This paper presents the ONCLE (One Clock Ensemble) solution for the Galileo time and frequency reference system with advanced features in terms of robustness, performance, continuity, and simplicity. Each component clock is frequency-steered to the ensemble time, which itself creates an average of those steered clock outputs, while clock faults are detected and corrected in real time within an integrated system. The feasibility of algorithm and hardware approaches has been demonstrated on an elegant breadboard and verified by an extended test and validation campaign at the Engineering Model (EM) level, developed for the Galileo next-generation onboard timing system under European GNSS Evolutions Program. Based on the progress for space application and the heritage on the Galileo ground precise timing facility (PTF), we propose a robust solution for the upgrade of PTF for ground application aiming to provide a fully continuous and performance-improved timescale under automated operation. The capability is demonstrated by preliminary simulation results.

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.

we report on the progress towards a miniaturized rubidium timing source based on double resonance (DR) in 85Rb. The clock technology is based on Orelia's commercial rubidium clock adapted for low power and volume. The rubidium plasma-lamp has been replaced with a low power VCSEL (Vertical Cavity Surface Emitting Laser), and the size of the glass-blown rubidium vapor cell has been reduced to fit the complete physics package inside a commercial DIL-14 hermetic package. The clock volume is 50.8×50.8×19.5 mm 3 (50 cm 3 ) with a power consumption of 350 mW at room-temperature.

The on-board timing subsystem is the key part of the Global Navigation Satellite System (GNSS) payloads. An Engineering Model (EM) of the Galileo on-board Clock Monitoring and Control Unit for the next generation (called “CMCU+”) has been developed. To meet the CMCU+requirements tasks of algorithms have been carried out: to generate the output frequency signal based on an ensemble of input clocks with optimized performance and improved robustness by clock anomalies handling. In addition, the algorithms should be feasible for an on-board implementation. In this paper, we present the design, development, and in particular performance verification of algorithms for CMCU+.

A robust clock ensemble is proposed for the time and frequency reference system to improve the robustness and performance of the system. Studies on the feasibility of hardware and algorithm approaches have been conducted. All clocks in the ensemble are locked in phase and frequency via the steering loop. The system performs corrections on the master clock in function of weighted averaging of clocks to generate one ensemble output, and the clock fault detection and compensation is implemented in real time with minimum three clocks powered. As the design has been demonstrated on an elegant breadboard of the Robust On-board Frequency Reference Subsystem, this concept is proposed for the next-generation of Precise Timing Facility. Simulation results have demonstrated its capability and simplicity to provide a smooth and reliable timing or frequency output even in presence of clock feared events.

This paper describes the achieved results of a new space Rubidium Atomic Frequency Standard (named Robust-RAFS) through the description of the internal coefficients influence reduction and the positive consequences in term of clock frequency stability and predictability improvements. Performances achievements during uninterrupted operation of several months demonstrate a monotonic behavior, a stability of 1×10 -14 @10 5 sec. and a drift per day of few 10 -14 .

Accurate and ultra-stable space qualified atomic clocks represent critical equipment for the precision Global Navigation Satellite Systems (GNSS). The Passive Hydrogen Maser (PHM), with its excellent frequency stability performance, is the master clock for European Navigation satellite payload, and is the most stable clock ever flown for GNSS. Nine PHMs have been flying onboard Galileo satellites (GIOVE-B and four IOV satellites) since Apr. 2008. More than 35 PHM flight Physics Packages (PP) have been manufactured and characterized by Orolia Switzerland SA (Spectratime), under the industrial consortium led by Selex ES S.p.A. Besides radiation effects on electronic components, lifetime on PHM depends mainly on which of PP. In the frame of the “Lifetime Qualification of the PHM” supported by ESA, two Qualification Model (QM) units had been subjected to test under vacuum since 2008. After the first 1.5-year test period as reported in previously published papers [1][2], one QM has been extended for another 2-year lifetime test. This paper provides test results over the overall period of QM1, and gives further comparison and analysis of key PP parameters over 3.5 years of operation (or 4.1 years including the stay-alive period). The extended test enhances the on-ground test statistics and provides better confidences in the PHM lifetime evaluation, which shows the instrument capability to comply with the lifetime requirement of 12 years.

Accurate and ultra-stable atomic clocks have been recognized as the critical equipment for the precision Global Navigation Satellite Systems (GNSS). SpectraTime (SpT) and T4Science (T4S) are space and ground clocks manufacturers of Rubidium Atomic Frequency Standard (RAFS) and Active & Passive Hydrogen Maser (HM) for various navigation systems (European, Chinese and Indian) and other programs. From Dec. 2005 to the beginning of 2012, both clock technologies have years of flight heritage through four Galileo and 11 Beidou satellites. Almost 90 SpT RAFS flight units and 25 Passive HM Physics Package flight units have been manufactured and characterized. As for ground application, more than 17 T4S Active HMs are involved in different ground segment worldwide, and one passive HM is in progress in the frame of a ground development program. This paper describes for space RAFS and HM the onground performances and life-time tests, as well as onboard achieved clock performances. A short overview of the ground GNSS timing reference segment with its active Masers and associated disciplining algorithms will be given. Even these standard Rubidium and maser technologies have been proven to be highly reliable and robust those could be subject to perturbations and could exhibit some anomalies ,especially when exposed to single event radiations , magnetic field perturbations etc.... With those elements in hands, a presentation of novel onboard techniques to generate highly robust timing signal directly from the satellite onboard ONe CLock Ensemble (ONCLE) is presented. Performances achievements in presence of perturbations, and frequency jumps are also shown allowing a continuous and uninterrupted operation of the satellite navigation signals.

Atomic Clock Ensemble in Space (ACES) is an ESA mission in fundamental physics based on a new generation of clocks operated in the microgravity environment of the International Space Station.Installed at the external payload facility of the Columbus module, ACES will accommodate two atomic clocks: PHARAO, a primary frequency standard based on samples of laser cooled Cs atoms, and the active H-maser SHM. The two on-board clocks will generate a time scale with fractional frequency instability and inaccuracy of a few parts in 10(16). The ACES frequency reference will be distributed to ground by a MicroWave Link (MWL) and used to compare distant clocks. These comparisons will allow precision tests of the Einstein's theory of general relativity, including a measurement of the gravitational red-shift, a search for time variations of fundamental constants, and tests of the standard model extension. ACES will also support applications in different areas of research, including geodesy and GNSS remote sensing. A link in the optical domain is also part of ACES for time transfer experiments, ranging, and analysis of atmospheric propagation delays.The engineering models of the ACES clocks and main subsystems have been successfully tested, and manufacturing of the flight models has been started. Mission concept, scientific objectives, and status of ACES will be presented together with the latest test results. (C) 2011 Elsevier Ltd. All rights reserved.

We present a compact physics package based on coherent population trapping (CPT) in rubidium 85. The architecture of the package is described along with results obtained using optical- and microwave spectroscopy. We also report the stability of the CPT resonance to variations in operating parameters such as microwave modulation power and vapour cell temperature. With this package we obtain a short term frequency stability σy(τ) = 7·10-11·τ-1/2 (τ <; 100 sec) and σy(τ) ~ 1·10-11 ( 100 sec <; τ <; 104 sec).

Thanks to its good trade-off between reliability, mass, performance and cost, the Rubidium clock technology is often selected for various missions as navigation, astrophysical cartography, scientific measurement or secure communication. The typical mass of such device is between 1 and 5 kg for a frequency stability below 1×10-13 over several hours including environmental effects.

Accurate and stable frequency reference sources are critical for commercial, navigation, military and scientific space applications. Several levels of frequency references are suitable for space applications. This paper discusses similarities and differences among single distributed oscillators for communications satellites, master oscillator groups for communications systems, and atomic clocks for military and navigation systems. This paper builds on reference [1] and broadly describes frequency sources on current and upcoming global navigation satellite systems (GNSS). The three current systems are the Global Navigation Satellite System (GLONASS), the Global Positioning System (GPS), and the Galileo system. The upcoming navigation systems are: China's Compass satellite positioning system, Japan's quasi-zenith satellite system (QZSS), India's regional navigation satellite system (IRNSS), GPS-IIF, and GPS-III.

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).

Galileo navigation program is in progress under the technical supervision of the European Space Agency (ESA). The preliminary activities related to GSTBV2 experimental satellite provide the first results and the implementation of the in orbit validation (IOV) phase are in progress. Atomic clocks represent critical equipment for the satellite navigation system and clocks development has been continuously supported by ESA. The rubidium atomic frequency standard (RAFS) and the passive hydrogen maser (PHM) are at present the baseline clock technologies for the Galileo navigation payload. For the PHM, initial ground technological project related to lifetime possible limitation of the clock was initiated in parallel to satellite experimentation (GIOVE-B). Long duration frequency stability performance tests were recorded on ground demonstrating 2*10-15 clock stability at one day (including the drift). This article gives an overview on the ground lifetime data and performance of the PHM. Extrapolation for the 12 years Galileo mission duration is discussed.

Current integrity concepts such as the GIC (Galileo Integrity Concept) and the SBAS integrity concept are based on integrity indicators that are computed exclusively on ground, uplink to the satellites from the Up-Link Stations (ULS) and broadcast from the satellites as originally computed in the ground segment.This paper presents two new concepts based on the possibility of intercepting the integrity information inside the satellite, the "Satellite autonomous on-board orbit monitoring" and the "Satellite autonomous on-board clock monitoring" concepts, which could be traced univocally to non-existing Space Segment functionalities and which would be in position of:Accessing to local information, not available on ground, which could be used to detect the existence of non-nominal Signal-In-Space (SIS). Such local information includes clock environment, other local clocks, inertial measurement units, satellite thrusters control information or Clock monitoring & Control Unit (CMCU) phase noise observables.Detecting non-nominal SIS sooner than with the current baseline, improving the time-to-alert (TTA).Potentially using inter-satellite links observables, which implies range and doppler measurements not affected by atmospheric delays and ground local degradation (including multipath and interference). Cleaner measurements could permit to define sharper barriers or making the Signal In Space Monitoring Accuracy compatible with lower a-priori probability of non-nominal SIS.Adding independent barriers for the detection of hazardously misleading SIS information.The new satellite on-board integrity monitoring algorithms will be described. Then, the results of the experimentation campaign, carried out with synthetic data generated with simulation tools defined and implemented by GMV and INRIM, will be presented to demonstrate the performance of the new satellite on-board monitoring algorithms.Furthermore, an introduction of 'on board clock ensemble' will be made. This clock ensemble shall offer very robust solutions, high reliability and autonomous integrity monitoring and autonomous redundancy. Since the switch-over of defective or unstable clocks of the ensemble could be made without upload of new clock model, this solution will keep the satellite operational in any conditions of clocks anomalies.Finally, the experimentation results will be to assess the capability of the new satellite autonomous integrity concepts to diminish the non-nominal Signal-In-Space "a priori" probability and to reduce the TTA; resulting as a consequence on a increase of service integrity availability.

A major part of satellite based navigation systems is the atomic frequency standard (AFS). The first satellite navigation was realized in the 1960's, with the US Navy's navigation satellite system known as TRANSIT. The TRANSIT satellites were launched with quartz crystal oscillators (XOs) for stable and precise frequency generation. In 1964 the Navy started the TIMATION program, a predecessor to GPS. The TIMATION developmental satellites (TIMATION-1 and -2) used high performance XOs and time referenced ranging signals. In 1974, TIMATION-3 extended the 'state-of-the-art' in satellite navigation by orbiting very precise AFSs. The superior frequency stability of the AFSs made satellite navigation a practical system to operate. This pioneering work provided the stimulus for developing reliable AFSs for space applications. This paper discusses AFSs on current and upcoming navigation systems. The two current systems are the Russian Global Navigation Satellite System (GLONASS) Global Positioning System (GPS). The upcoming navigation systems with AFSs are: the Galileo system, China's Beidou (a.k.a. Compass) satellite positioning system, and Japan's quasi-zenith satellite system (QZSS). Other systems with AFS are introduced. These including the Gravity Probe-A (GP-A) experiment, the military strategic and tactical relay (Milstar), the Advanced EHF (AEHF) program, the navigation experiment (NAVEX), the Cassini- Huygens mission, the cesium clock in the primary atomic clock in space (PARCS) mission and the Projet d'Horloge atomique par refroidissement d'atomes en orbite (PHARAO) project. The future of AFSs are discussed including subminiature Rb, smart clock technology, optically pumped cesium, coherent population trapping (CPT) technology. The Advanced Technology Atomic Frequency Standard (ATAFS) program and the DARPA Chip Scale Atomic Clock (CSAC) program are mentioned as well at the hydrogen maser and the developments with trapped ion, optical, and cold atom clocks.

The pointing accuracy of satellite navigation systems relies to a great extent on the stability of the on-board atomic clocks.The Passive Hydrogen Maser (PHM) and the Rubidium Atomic Frequency Standard (RAFS) constitute respectively the master and the hot-redundant clock of Galileo Satellite Navigation System. Their development has been continuously supported by ESA.This article gives a general overview on the RAFS and the PHM current status and the new developments foreseen.

Two hydrogen masers (HMs) are used in the Precise Timing Facility to provide the physical realization of Galileo System Time, insuring the extremely high short-term stability required for the navigation functions. In order to allow a smooth switch over between backup and primary HMs, the “backup HM steering algorithm” is developed. This acquires the phase difference measured between two HMs, computes a steering correction, and generates the steering correction to the backup HM via a PicoStepper with a 0.1-picosecond resolution. The algorithm design is based on outlier removal and a proportional-integral filtering controller. To verify the steering operability and the loop performance, the overall backup HM steering system is simulated using real HM-HM measurements, and with simulated anomalies (phase/frequency spikes, jumps, and drift).