International and National Journals
S. Micalizio, A. Godone, F. Levi, J. Vanier
Spin-exchange frequency shift in alkali-metal-vapor cell frequency standards
Physical Review A, Vol. 73, pp. 033414.
In this paper we calculate the effect of spin-exchange collisions in alkali-metal vapors. In the framework of the high-energy approximation, we evaluate the spin-exchange cross sections related to the line broadening and to the frequency shift of the ground state hyperfine transition. We do the calculation for the four isotopes, 23Na, 39K, 87Rb, and 133Cs. The results are used in particular to evaluate the spin-exchange frequency shift in Rb vapor cell frequency standards used in many applications. It turns out that, due to possible fluctuations in the atomic density, spin exchange may affect significantly the medium and long term frequency stability of the frequency standard.
A. Godone, S. Micalizio, F. Levi, C. Calosso
Physics characterization and frequency stability of the pulsed rubidium maser
Physical Review A, Vol. 74, 043401, 2006.
In this paper we report the theoretical and experimental characterization of a pulsed optically pumped vapor-cell frequency standard based on the detection of the free-induction decay microwave signal. The features that make this standard similar to a pulsed passive maser are presented. In order to predict and optimize the frequency stability, thermal and shot noise sources are analyzed, as well as the conversions of the laser and microwave fluctuations into the output frequency. The experimental results obtained with a clock prototype based on 87Rb in buffer gas are compared with the theoretical predictions, showing the practical possibility to implement a frequency standard limited in the medium term only by thermal drift. The achieved frequency stability is σy(τ)=1.2×10-12τ-1/2 for measurement times up to 105 s. It represents one of the best results reported in literature for gas cell frequency standards and is compliant with the present day requirements for on board space applications.
A. Godone, S. Micalizio, C. E. Calosso, F. Levi
The pulsed rubidium clock
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control (UFFC), Vol. 53, no. 3, pp. 525-529, 2006.
In this paper, we describe a laboratory prototype of pulsed optically pumped clock based on a rubidium vapor cell with buffer gas. The measured frequency stability (overlapping Allan deviation) is σy(τ)=3×10-12τ-1/2 and the level of 4 × 10-15 is reached for averaging time of t = 3 × 104 s. For the same set of data, the statistical tool Theol predicts a frequency stability of 2 × 10-15 for t = 105 s. This result confirms the theoretical predictions regarding this kind of frequency standard and makes it very attractive for satellite navigation and space applications in which a simple and reliable implementation is required, and the short and medium term stability (till one day) is the main concern.
P. Tavella, M. Priel
Calibration intervals and methods for monitoring the measurement processes
Metrology in industry - The key for quality, edited by French College of Metrology, pp. 149-162, 2006.
Metrology is an integral part of the structure of today's world: navigation and telecommunications require highly accurate time and frequency standards; human health and safety relies on authoritative measurements in diagnosis and treatment, as does food production and trade; global climate studies also depend on reliable and consistent data. Moreover, international trade practices increasingly require institutions to display demonstrated conformity to written standards and specifications. As such, having relevant and reliable results of measurements and tests in compliance with mutually recognised standards can be a technical, commercial and statutory necessity for a company. This book, the results of a working group from the French College of Metrology and featuring chapters written by a range of experts from a variety of European countries, gives a comprehensive and international treatment of the subject. Academics involved in metrology as well as people involved in the metrology capacities of companies and institutions will find this book of great interest.
G. Panfilo, P. Tavella, E. Nunzi, P. Carbone, D. Petri
Optimal calibration interval in case of integrated brownian behaviour: the example of a rubidium frequency standard
IEEE Transactions on Instrumentation and Measurement (IEEE Trans. IM), Vol. 55, no. 5, pp 1713-1719, 2006.
In this paper we consider two different techniques in order to determine the optimal calibration interval: the intervals obtained using a mathematical model and those calculated with an iterative technique referred to as Simple Response Method (SRM). It is shown that both techniques provide useful information for an accurate calibration of a Rubidium frequency standard. We have chosen this example because the degradation of its calibration condition can be modelled by the use of a quite complex stochastic process called Integrated Brownian Motion (IBM) also of interest in other different contexts. The Rubidium frequency standard is quite diffused in industrial calibration laboratories.
F. Levi, J. H. Shirley, T. P. Heavner, Dai-Hyuk Yu, S. R. Jefferts
Power dependence of the frequency bias caused by spurious components in the microwave spectrum in atomic fountains
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control (UFFC), Vol. 53, no. 9, pp. 1584-1589, 2006.
The presence of spurious spectral components in the microwave excitation may induce frequency shifts in an atomic fountain frequency standard. We discuss how such shifts behave as a function of power variations of the excitation carrier and in the spur-to-carrier ratio. The discussion here is limited to the case of single-sideband spurs, which are generally much more troublesome due to their ability to cause frequency shifts. We find an extremely rich and unintuitive behavior of these frequency shifts. We also discuss how pulsed operation, typical of today's fountain frequency standards, relates to frequency shifts caused by spurs in the microwave spectrum. The conclusion of these investigations is that it is, at best, difficult to use elevated power microwaves in fountain frequency standards to test for the presence of spurs in the microwave spectrum.
S. R. Jefferts, F. Levi
Comment on 'Evaluation of the primary frequency standard NPL-CsF1'
Metrologia, Vol. 43, pp. L11-L13, 2006 (short communication).
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 × 10-16attributed 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.
W. Lewandowski, D. Matsakis, G. Panfilo, P. Tavella
The evaluation of uncertainties in [UTC - UTC(k)]
Metrologia, Vol. 43, pp. 278-286, 2006.
This work presents a study of the determination of uncertainties in [UTC - UTC(k)] needed for publication in the Bureau Inernational des Poids et Mesures's (BIPM's) Circular T and the Key Comparison Database, as required by the Mutual Recognition Arrangement. In the first part of the paper, an analytical solution based on the law of the propagation of uncertainty is derived. In the second part, the solution is verified numerically using the software used by the BIPM for the generation of UTC.
A. Bauch, J. Achkar, S. Bize, D. Calonico, R. Dach, R. Hlavac, L. Lorini, T. Parker, G. Petit, D. Piester, K. Szymaniec, P. Uhrich
Comparison between frequency standards in Europe and the USA at the 10-15 uncertainty level
Metrologia, Vol. 43, pp. 109-120, 2006.
Istituto Elettrotecnico Nazionale Galileo Ferraris (IEN), National Institute of Standards and Technology (NIST), National Physical Laboratory (NPL), Laboratoire National de Métrologie et d'Essais-Observatoire de Paris/Systèmes de Référence Temps Espace (OP) and Physikalisch-Technische Bundesanstalt (PTB) operate cold-atom based primary frequency standards which are capable of realizing the SI second with a relative uncertainty of 1 × 10-15or even below. These institutes performed an intense comparison campaign of selected frequency references maintained in their laboratories during about 25 days in October/November 2004. Active hydrogen maser reference standards served as frequency references for the institutes' fountain frequency standards. Three techniques of frequency (and time) comparisons were employed. Two-way satellite time and frequency transfer (TWSTFT) was performed in an intensified measurement schedule of 12 equally spaced measurements per day. The data of dual-frequency geodetic Global Positioning System (GPS) receivers were processed to yield an ionosphere-free linear combination of the code observations from both GPS frequencies, typically referred to as GPS TAI P3 analysis. Last but not least, the same GPS raw data were separately processed, allowing GPS carrier-phase (GPS CP) based frequency comparisons to be made. These showed the lowest relative frequency instability at short averaging times of all the methods. The instability was at the level of 1 part in 1015at one-day averaging time using TWSTFT and GPS CP. The GPS TAI P3 analysis is capable of giving a similar quality of data after averaging over two days or longer. All techniques provided the same mean frequency difference between the standards involved within the 1σ measurement uncertainty of a few parts in 1016. The frequency differences between the three fountains of IEN (IEN-CsF1), NPL (NPL-CsF1) and OP (OP-FO2) were evaluated. Differences lower than the 1σ measurement uncertainty were observed between NPL and OP, whereas the IEN fountain deviated by about 2σ from the other two fountains.
E. Bava, G. Galzerano, C. Svelto
Amplitude and frequency noise sensitivities of optical frequency discriminators based on Fabry-Pérot interferometers and the frequency modulation technique
Online Publication in Review of Scientific Instruments, Vol. 77, no. 12, 123106.
We report on the sensitivities to frequency and amplitude noise of optical frequency discriminators based on Fabry-Pérot interferometers and the frequency modulation (Pound-Drever-Hall) technique. The response to frequency and amplitude noise is obtained by analyzing the demodulated transients to phase or amplitude steps impressed on the incident frequency modulated light. The responses in the Fourier frequency domain are then obtained by performing the Laplace transform of the demodulated transients. As expected, when the laser frequency is coincident with the Fabry-Pérot resonance, this discriminator is completely immune to amplitude noise. To simulate real operations, the amplitude noise immunity is evaluated in the presence of small detunings between the laser frequency and the Fabry-Pérot resonance. Diagrams of the responses to both types of noise are reported for symmetric and asymmetric resonators. The Pound-Drever-Hall discriminator turns out to be insensitive to the asymmetry of the Fabry-Pérot resonator. The performed evaluations lead to a simple estimation of the minimum detectable frequency noise in terms of a given laser intensity noise.
P. Tavella
Time scales
MAPAN Journal of Metrology Society of India, Vol. 21, no. 4, pp. 213-222, 2006 (invited paper).
This work presents a tutorial on atomic time scales starting from an historical perspective and giving insight on the current atomic time scales, their definition, realisation, and the most demanding applications.
W. H. Oskay, S. A. Diddams, E. A. Donley, T. M. Fortier, T. P. Heavner, L. Hollberg, W. M. Itano, S. R. Jefferts, M. J. Delaney, K. Kim, F. Levi, T. E. Parker, J. C. Bergquist
Single-Atom Optical Clock with High Accuracy
Physical Review Letters, Vol. 97, no. 2, art. No. 020801, 2006.
For the past 50 years, atomic standards based on the frequency of the cesium ground-state hyperfine transition have been the most accurate time pieces in the world. We now report a comparison between the cesium fountain standard NIST-F1, which has been evaluated with an inaccuracy of about 4×10-16, and an optical frequency standard based on an ultraviolet transition in a single, laser-cooled mercury ion for which the fractional systematic frequency uncertainty was below 7.2×10-17. The absolute frequency of the transition was measured versus cesium to be 1.064.721.609.899.144,94 (97) Hz, with a statistically limited total fractional uncertainty of 9.1×10-16, the most accurate absolute measurement of an optical frequency to date.
J. H. Shirley, F. Levi, T. P. Heavner, D. Calonico, Dai-Hyuk Yu, S. R. Jefferts
Microwave leakage-induced frequency shifts in the primary frequency standards NIST-F1 and IEN-CSF1
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 53, no. 12, pp. 2376- 2385, 2006.
In atomic fountain primary frequency standards, the atoms ideally are subjected to microwave fields resonant with the ground-state, hyperfine splitting only during the two pulses of Ramsey's separated oscillatory field measurement scheme. As a practical matter, however, stray microwave fields can be present that shift the frequency of the central Ramsey fringe and, therefore, adversely affect the accuracy of the standard. We investigate these uncontrolled stray fields here and show that the frequency errors can be measured, and indeed even the location within the standard determined by the behavior of the measured frequency with respect to microwave power in the Ramsey cavity. Experimental results that agree with the theory are presented as well.
F. Lahaye, D. Orgiazzi, P. Tavella, G. Cerretto
GPS time transfer using precise point positioning for clock comparisons
GPS World, Vol. 17, no. 11, pp. 44-49, 2006.
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.
Conference Proceedings
S. R. Jefferts, J. H. Shirley, E. A. Donley, T. P. Heavner, F. Levi
On the power dependence of extraneous microwave fields in atomic frequency standards
Proceedings of the 2005 IEEE International Frequency Control Symposium and Exposition (IEEE-FCS 2005), pp.105-110, 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.
T. P. Heavner, 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 IEEE International Frequency Control Symposium and Exposition (IEEE-FCS 2005), pp. 308-311, 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.
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 IEEE International Frequency Control Symposium and Exposition (IEEE-FCS 2005), pp. 292-296, 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.
S. R. Jefferts, J. H. Shirley, N. Ashby, T. P. Heavner, E. A. Donley, F. Levi, E. A. Burt, G. J. Dick
Frequency biases associated with distributed cavity phase and microwave leakage in the atomic fountain primary frequency standards IEN-CSF1 and NIST-F1
Proceedings of the 20th European Frequency and Time Forum (EFTF), Braunschweig (Germany), March 2006 (on cd rom).
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.
F. Levi, J. H. Shirley, T. P. Heavner, Dai-Hyuk Yu, S. R. Jefferts
Power Dependence of the Shift Caused by Spurious Spectral Components in Atomic Fountain
Proceedings of the 20th European Frequency and Time Forum (EFTF), pp. 181-185, Braunschweig (Germany), March 2006 (on cd rom).
In this paper we analyze the behavior of the frequency shift caused by a spurious spectral component in the microwave spectrum against variation of the excitation microwave field. The theory of the shift caused by the presence of spurs in the microwave spectrum was investigated in depth over many years, however for historical reasons the behavior was never analyzed when microwave power is varied far above optimum. The theoretical predictions are significantly different form the thermal beam case. Furthermore the pulsed operation of a Fountain, with generally a well defined cycle time, set some new constraints on the size of carrier sidebands even when they are symmetric.
F. Levi, S. Micalizio, A. Godone, C. Calosso, E. Bertacco
Realization of a Pulsed Optically Pumped Rubidium Frequency Standard
Proceedings of the 20th European Frequency and Time Forum (EFTF), pp. 229-232, Braunschweig (Germany), March 2006 (on cd rom).
In this paper we present the more recent developments of a vapor cell frequency standard based on a pulsed technique. The laboratory prototype we implemented has shown very interesting metrological features, such as negligible light-shift and strongly reduced cavity-pulling. Moreover, a frequency stability of σy(τ)=1.2×10-12τ-1/2 for measurement times up to τ ≈ 100000 s has been achieved.
D. Piester, J. Achkar, J. Becker, B. Blanzano, K. Jaldehag, G. de Jong, O. Koudelka, L. Lorini, H. Ressler, M. Rost, I. Sesia, P. Whibberley
Calibration of Six European TWSTFT Earth Stations Using a Portable Station
Proceedings of the 20th European Frequency and Time Forum (EFTF), pp. 460-467, Braunschweig (Germany), March 2006 (on cd rom).
Two-way satellite time and frequency transfer (TWSTFT) has become an important component in the international network for comparing time scales. To employ the full potential of the technique a calibration of the internal delay of each ground station is necessary. Only a few calibration campaigns have previously been carried out in the European network of TWSTFT links. We report on the first recalibration of TWSTFT links during a campaign involving six European time institutes. The campaign was performed using a portable ground station assembled and operated by TUG/Joanneum Research, Graz, that visited the sites of INRIM, NPL, OP, PTB, SP, and VSL, travelling a total distance over 7000 km during a three-week period in October/November 2005. Differential delays of earth stations were determined in the common clock mode relative to the portable station. Combined uncertainties ranging from 0.9 ns to 1.3 ns for all calibrated links were achieved in this campaign.
G. Cerretto, D. Orgiazzi, P. Tavella, P. Collins, F. Lahaye
Rapid Transatlantic Time Transfer Surveying the Link between USNO and IEN within the IGS Global Real-Time Network
Proceedings of the 20th European Frequency and Time Forum (EFTF), pp. 517-523, Braunschweig (Germany), March 2006 (on cd rom).
Natural Resources Canada (NRCan) computes real-time wide area differential GPS corrections, known as GPSºC. These corrections allow real-time Canadian users of GPS to reduce the errors in the broadcast satellite position and clock states and achieve improved positioning accuracy. One by product of the estimation process is receiver clock states for all stations used in the corrections computation. In this paper we report on the capability of rapid/frequency transfer, surveying the transatlantic link between timing laboratories of the U.S Naval Observatory (Washington D.C., USA) and the IEN. Over 6 months period, the near-real-time estimation of the UTC(USNO) to UTC(IEN) offset is then directly compared with BIPM Circular T data and other independent synchronization techniques, such as TWSTFT, putting this new technique into perspective aiming to support the real-time institutional activities of timing laboratories.
M. Norgia, A. Pesatori, G. Galzerano, E. Bava, C. Svelto
Absolute Laser Frequency Stabilization to R(3) Absorption Line of HI at 1541.06 nm
Proceedings of the 23rd IEEE Instrumentation and Measurement Technology Conference (IMTC 2006), pp. 458-461, Sorrento (Italy), April 2006.
An Extended-Cavity Laser Diode was used to perform high resolution spectroscopy of molecular absorption lines of hydrogen iodide. For the first time, this laser source was frequency locked to an HI transition at 1.5 µm using the wavelength modulation technique. Preliminary measurements of the closed-loop error signal provided a relative frequency stability of 2×10-9 in a 10 Hz acquisition bandwidth.
T.P. Heavner, J.H. Shirley, F. Levi, D. Yu, S.R. Jefferts
Frequency Biases in Pulsed Atomic Fountain Frequency Standards Due to Spurious Components in the Microwave Spectrum
Proceedings of the IEEE International Frequency Control Symposium and Exposition, pp. 273-276, Miami (USA), June 2006.
A mechanism by which spurs in the microwave spectrum may cause biases in pulsed atomic fountain frequency standards was identified in [1]. Here we present models for the cases of phase modulating (PM) and amplitude modulating (AM) spurs in the spectrum. In the case of a PM spur, the bias is first-order in the spur amplitude. Under common experimental conditions, this effect may be much larger than the second-order bias presented in [2]. We present measurements made using NIST-F1 whereby PM spurs of known amplitude were added onto the microwave signal used to interrogate the cesium (Cs) clock transition. The measured biases are compared to the predictions of the model. Finally, we discuss the difficulties of evaluating the frequency bias in fountain frequency standards using traditional techniques such as operating at elevated microwave powers.
J. E. Stalnaker, S. A. Diddams, K. Kim, L. Hollberg, E. A. Donley, T. P. Heavner, S. R. Jefferts, F. Levi, T. E. Parker, J. C. Bergquist, W. M. Itano, M. J. Jensen, L. Lorini, W. H. Oskay
Absolute optical frequency measurements with a fractional frequency uncertainty at 1 × 10-15
Proceedings of the IEEE International Frequency Control Symposium and Exposition, pp. 462-469, Miami (USA), June 2006.
We report the technical details specific to our recent measurements of the optical frequency of the mercury single-ion frequency standard in terms of the SI second as realized by the NIST-F1 cesium fountain clock. In these measurements the total fractional uncertainty is ≈ 10-15, limited by the statistical measurement uncertainty. In this paper we will address the techniques employed for the optical-to-microwave comparison itself, which had an estimated fractional uncertainty of 3 × 10-16, limited by the stability of the electronics used for the comparison.
R. Píriz, P. Tavella, J. H. Hahn
GPS/Galileo interoperability: GGTO, timing biases and Giove-A experience
Proceedings of the 38thAnnual Precise Time and Time Interval Systems and Application Meeting (PTTI 2006), (oral session), Washington DC (USA), December 2006.
The future European radio navigation system Galileo will use its own timescale for the synchronization and dissemination of the predicted satellite clocks. Broadcast satellite clock predictions must be referred to a common and stable time reference in order for the user to be able to obtain an accurate positioning solution. The new Galileo timescale is called Galileo System Time (GST) and is the equivalent of the GPS Time (GPST) scale used in the Global Positioning System. For timing applications both the GPS and Galileo systems broadcast as well the difference between their respective system times (GPST and GST) and the universal timescale (UTC), with a maximum deviation requirement of the order of 1 microsecond for GPS (although in practice the deviation is currently below 10 nanoseconds) and 50 nanoseconds for Galileo. Furthermore, for GPS/Galileo interoperability the Galileo and GPS systems are planning to transmit within their navigation messages the so-called GPS to Galileo Time Offset (GGTO), i.e. the predicted difference between the GPST and GST system times. This paper analyses the different issues involved in GPS/Galileo interoperability for positioning and timing, including GGTO and timing biases, and presents practical experience and solutions from the data processing of GIOVE-A, the first experimental Galileo satellite.
A. Svelto, A. Pesatori, M. Norgia, G. Galzerano, E. Bava
Caratterizzazione a temperatura ambiente di un laser Yb:KYF4 pompato a diodo
Atti del XXIII Congresso Nazionale "Gruppo Misure Elettriche ed Elettroniche (GMEE 2006)", pp. 11-12, L'Aquila (Italy), settembre 2006.
Un'azione laser in regime continuo è stata ottenuta per la prima volta in un materiale attivo Yb:KYF4. In questo lavoro sono riportate le prime misure sperimentali di caratterizzazione di questa nuova sorgente ottica. Una potenza massima di uscita di 505 mW, un'efficienza differenziale del 43% e un'accordabilità in lunghezza d'onda di ben 65 nm, da 1013 nm a 1078 nm, sono stati misurate a temperatura ambiente.
E. Nunzi, L. Galleani, P. Tavella, P. Carbone
Rivelazione di anomalie nel comportamento degli orologi atomici
Atti del XXIII Congresso Nazionale "Gruppo Misure Elettriche ed Elettroniche (GMEE 2006)", pp. 311-319, L'Aquila (Italy), settembre 2006.
Lo scopo di questo studio, condotto in collaborazione tra Unità di ricerca di Perugia, Politecnico di Torino e Istituto Nazionale di Ricerca Metrologica, è quello di identificare eventuali variazioni comportamentali di orologi atomici applicando metodologie diverse. In particolare, i dati sperimentali relativi ad un orologio atomico al Rubidio sono stati analizzati mediante tecniche classiche adibite allo scopo, quali la varianza di Allan classica e dinamica, mettendo in evidenza la natura intrinsecamente non stazionaria dei dati. I dati sono stati elaborati anche mediante una tecnica di rilevazione dei guasti basata sul test del rapporto di verosimiglianza generalizzato (GLRT). In questo lavoro, le tecniche vengono applicate ai dati di misura di un orologio atomico che presenta delle anomalie di tipo aleatorio rispetto al modello comportamentale previsto. Il risultato di tale analisi è che i metodi proposti, pur con caratteristiche e prestazioni diversi, rilevano le variazioni di media e varianza dei dati sperimentali in maniera efficiente e quindi possono essere proposti per la rilevazione di cambiamenti nel comportamento degli orologi atomici.
Technical Reports
E. Valenti
La scala di tempo del sistema Galileo
Monografia di tirocinio presso IEN, 45+13 pp., Polit. di Torino - III Facoltà di Ingegneria - Corso di laurea in Ingegneria dell'Informazione; tutori: A. Vallan e P. Tavella; a.a. 2004/2005.
R. Costa, V. Pettiti
Elaborazione dei dati di misura GPS e SRC degli oscillatori tarati in modo remoto dall'Istituto
Rapporto Tecnico INRIM n. 27, luglio 2006.
Il rapporto descrive la procedura ed i programmi di calcolo utilizzati per elaborare i dati di misura GPS (Global Positioning System) e SRC (Segnale RAI Codificato), forniti dai sistemi associati agli oscillatori di riferimento operanti presso i laboratori metrologici. Questi oscillatori vengono tarati dall'INRIM in modo remoto utilizzando i dati di misura inviati periodicamente dai laboratori stessi. In particolare, dopo un'introduzione ai diversi sistemi di misura utilizzati ed ai corrispondenti formati di dati forniti, si illustrano le modalità d'utilizzo e le operazioni eseguite dal programma di calcolo utilizzato per organizzare i dati e confrontarli con UTC(IT). Vengono poi descritte le ulteriori elaborazioni, effettuate mediante l'uso di un foglio elettronico, per produrre i grafici e calcolare i parametri caratteristici da inserire nei certificati di taratura. Infine, è presentato un esempio di certificato di taratura prodotto trimestralmente dall'Istituto, emesso nei confronti dei laboratori metrologici che richiedono la riferibilità del campione locale di frequenza alla scala di tempo nazionale.
R. Costa, F. Cordara, V. Pettiti
La scala di tempo nazionale UTC (IT): generazione mediante maser all'idrogeno, sistemi di disseminazione e di monitoraggio
Rapporto Tecnico INRIM n. 28, ottobre 2006.
Con questo rapporto si intende descrivere la nuova catena di generazione della scala di tempo nazionale UTC(IT) mediante l'utilizzo di un maser all'idrogeno e dell'Auxiliary Offset Generator (AOG). Inoltre si illustrano le tecniche utilizzate per la previsione di anomalie e guasti negli oscillatori al cesio e al maser all'idrogeno in dotazione e si riproducono i dati sperimentali ottenuti. Quindi si descrive la tecnica di elaborazione dei dati GPS, utilizzata dal laboratorio di tempo e frequenza, al fine di consentire a laboratori metrologici secondari di riferire il proprio oscillatore alla scala UTC(IT). Infine, vengono descritti i criteri e le tecniche utilizzati per il controllo dell'incertezza di generazione della scala di tempo nazionale UTC(IT) e di monitoraggio dei servizi di disseminazione di tempo campione dell'INRIM.