanted cavity frequency variations are expected to be caused mainly by acceleration effects due to residual drag and vibration, temperature variations, and fluctuations in the energy stored in the cavity. At present, acceleration effects appear to be the predominant limit, and a new cavity support system has been fabricated to reduce the effect, and three assemblies are in test. Fractional frequency stabilities in the low 10−16 range are expected on time scales of minutes.

This is a review of those experiments in the area of Fundamental Physics that are either approved by ESA and NASA, or are currently under development, which are to be performed in the microgravity environment of the International Space Station. These experiments cover the physics of liquid Helium (SUE, BEST, MISTE, DYNAMX, and EXACT), ultrastable atomic clocks (PHARAO, PARCS, RACE), ultrastable microwave resonators (SUMO), and particle detectors (AMS and EUSO). The scientific goals are to study more precisely the universality properties of liquid Helium under microgravity conditions, to establish better time standards and to test the universality of the gravitational red shift, to make more precise tests of the constancy of the speed of light, and to measure the particle content in space directly without disturbances from the Earth's inner atmosphere.

Recently the European Laser Timing experiment is under construction. It is an optical link prepared in the frame of the European Space Agency mission Atomic Clock Ensemble in Space. The objective of this laser time transfer is the synchronization of the ground based clocks and the clock on board the International Space Station with precision of the order of units of picoseconds and the accuracy of 50 ps. We are reporting on a progress and results in calibration of the system delays involved.

PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite), which is being developed by the French space agency CNES, is the first primary frequency standard specially designed for operation in space. PHARAO is the main instrument of the ESA mission ACES (Atomic Clock Ensemble in Space) [1]. ACES payload will be installed on-board the International Space Station to perform fundamental physics experiments. Last year [2], some results on two flight model (FM) sub-systems have been presented: Microwave Source performances and Cesium Tube operating as a cold atom clock by using the other engineering model sub-systems. All the FM sub-systems have now passed the qualification process and the whole FM of the cold cesium clock, PHARAO, has been assembled and will undergo extensive tests during the first semester of 2014. The results on the cold atoms manipulation and the metrological evaluation are presented.

The successful miniaturization of extremely accurate atomic clocks invites prospects for satellite missions to perform precise timing experiments. This will allow effects predicted by general relativity to be detected in Earth's gravitational field. In this paper we introduce a convenient formalism for studying these effects, and compute the fractional timing differences generated by them for the orbit of a satellite capable of accurate time transfer to a terrestrial receiving station on Earth, as proposed by planned missions. We find that (1) Schwarzschild perturbations would be measurable through their effects both on the orbit and on the signal propagation, (2) frame-dragging of the orbit would be readily measurable, and (3) in optimistic scenarios, the spin-squared metric effects may be measurable for the first time ever. Our estimates suggest that a clock with a fractional timing inaccuracy of $10^{-16}$ on a highly eccentric Earth orbit will measure all these effects, while for a low Earth circular orbit like that of the Atomic Clock Ensemble in Space Mission, detection will be more challenging., Comment: Accepted for publication in Physical Review D. Typos corrected

In 1994, the CNES initiated an ambitious Technology and Research Program in preparation of a new space clock development, the PHARAO project (PHARAO: Projet d'Horloge Atomique a Refroidissement d'Atomes en Orbite). PHARAO has been selected by ESA to be flown on an external Express Pallet of the International Space Station during the early utilisation phase. PHARAO is one of the main components of the European payload called ACES (Atomic Clock Ensemble in Space). This Cesium clock, using specific laser technics for cooling the atoms, is able to reach levels of stability and accuracy in the range 10 −16 – 10 −17 , which have never been achieved yet. But such performances do require an advanced technology, because off-the-shelf equipment would not fulfill the drastic specifications required for PHARAO. The paper presents the preliminary design of the PHARAO flight version which is currently based on new developments such as: • - an ultra-stable quartz oscillator at 10 MHz • - a low phase noise synthesis chain • - a laser with 100 KHz linewidth.

Original proposals and experiments on gravitation and fundamental metrology on the space station are described. These experiments were formulated in the Metrology and Gravitation Science Team, in two ESA industrial study contracts, on microsatellites and on time and frequency science, within the space station scenario. Although limited by the design constraints of the space station, the experiments range from clock-based tests on special and general relativity to, with additional infrastructure, the equivalence principle and the detection of gravitational waves. Supporting technology, such as damping systems and microgravity cooled atom clocks, is also described. Finally, the major scientific goals, the experiments, hardware and the status are summarized. This work represents the first coordinated attempt, at least within the European space programmes, to consider experiments on relativity and fundamental physics without resorting to experiment dedicated space missions. For details on specific issues a large bibliography is referred to.

The laser time transfer link is under construction for the European Space Agency for its application in the experiment Atomic Clock Ensemble in Space. We have developed and tested the photon counting detector optimized for operation in space for this project. In this paper are reported the main characteristics which changed after the irradiation by proton and gamma ionizing radiation. The resulting change of the detection delay with temperature is typically below 500 fs/K. The detector has been approved for application in the European Laser Timing experiment, the flight module is under manufacturing now.

PHARAO (Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite), which is being developed by the French space agency CNES, is the first Primary Frequency Standard (PFS) specially designed for operation in space. PHARAO is a main instrument of the ACES (Atomic Clock Ensemble in Space) mission. ACES is being developed by ESA and the payload will be installed on-board the International Space Station. The mission is based on comparisons with ground based clocks to perform fundamental physics experiments: gravitational redshift measurements, analysis of the stability of the fundamental constants and anisotropy of light. Planned duration of the mission is 18 months with a possible extension to 36 months. The frequency accuracy requirement for PHARAO is less than 3 × 10-16 and the expected frequency stability is 10-13Tau-1/2. An engineering model of PHARAO has been constructed and fully tested to validate the clock architecture. Two sub-systems for the flight model FM) have been delivered: the cesium tube where the atoms are manipulated and the microwave source which generates the 9.2 GHz signals. The other sub-systems, the laser source and the computer, will be delivered this year. Some performances tests have been performed on the microwave source and on the cesium tube assembled with the other EM subsystems. The whole FM clock will be assembled in 2013.

In many fundamental physical experiments time plays an important role. The standard way for the comparison of time and frequency is the application of GNSS signals and the Two-Way Satellite Time and Frequency Transfer - TWSTFT. This technique is based on radiofrequency signal transmission. Recently, there was a rapid increase of optical time comparison development, which uses the Satellite Laser Ranging network (SLR). Currently the French project T2L2 is in operation on board of Jason 2 and the European Space Agency project ELT in support of the Atomic Clock Ensemble in Space (ACES) is under development. The goal of both projects is the time synchronization with a precision below 40 ps rms and an absolute error well below 100 ps. Comparing the results of the optical time transfer with the GNSS time comparison requires unprecedented control of the local ties between the different observation techniques. One of the possible methods is the application of the Two Way Time Transfer (TWTT) on a single coaxial cable. Such a system can be implemented using two or more event timers, which are interconnected by a standard coaxial cable. The event timers are exchanging pulses between each other and time tagging them. Out of the measured result one can evaluate the difference of time scale represented by the event timers. It was shown that such a technique can be used for time transfer with a precision of a few picoseconds of rms and an absolute error below 20 ps for distances reaching several hundreds of meters. We have implemented this technique for establishing and monitoring the absolute time delays between the SLR system and the time keeping laboratory on the Geodetic Observatory Wettzell. The event timers were developed at the Czech Technical University. The measurement principal is based on SAW filter excitation. The Event Timers were located in different buildings 50 meters apart and with the help of the TWTT technique, the delay between them was measured. The second input sockets of the event timers were used to monitor the respective local timescale.

Atomic clocks are fundamental elements of a Global Navigation Satellite System (GNSS). Currently, several GNSSs are operational and the different clock technologies employed onboard their satellites benefit from the technological improvements achieved during the last decades. To ensure the timing capabilities needed for correct positioning, the analysis of GNSS clock performances is essential. We have recently started a performance analysis for GPS and GLONASS clocks, by using the satellite clock estimates produced by the Information-Analytical Centre (IAC) of the Russian Federal Space Agency. In this paper we discuss a few preliminary results of this analysis. We analyze the time deviation, frequency deviation, and frequency stability of the Cesium and Rubidium clocks onboard three GPS and GLONASS satellites. The obtained results highlight the presence of two common space clock anomalies, namely, deterministic oscillations and frequency jumps. Our final goal is to build a detailed statistics of the clock anomalies for all GNSSs.

This paper reviews key characteristics and requirements for Two-Way Satellite Time and Frequency Transfer (TWSTFT) links. The outline and perspective of two concepts are described: 1. The classical bend-pipe transponder, without special equipment on board, with the potential of 10-17/day frequency transfer uncertainty; 2. The ACES-MWL (Atomic Clock Ensemble in Space Microwave Link) like active on-board equipment, with the potential down to few 10-19/day, using both high-rate modulation and carrier-phase. An outlook to advanced links is given. The aim of these proposed links is to provide a unique platform for the demanding requirements in the field of time and frequency metrology and to support improved orbit determination and relativistic geodesy for the next decades.

The laser time transfer link is under construction for the European Space Agency in the frame of Atomic Clock Ensemble in Space. We have developed and tested the flying unit of the photon counting detector optimized for this space mission. The results are summarized in this Note. An extreme challenge was to build a detector package, which is rugged, small and which provides long term detection delay stability on picosecond level. The device passed successfully all the tests required for space missions on the low Earth orbits. The detector is extremely rugged and compact. Its long term detection delay stability is excellent, it is better than ±1 ps/day, in a sense of time deviation it is better than 0.5 ps for averaging times of 2000 s to several hours. The device is capable to operate in a temperature range of -55 °C up to +60 °C, the change of the detection delay with temperature is +0.5 ps/K. The device is ready for integration into the space structure now.

The ESA (European Space Agency) fundamental physics mission ACES (atomic clock ensemble in space) on the ISS (International Space Station) has a high demand on orbit determination and orbit prediction accuracy to fulfill its scientific goals. Therefore the experiment will be equipped with a multi-frequency JAVAD GNSS receiver. Due to constructional reasons, the GNSS antenna will be tilted from the zenith by 60° at an azimuth of 50°. In addition the field of view of the antenna will be severely obstructed by the solar panels of the ISS. A simulation study using a GPS signal simulator and a JAVAD GNSS receiver demonstrates the impact of these limitations on a GPS orbit solution of the ISS. The observations recorded by the GPS receiver show small data gaps, but in general the data quality is good enough to generate a continuous orbit and reach the required accuracy.

The International Space Station is equipped with Global Positioning System receivers that provide real-time position information at the 10m accuracy level. In preparation of the Atomic Clock Ensemble in Space experiment, measurements from Russian and American receivers have been used to assess the navigation accuracy that can be achieved through postprocessing of navigation solutions and raw data in a precise orbit determination process. In addition, the capability to accurately forecast the space station orbit for operation of microwave and laser terminals has been studied. It is shown that the orbit can be reconstructed with a 1 m position accuracy and a 1 mm/s velocity accuracy even from single-frequency Global Positioning System measurements. For the test period in mid 2006, short arc orbit predictions with a median error of 20 and 70 m could be obtained over forecast intervals of 6 and 12 h, respectively. The navigation accuracy obtained is compatible with the mission requirements for the relativistic correction of the atomic clocks and the quick look clock performance monitoring.

We are presenting the work progress and recent results in the development and construction of the photon counting receiver, which is prepared for the European Laser Timing experiment in space. It is an optical link prepared in the frame of the ESA mission Atomic Clock Ensemble in Space. The ultra short laser pulses will be used to synchronize the time scales ground to space with picosecond precision. To minimize the timing biases the photon counting concept of the space born receiver was selected. The requirements put on the photon counting receiver are quite challenging in terms of the long term detection delay stability, wide operation temperature range, extremely high background photon flux and others. Recently, the bread board version of the detector has been constructed and is under extensive test in our labs. The concept and construction will be presented along with the achieved device parameters.

In the global network of institutions engaged with the realization of International Atomic Time (TAI), atomic clocks and time scales are compared by means of the Global Positioning System (GPS) and by employing telecommunication satellites for two-way satellite time and frequency transfer (TWSTFT). The frequencies of the state-of-the-art primary caesium fountain clocks can be compared at the level of 10−15 (relative, 1 day averaging) and time scales can be synchronized with an uncertainty of one nanosecond. Future improvements of worldwide clock comparisons will require also an improvement of the local signal distribution systems. For example, the future ACES (atomic clock ensemble in space) mission shall demonstrate remote time scale comparisons at the uncertainty level of 100 ps. To ensure that the ACES ground instrument will be synchronized to the local time scale at the Physikalisch-Technische Bundesanstalt (PTB) without a significant uncertainty contribution, we have developed a means for calibrated clock comparisons through optical fibers. An uncertainty below 40 ps over a distance of 2 km has been demonstrated on the campus of PTB. This technology is thus in general a promising candidate for synchronization of enhanced time transfer equipment with the local realizations of Coordinated Universal Time UTC. Based on these experiments we estimate the uncertainty level for calibrated time transfer through optical fibers over longer distances. These findings are compared with the current status and developments of satellite based time transfer systems, with a focus on the calibration techniques for operational systems.

Atomic Clock Ensemble in Space (ACES) is a mission using high-performance clocks and links to test fundamental laws of physics in space. The ACES microwave link (MWL) will make the ACES clock signal available to ground laboratories equipped with atomic clocks. The ACES MWL will allow space-to-ground and ground-to-ground comparisons of atomic frequency standards. The MWL comprises the Flight Segment (FS) as part of the ACES payload as well as a distributed set of Ground Terminals (GT), collocated with the ground atomic clocks. MWL is a two-way, two-frequency link transmitting a carrier signal modulated by a PN code, both phase coherent with the local clocks. MWL is designed to reach a time resolution (TDEV) of 0.3 ps in less than 300 s of integration time. In addition to the MWL, operating in microwave domain, ELT is an optical link based on the exchange of laser pulses detected and time stamped in MWL in the local time scale in space and on ground. The engineering model of the MWL FS and the first prototypes of the MWL GT will be used for end-to-end testing of the MWL performance. The end-to-end test is using a set-up, where the MWL FS and GT are connected by cables through a test equipment that allows the full simulation of the signal dynamics during an overflight of the ISS over a MWL GT. The MWL design and test results of the individual elements will be briefly reported. The results of the end-to-end test will be presented and perspectives for clock comparisons with ACES discussed.

We are presenting a progress in a construction and indoor tests of the photon counting detector for the European Laser Timing (ELT) experiment. ELT is an optical link prepared in the frame of the ESA mission "Atomic Clock Ensemble in Space" (ACES). The objective of this laser time transfer is the synchronization of the ground based clocks and the clock on board the space station with precision of the order of units of picoseconds and the accuracy of 50 ps. The requirements put on the detector package are quite high — temperature stability of the delay better than 20 ps peak to peak within one satellite orbit, operation within a broad temperature, absolute calibration of the photon to electrical signal delay with precision 25 ps and others.

We are presenting the concept, design and construction of an optical receiver of the space segment for the time transfer ground to space using optical pulses and photon counting detection. Laser time transfer link is under construction for the European Space Agency (ESA) for its application in the experiment Atomic Clock Ensemble in Space (ACES). The device is expected to be launched toward the International Space Station in 2013. The objective of this laser time transfer is the synchronization of the ground based clocks and the clock on board the station with precision of the order of units of picoseconds and the accuracy of 50 picoseconds. The photon counting approach has been selected for on-board optical detection in order to reduce the systematic biases as much as possible. However, the background photon flux of the solar light scattered by the Earth atmosphere and the Earth surface with wide field of view is a challenge in the optical detector design. The optical receiver concept and the first experiment results of indoor tests are presented.

The Atomic Clock Ensemble in Space (ACES) is composed of two atomic clocks: one cold Caesium clock (PHARAO) and one active Space Hydrogen Maser (SHM). PHARAO is necessary to ensure outstanding long-term frequency stability (τ ≥ 3000 s) and accuracy, while SHM is mandatory for its ultimate frequency stability in the mid-term range (3 s ≤ τ ≤ 3000 s). The combination of the two clocks via a double servo loop (short-term and long-term), will ensure an ultimate frequency stability. This configuration takes advantage of the best frequency stability for each integration time (PHARAO frequency locked to SHM for the short and mid-term, and SHM frequency steered to PHARAO for the long-term).

The ESA mission Atomic Clock Ensemble in Space (ACES) will operate a new generation of atomic clocks on board the International Space Station (ISS) in 2013-2015 timeframe. The ACES payload will be attached externally to the European Columbus module. The ACES clock signal will reach fractional frequency stability and accuracy of 1 part in 10-16. A GNSS receiver will be connected to the ACES clock signal. Primarily, the GNSS receiver will ensure orbit determination of the ACES clocks using GPS, GALILEO/GIOVE, and possibly GLONASS satellite signals in the L1, L2, and L5/E5a bands. Orbit determination is important for the correct evaluation of relativistic corrections in the space-to-ground comparison of clocks. Secondarily, the receiver offers the potential to support additional functionality for remote sensing applications in the field of GNSS radio-occultation and GNSS reflectometry, exploiting opportunities arising from the new GPS and GALILEO/GIOVE signals. The ACES GNSS instrument consists of a state-of-the-art commercial-of-the-shelf JAVAD GNSS Triumph TRE-G3T receiver board. The receiver is connected to a GNSS antenna which will be directly mounted at the corner of the ACES payload. Antenna boresight is pointing +50° off the ISS flight direction and is tilted 30° toward the zenith direction. This offers ideal conditions to receive coherent reflected GNSS signals and improves radio occultation measurements. Within the ACES project the receiver will be ruggedized and tested for space environment. Initial tests performed by DLR with the Co-60 source in Euskirchen, Germany, indicate a high tolerance to total ionizing dose. The receiver sensitivity to harmful single event effects of ionizing radiation including single event upset (SEU) and latch-up (LU) has been characterized in SEE testing using the radiation test facility of Groningen, NL. The results will be used to design the protection system counteracting these effects. In addition the receiver will be accommodated in a double redundant architecture. Under simulated low Earth orbit (LEO) conditions the JAVAD Triumph receiver firmware demonstrated fast acquisition of GPS signals and respectable orbit accuracy/ performance. Current status and test results of the ACES GNSS instrument will be presented in this paper.

We are presenting the work progress and recent results in the field of the European Laser Timing instrumentation for the ACES time transfer using laser pulses. European Laser Timing (ELT) is an optical link presently under preparation in the frame of the ESA mission “Atomic Clock Ensemble in Space” (ACES). The on-board hardware consists of a comer cube retro-reflector (CCR), an optical receiver based on a single-photon avalanche diode (SPAD) and an event timer board connected to the ACES time scale. Short laser pulses fired towards ACES by a laser ranging station will be time tagged with respect to the ground time scale. They will also be detected in space by the SPAD diode and time tagged in the ACES time scale. At the same time, the CCR will re-direct the laser pulse towards the ground station providing precise ranging information. This procedure provides ground-to-space and ground-to-ground time transfer with a precision and accuracy outperforming the radiofrequency techniques. Extensive work has been done on the photon counting detector. Three different versions of the photon counter for space application have been developed and tested. The main areas of interest were: minimal power consumption requirements of the optical detector package, long term stability of detection delay and a broad operational temperature range. The detector shall be capable to operate within a temperature range of - 50° to + 50° C while keeping the detection delay stability on the picosecond level for temperature variations of 6.5 K peak-to-peak. The detection delay shall be characterized and controlled - e.g. delay between the event of photon absorption and the appearance of the electrical pulse on the detector output. A configuration of the optical receiver able to maintain uniform sensitivity over a wide field of view of 120 degrees was developed. The parameters of the optical receiver obtained in the ground tests meet safely all the requirements for the ELT mission.

Atomic Clock Ensemble in Space (ACES) is a pioneering mission that will use high performance atomic clocks to test fundamental laws of physics. Operated in the microgravity environment of the International Space Station, the ACES clocks, PHARAO and SHM, will generate a frequency reference reaching instability and inaccuracy at the 1·10-16 level. A high-performance link in the microwave domain (MWL) will distribute on ground the ACES signal allowing for comparisons of distant clocks to a frequency resolution of 1·10-17. ACES will connect ground clocks based on different atoms and atomic transitions in a worldwide network that will probe fundamental laws of physics to high accuracy. Space-to-ground and ground-to-ground comparisons of atomic frequency standards will be used to test Einstein's theory of general relativity including a precision measurement of the gravitational red-shift, a search for time variations of fundamental constants, and tests of special relativity. Applications in geodesy, GNSS remote sensing, optical time transfer, and ranging (via the ELT optical link) will also be supported. ACES has presently reached an advanced status of development, with engineering models of key instruments and subsystems completed and tested. The ACES Payload Critical Design Review and the Ground Segment Preliminary Design Review were recently completed. The ACES engineering model workbench has been integrated and characterized in a test campaign recently conducted at CNES facilities in Toulouse. The first prototype of the ACES MWL ground terminal is being assembled ready for delivery. The ACES ground segment architecture has been defined. Based on an extension of the standard Columbus USOC (User Support and Operations Center) located in CADMOS - Toulouse, the ACES USOC will remotely control the network of MWL ground terminals, and it will provide the necessary interface with the Columbus Control Center and the ACES user community. The current development status of the ACES mission elements will be presented and discussed. An overview of future planning will be given.