International and National Journals
C. Zucca, P. Tavella
The clock model and its relationship with the Allan and related variances
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control (UFFC), Vol. 52, no. 2, pp. 289-296, 2005.
The clock errors are modeled by stochastic differential equations (SDE) and the relationships between the diffusion coefficients used in SDE and the Allan variance, a typical tool used to estimate clock noise, are derived. This relationship is fundamental when a mathematical clock model is used, for example in Kalman filter, noise estimation, and clock prediction activities.
E. Nunzi, G. Panfilo, P. Tavella, P. Carbone, D. Petri
Stochastic and Reactive Methods for the Determination of Optimal Calibration Intervals
IEEE Transactions on Instrumentation and Measurement, vol. 54, no. 4, pp. 1565-1569, 2005.
The recommended length of calibration intervals of measurement instrumentation can be determined by means of several techniques. In this paper, three different methods are compared for the establishment of optimal calibration intervals of atomic clocks. The first one is based on a stochastic model and provides the estimation of the calibration interval also in the transient situation, while the others pertain to the class of the so-called reactive methods, which determine the value of the optimal interval on the basis of the last calibration outcomes. Algorithms have been applied to experimental data, and the obtained results have been compared in order to determine the most effective technique. Since the analyzed reactive methods present a long transient time, a new algorithm is proposed and applied to the available data.
A. Godone, D. Calonico, F. Levi, S. Micalizio, C. Calosso
Stark-shift measurement of the 2S½, F = 3 - F = 4 hyperfine transition of 133Cs
Physical Review A Vol. 71, no. 6, pp. 063401-1/7, 2005.
We report in this paper the measurement of the Stark frequency shift of the ground state hyperfine transition of 133Cs observed through a Λ excitation scheme in a vapor cell. The measured value of the quadratic Stark coefficient is k = (-2.05±0.04)·10-10 Hz/(V/m)2 when the electric field is perpendicular to the quantization axis. A comparison with the values reported in literature is also given in view of its interest for the evaluation of the black-body radiation shift in the primary frequency standards.
E.A. Donley, T.P. Heavner, F. Levi, M.O. Tataw, S.R. Jefferts
Double-pass acousto-optic modulator system
Review of Scientific Instruments, Vol. 76, no. 6, pp. 063112-1/6, 2005.
A practical problem that arises when using acousto-optic modulators (AOM) to scan the laser frequency is the dependence of the beam diffraction angle on the modulation frequency. Alignment problems with AOM-modulated laser beams can be effectively eliminated by using the AOM in thedouble-pass configuration, which compensates for beam deflections. On a second pass through the AOM, the beam with its polarization rotated by 90° is deflected back such that it counterpropagates the incident laser beam and it can be separated from the input beam with a polarizing beam splitter. Here we present our design for a compact, stable, double-pass AOM with 75% double-pass diffraction efficiency and a tuning bandwidth of 68 MHz full width at half maximum for light transmitted through a single-mode fiber. The overall efficiency of the system (defined as the optical power out of the single-mode fiber divided by the optical power into the apparatus) is 60%.
C.E. Calosso, F. Levi, E.K. Bertacco, A. Godone, S. Micalizio
Low noise electronic design for the 87Rb coherent population trapping maser
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 52, no. 11, pp. 1923-1930, 2005.
In this paper we present the low noise electronics for a prototype of rubidium maser based on the Coherent Population Trapping (CPT) phenomenon. After an overview of the general architecture, we will focus our description on the main blocks of the equipment we implemented: the microwave synthesis chain, the detection apparatus, the clock servo system and the electronics devoted to control the laser and the temperature. For each part, we present the design, the implementation and the characterization measurements we performed. The contribution to the CPT maser frequency stability of each part has been also evaluated and the frequency stability of the clock is finally reported.
S. Leschiutta
The definition of the "atomic" second
Metrologia, no. 42, pp. S10-S19, 2005.
When the reference of a unit is changed, the task remains of expressing the new definition in the terms of the previous one. In two centuries, for instance, the definition of the metre was changed six times. Around 1955, the definitions of the second as the SI unit of time were changed from astronomical ones, based on the rotation (or revolution) of the Earth, to a definition stemming from a quantum phenomenon. The expression of the new atomic second in terms of the previous definitions required a number of actions and measurements. The aim of this paper is to review these actions, some of which, fifty years later, are being forgotten for a lack of adequate documentation.
S.R. Jefferts, F. Levi
Comment on "Evaluation of the primary frequency standard NPL-CsF1"
Published as short communication in Metrologia, vol. 43, pp. L11-L13, 2006.
A recent evaluation of the accuracy of the National Physical Laboratory (NPL) primary frequency standard NPL-CsF1 by K Szymaniec et al (2005 Metrologia 42 49-57) reported an overall frequency uncertainty of δv/v0 = 1 · 10-15. This stated uncertainty includes a correction of a frequency bias of δv/v0 = 8 · 10-16 ± 3 · 10-16 attributed by the authors to microwave leakage. We believe that the stated cause of the frequency bias, its magnitude and its stated uncertainty are in error.
Conference Proceedings
E.A. Donley, T.P. Heavner, M.O. Tataw, F. Levi, S.R. Jefferts
Progress towards the second-generation atomic fountain clock at NIST
Proceedings of the 2004 IEEE International Frequency Control Symposium and Exposition, pp 82-86, 2005.
We present results on the performance of two important subsystems for NIST-F2 - the second-generation atomic fountain clock at NIST. Firstly, we demonstrate the efficient capture of cesium atoms from a low-velocity intense source (LVIS) of atoms into an optical molasses. Our typical LVIS flux was 1010 s-1. The initial molasses fill rate was Rt=0 =3.8 (5)·109 s-1, which is consistent with 100% atom capture given the lifetime of our vacuum system. Secondly, recent results for our launch performance in the (1,1,1) fountain geometry are presented. We were able to cool the atoms to a temperature of 1.5 µK with our current design. Our plans for improving the laser cooling performance are also presented.
S. Facioni, G. Graglia, M. Mascarello, F. Cordara, R. Costa, M. Farina, L. Lorini, D. Orgiazzi, V. Pettiti, I. Sesia, P. Tavella, M. Falcone, J. Hahn, M. Brunet
The first months of operation for the Experimental Precise Timing Station in the GSTB-V1 project
Proceedings of the Global Navigation Satellite Systems meeting, Rotterdam (NL), May 2004 (on cd rom).
The Galileo System Test Bed (GSTB) V1 is the first experimental phase in the Galileo project supported by the European Space Agency (ESA). The aim is testing the Galileo algorithms in a mixed configuration where the space segment is given by the existing GPS constellation, while the ground segment is an experimental set-up as close as possible to the Galileo architecture. In the framework of The Galileo System Test Bed (GSTB) V1, Alenia Spazio and the Istituto Elettrotecnico Nazionale (IEN), have the responsibility to design and realize an infrastructure able to generate the Experimental Galileo System Time (E-GST) and then provide a set of experiments dedicated to an assessment of the Galileo timescale. The infrastructure that allows the realization of E-GST is the Experimental Precise Timing Station (E-PTS), that is implemented at the IEN facilities, Turin, Italy. E-GST is obtained by a set of algorithms implemented in a dedicated SW that daily generates the time scale of the GSTB V1 system. The major experiment aims to test the real-time E-GST generation and the steering algorithms in a real environmental situation. In addition, off-line, alternative time scale algorithms as well as different clocks ensembles, measurement systems, remote synchronization techniques are used to test the stability and accuracy performances of the resulting time scale. The paper reports the performed activity for the complete installation, calibration, and verification of the E-PTS during the second part of 2003 and the first months of experimentation (since March 2004) with the obtained results. The experimental period is planned to last for the whole 2004 and the first months of 2005. The first experimental results have been reported and discussed, pointing out the link with the Galileo C0 phase, the criticalities encountered and the solutions found in reaching the demanding GSTB V1 goals.
P. Tavella
Experimental Galileo System Time: results in the Galileo System Test Bed V1
Proceedings of the International Workshop on Galileo Time, Modelling and Characterization of Atomic Clock, Torino, September 2004 (on cd rom).
Galileo System Test Bed (GSTB) V1 is the first experimental phase in the Galileo project supported by the European Space Agency (ESA).The aim is testing the Galileo algorithms in a mixed configuration where the space segment is given by the existing GPS constellation, while the ground segment is an experimental set-up as close as possible to the Galileo architecture. In the framework of the GSTB V1, Alenia Spazio, in collaboration with the Istituto Elettrotecnico Nazionale (IEN), had the responsibility to design and realize an infrastructure able to generate the Experimental Galileo System Time (E-GST) which is the reference time for all the Galileo system. The infrastructure that allows the realization if E-GST is the Experimental Precise Timing Station (E-PTS), that is implemented at the IEN facilities, Turin, Italy. The time scale E-GST is obtained with a set of algorithms computed by a dedicated SW that daily generates the time scale used as reference in all the GSTB V1. The major experiment aims to test the real-time E-GST generation and the steering algorithms in a real environmental situation. In addition, off-line, alternative time scale algorithms as well as different clock ensembles, measurement systems, remote synchronization techniques are used to test the stability and accuracy performances of the resulting time scale. The first months of operation of the GSTB V1 are giving very interesting and promising results and also pointing out the criticalities in reaching the demanding Galileo goals.
E. Bava
Atomic clocks
Proceedings of the International Workshop on Galileo Time, Modelling and Characterization of Atomic Clock, Torino, September 2004 (on cd rom).
W. Lewandowski, D. Matsakis, G. Panfilo, P. Tavella
First evaluation and experimental results on the determination of uncertainties in [UTC - UTC (k)
Proceedings of the 36th Precise Time and Time Interval Meeting, Washington DC, November 2004 (36th PTTI 2004) (on cd rom).
In this work, we present a preliminary study of the uncertainty of [UTC - UTC (k)]. In the first part of the paper, we consider an analytical solution considering the law of the propagation of uncertainty. In the second part, we verify the analytical results numerically, using the software used for the generation of UTC.
F. Cordara, R. Costa, L. Lorini, D. Orgiazzi, V. Pettiti, I. Sesia, P. Tavella, P. Elia, M. Mascarello, M. Falcone, J. Hahn
Experimental Galileo System Time (E-GST): one year of real-time experiment
Proceedings of the 36th Precise Time and Time Interval Meeting, Washington DC, November 2004 (36th PTTI 2004) (on cd rom).
The Galileo System Test Bed (GSTB) V1 is the first experimental phase in the Galileo project supported by the European Space Agency (ESA). The aim is testing the Galileo algorithms in a mixed configuration where the space segment is given by the existing GPS constellation, while the ground segment is an experimental set-up as close as possible to the Galileo architecture. In the framework of the GSTB V1, Alenia Spazio, in collaboration with the Istituto Elettrotecnico Nazionale (IEN), had the responsibility to design and realize an infrastructure able to generate the Experimental Galileo System Time (E-GST) which is the reference time for all the Galileo system. The infrastructure that allows the realization of E-GST is the Experimental Precise Timing Station (E-PTS), which is implemented at the IEN facilities, Turin, Italy. The paper reports the main results of the E-PTS experimentation carried out during the whole 2004 addressing the E-GST overall performance with respect to the demanding GSTB V1 goals. Besides, the lessons learnt from the experiment are also pointed out in the paper, together with some recommendations to be taken into account in the design of the Precise Timing Facilities (PTFs) of the Galileo ground segment.
G. Panfilo, P. Tavella (poster session)
Atomic clock prediction for the generation of a time scale
Published as short communication in Proceedings of the VII Advanced Mathematical and Computational Tools in Metrology (AMCTM) 2005, P. Ciarlini, E. Filipe, A. B. Forbes, F. Pavese, C. Perruchet, B. Siebert (editors), pp. 320-324, World Scientific Publishing 2006, Caparica (Lisbona, Portugal), June 2005.
In the generation of a time scale, the prediction of the atomic clock behaviour plays an important role. The international time reference "Universal Coordinated Time" (UTC) is in fact available a posteriori, while the national time scales are realised in real time and they need to be good local approximations of the international UTC. In this case a prediction of the local clock behavior for a period of about 45 days is needed. In this paper two methods to predict are presented; extrapolation from past data and study of the stochastic behavior of the atomic clock error.
S. Micalizio, A. Godone, C. Calosso, F. Levi, D. Calonico
Gas cell frequency standards for space application at IEN
Proceedings of the 1st ESA International Workshop on Optical Clocks for Space Applications, Noordwijk (Olanda), pp. 9-16, June 2005.
In this paper the recent activity we carried out at IEN in the field of vapor cell frequency standards is presented. In particular, we summarize the results we obtained with the Coherent Population Trapping (CPT) maser and we show the preliminary results related to the Pulsed Optically Pumped (POP) rubidium clock. A suitable implementation of this standard may lead to a compact vapor cell clock with very interesting short and medium term frequency stability and consequently may be of great interest in radio-navigation systems.
D. Orgiazzi, P. Tavella, F. Lahaye (oral session)
Experimental assessment of the time transfer capability of Precise Point Positioning (PPP)
Proceedings of 2005 Joint IEEE International Frequency Control Symposium (FCS) and Precise Time and Time Interval (PTTI) Systems and Applications Meetings, pp. 337-345, Vancouver (Canada), August 2005.
In recent years, many national timing laboratories have installed geodetic Global Positioning System (GPS) receivers together with their traditional GPS/GLONASS Common View (CV) receivers and Two Way Satellite Time and Frequency Transfer (TWSTFT) equipment. A method called Precise Point Positioning (PPP) is in use in the geodetic community allowing precise recovery of geodetic GPS receiver position, clock phase and tropospheric delay by taking advantage of the International GNSS Service (IGS) precise products. Natural Resources Canada (NRCan) has developed software implementing the PPP and a previous assessment of the PPP as a promising time transfer method was carried out at Istituto Elettrotecnico Nazionale (IEN) in 2003. This paper reports on a more systematic work performed at IEN and NRCan to further characterize the PPP method for time transfer application, involving data from nine national timing laboratories. Dual-frequency GPS observations (pseudorange and carrier phase) over the last ninety days of year 2004 were processed using the NRCan PPP software to recover receiver clock estimates at five minute intervals, using the IGS Final satellite orbit and clock products. The quality of these solutions is evaluated mainly in terms of short-term noise. In addition, the time and frequency transfer capability of the PPP method were assessed with respect to independent techniques, such as TWSTFT, over a number of European and Transatlantic baselines.
W. Lewandowski, D. Matsakis, G. Panfilo, P. Tavella
Refining the evaluation of uncertainties in [UTC - UTC (k)]
Proceedings of the 2005 Joint IEEE FCS Symposium and PTTI Systems and Applications Meetings, pp. 383-391, Vancouver (Canada), August 2005.
We refine the evaluation of the uncertainty on [UTC - UTC (k)], by taking into account in particular the contribution of the correlations. To easily handle the different computational requirements we use a matrix formulation. Using this matrix formalism we re-analyze the link-based uncertainties and we obtain the same solution as given in [1]. Next we evaluate the site-based uncertainties and compare the results with the link-based uncertainties.
P. Tavella, L. Galleani
Tracking nonstationarities in clock noises using the Dynamic Allan Variance
Proceedings of the 2005 Joint IEEE FCS Symposium and PTTI Systems and Applications Meetings, pp. 392-396, Vancouver (Canada), August 2005.
In a previous paper we have introduced the concept of dynamic Allan variance, an extension of the classical Allan variance that is commonly used to evaluate the stability of atomic clocks. The Allan variance assumes the stationarity of the (increment of the) clock error signal, a condition that is valid for ideal clocks only. For real clocks one has to pay attention in the evaluation of the clock stability, because even for short time intervals the clock can exhibit a nonstationary behavior. Possible reasons for the lack of stationarity are sudden breakdowns, or, in the long term, clock ageing. Even cyclostationary behaviors can be observed due to daily or seasonal variation of temperature, humidity and other physical quantities that have a direct influence on the clock behavior. The main purpose of the Dynamic Allan variance is to describe the variation in time of the clock stability. In this paper we give a mathematical definition of this quantity. We apply our method to simulated data and to real data coming from a Rubidium clock. The results are very interesting, and they show that the proposed method can track and reveal in a clear and intuitive manner the changes in the behavior of atomic clock data.
D. Orgiazzi, P. Tavella, G. Ceretto, P. Collins, F. Lahaye (poster session)
First evaluation of a rapid time transfer within the IGS Global Real-Time Network
Proceedings of the 2005 Joint IEEE FCS Symposium and PTTI Systems and Applications Meetings, pp. 721-727, Vancouver (Canada), August 2005.
This paper presents a preliminary assessment of the quality of station clock states, estimated in "near-real-time" by the process running at Natural Resources Canada (NRCan). The evaluation is mainly of the short and long-term frequency stability and the accuracy thereof with respect to the IGS Rapid and Final clock products, provided by the International GNSS Service (IGS). The aim of this work is to evaluate the feasibility of time and frequency transfer in real-time and to show the potential of such a process once access to a global IGS network of real-time GPS tracking stations becomes a reality.
S.R. Jefferts, J.H. Shirley, N. Ashby, T.P. Heavner, E.A. Donley, F. Levi
On the Power Dependence of Extraneous Microwave Fields in Atomic Frequency Standards
Proceedings of the 2005 Joint IEEE FCS Symposium and PTTI Systems and Applications Meetings, pp. 105-110, Vancouver (Canada), August 2005.
We show that the frequency bias caused by distributed cavity phase has a strong dependence on microwave power. We also show that frequency biases associated with microwave leakage have distinct signatures in their dependence on microwave power and the physical location of the leakage interaction with the atom.
E.A. Donley, T.P. Heavner, J.W. O'Brien, S.R. Jefferts, F. Levi
Laser cooling and launching performance in a (1,1,1)-geometry atomic fountain
Proceedings of the 2005 Joint IEEE FCS Symposium and PTTI Systems and Applications Meetings, pp. 292-296, Vancouver (Canada), August 2005.
We have successfully cooled and launched cesium atoms in the (1,1,1) atomic fountain geometry at temperatures as low as 0.8 µK. This corresponds to a root-mean-square thermal velocity of 0.7 cm/s, which is two times the photon recoil velocity of 0.35 cm/s. Factors that limit the final temperature are presented in detail.
T.P. Heavner, S.R. Jefferts, E.A. Donley, T.E. Parker, F. Levi
A New Microwave Synthesis Chain for the Primary Frequency Standard NIST-F1
Proceedings of the 2005 Joint IEEE FCS Symposium and PTTI Systems and Applications Meetings, pp. 308-311, Vancouver (Canada), August 2005.
We present the design and measurements of the microwave synthesis chain presently used in NIST-F1, the laser-cooled cesium fountain primary frequency standard in operation at NIST, Boulder, CO. The chain has been used in two accuracy evaluations of NIST-F1 (January 2005 and July 2005), each of which had a combined (Type A and Type B) fractional frequency uncertainty of < 0.5 · 10-15. Additionally, this synthesis chain was in use during a recent calibration of the 199Hg+ optical clock transition against Cs, which had a fractional uncertainty of 9.1 · 10-16.
F. Cordara, V. Pettiti
Riferibilità di un laboratorio metrologico mediante i segnali GPS: come stabilirla e verificarla
Atti del Congresso Metrologia e Qualità, pp. 344-349, Torino, febbraio 2005.
Il sistema di radionavigazione satellitare GPS (Global Positioning System), è attualmente il principale strumento per la riferibilità di un laboratorio metrologico nelle misure di tempo e frequenza. Accanto ad una rassegna delle tecniche attualmente utilizzate per la riferibilità nazionale ed internazionale, saranno analizzate le procedure di validazione o verifica, adatte a stabilire e a confermare i livelli d'incertezza con cui i laboratori metrologici possono essere riferiti al campione nazionale di tempo.
V. Pettiti, F. Cordara
Misure d'intervallo di tempo, di durata e di tempo di salita di segnali mediante contatore elettronico
Atti del Congresso Metrologia e Qualità, pp. 364-369, Torino, febbraio 2005.
Le misure d'intervallo di tempo, di durata e di tempo di salita di segnali mediante contatore elettronico, costituiscono una quota significativa delle possibilità offerte comunemente dai contatori. La stima dell'incertezza associata a queste misure, che utilizzano due canali d'ingresso del contatore, richiede la conoscenza del ritardo differenziale tra detti canali; in caso contrario, esso può costituire la maggior fonte d'incertezza.
C. Svelto, G. Galzerano, M. Norgia, A. Pesatori, E. Bava
Spettroscopia laser della molecola di ioduro d'idrogeno a 1.54 µm
Atti del GMEE 2005 - XXII Congresso Nazionale Associazione Italiana "Gruppo di Misure Elettriche ed Elettroniche", pp. 29-30, Palermo (Italy), settembre 2005.
Sono state compiute misure di spettroscopia laser della molecola di HI alla lunghezza d'onda di 1.54 µm mediante un diodo laser a cavità esterna e impiegando tecniche di misura sia diretta che FM. L'allargamento di riga per l'HI è stato misurato per differenti pressioni del gas e sono state effettuate le prime misure dei corrispondenti coefficienti di allargamento pressorio. In particolare, sono stati acquisiti i profili di assorbimento per la riga di transizione R(3) a 1541.06 nm, ottenuti attraverso la spettroscopia FM e l'integrazione numerica del segnale in derivata prima.
Technical Reports
V. Pettiti, R. Costa
Instabilità di fase di segnali di tempo e frequenza inviati su linea in fibra ottica
Rapporto Tecnico IEN n. 688, giugno 2005.
The aim of this IEN Technical Report is to analyze the short and long term stability of an optical fiber link by means of a 10 MHz signal supplied by a frequency standard. Furthermore, another aim is to estimate the deterioration of time and frequency signals sent through an optical network for data transmission (managed by FastWeb company), about 30 Km long. The experiment was performed by the Time and Frequency Laboratory of IEN, using the facilities of PhotonLab (ISMB - Istituto Superiore Mario Boella - Turin), between December 2004 and January 2005.
F. Bandel, G. Cerretto, D. Orgiazzi, V. Pettiti
Il Precise Point Positioning: confronto a distanza di orologi mediante ricevitori GPS per geodesia
Rapporto Tecnico IEN n. 689, giugno 2005.
Scope of this Technical Report is to describe the basis of work of a system called Precise Point Positioning (PPP), involved in the remote comparison of standard clocks. In the following the mathematical model that is under the functioning of PPP is described as well as the use of this technique in timing applications. Furthermore are described the role of the International GNSS Service (IGS) as a provider of PPP oriented products (satellite orbits and GPS satellite clocks offset) and the IGS Time Scale, which is the time reference generated by IGS.
G. Cerretto, D. Orgiazzi, V. Pettiti
Campagna di sperimentazione del PPP a cura dello IEN e di NRCan
Rapporto Tecnico IEN n. 690, giugno 2005.
Scope of this Technical Report, performed in collaboration with the Geodetic Survey Division of Natural Resources Canada (GSD NRCan), is to describe the process that was adopted to asses the estimates generated by the Precise Point Positioning (PPP) technique, in order to provide remarks for a possible upgrade of the software that was used to generate the estimates. The experiment involved 9 primary metrological institutes (USNO, NIST, PTB, NPL, OP, IEN, NICT, NRC, ORB) and had been analysed PPP estimates for each station, for couples of stations and were directly compared PPP with TWSTFT estimates.