Showing papers in "Metrologia in 2011"

Counting the atoms in a 28Si crystal for a new kilogram definition

Abstract: This paper concerns an international research project aimed at determining the Avogadro constant by counting the atoms in an isotopically enriched silicon crystal. The counting procedure was based on the measurement of the molar volume and the volume of an atom in two 1 kg crystal spheres. The novelty was the use of isotope dilution mass spectrometry as a new and very accurate method for the determination of the molar mass of enriched silicon. Because of an unexpected metallic contamination of the sphere surfaces, the relative measurement uncertainty, 3 × 10−8 NA, is larger by a factor 1.5 than that targeted. The measured value of the Avogadro constant, NA = 6.022 140 82(18) × 1023 mol−1, is the most accurate input datum for the kilogram redefinition and differs by 16 × 10−8 NA from the CODATA 2006 adjusted value. This value is midway between the NIST and NPL watt-balance values.

The 87 Sr optical frequency standard at PTB

Abstract: With 87Sr atoms confined in a one-dimensional optical lattice, the frequency of the optical clock transition 5s2 1S0–5s5p 3P0 has been determined to be 429 228 004 229 872.9(5) Hz. The transition frequency was measured with the help of a femtosecond-frequency comb against one of Physikalisch-Technische Bundesanstalt (PTB's) H-masers whose frequency was measured simultaneously by the PTB Cs-fountain clock CSF1. The Sr optical frequency standard contributes with a fractional uncertainty of 1.5 × 10−16 to the total uncertainty. The agreement of the measured transition frequency with previous measurements at other institutes supports the status of this transition as the secondary representation of the second with the currently smallest uncertainty.

Improved accuracy of the NPL-CsF2 primary frequency standard: evaluation of distributed cavity phase and microwave lensing frequency shifts

Abstract: We evaluate the distributed cavity phase (DCP) and microwave lensing frequency shifts, which were the two largest sources of uncertainty for the NPL-CsF2 caesium fountain clock. We report measurements that confirm a detailed theoretical model of the microwave cavity fields and the frequency shifts of the clock that they produce. The model and measurements significantly reduce the DCP uncertainty to 1.1 ? 10?16. We derive the microwave lensing frequency shift for a cylindrical cavity with circular apertures. An analytic result with reasonable approximations is given, in addition to a full calculation that indicates a shift of 6.2 ? 10?17. The measurements and theoretical models we report, along with improved evaluations of collisional and microwave leakage induced frequency shifts, reduce the frequency uncertainty of the NPL-CsF2 standard to 2.3 ? 10?16, nearly a factor of two lower than its most recent complete evaluation.

Determination of the Planck constant with the METAS watt balance

Abstract: The METAS watt balance project was initiated slightly more than a decade ago. Over this time, the apparatus has been through an uninterrupted series of upgrades that have improved its reliability to a point where continuous series of measurements can be taken fully automatically over periods of several months. A comprehensive analysis of possible systematic errors has now been completed and a large set of data has been analysed to calculate a value for the Planck constant h. This paper describes the watt balance in detail, explains the data acquisition and analysis thoroughly and presents the uncertainty budget. The value of the Planck constant determined with our apparatus is h = 6.626?069?1(20) ? 10?34?J?s with a relative standard uncertainty of 0.29 ? 10?6. This value differs from the 2006 CODATA adjustment by 0.024??W?W?1.

BeiDou Navigation Satellite System and its time scales

Abstract: The development and current status of BeiDou Navigation Satellite System are briefly introduced. The definition and realization of the system time scales are described in detail. The BeiDou system time (BDT) is an internal and continuous time scale without leap seconds. It is maintained by the time and frequency system of the master station. The frequency accuracy of BDT is superior to 2 ? 10?14 and its stability is better than 6 ? 10?15/30 days. The satellite synchronization is realized by a two-way time transfer between the uplink stations and the satellite. The measurement uncertainty of satellite clock offsets is less than 2?ns. The BeiDou System has three modes of time services: radio determination satellite service (RDSS) one-way, RDSS two-way and radio navigation satellite service (RNSS) one-way. The uncertainty of the one-way time service is designed to be less than 50?ns, and that of the two-way time service is less than 10?ns. Finally, some coordinate tactics of UTC from the viewpoint of global navigation satellite systems (GNSS) are discussed. It would be helpful to stop the leap second, from our viewpoint, but to keep the UTC name, the continuity and the coordinate function unchanged.

An electronic measurement of the Boltzmann constant

Abstract: The Boltzmann constant k was measured by comparing the Johnson noise of a resistor at the triple point of water with a quantum-based voltage reference signal generated with a superconducting Josephson-junction waveform synthesizer. The measured value of k = 1.380 651(17) × 10−23 J K−1 is consistent with the current CODATA value and the combined uncertainties. This is our first measurement of k with this electronic technique, and the first noise-thermometry measurement to achieve a relative combined uncertainty of 12 parts in 106. We describe the most recent improvements to our Johnson-noise thermometer that enabled the statistical uncertainty contribution to be reduced to seven parts in 106, as well as the further reduction of spurious systematic errors and electromagnetic interference effects. The uncertainty budget for this measurement is discussed in detail.

Surface layer determination for the Si spheres of the Avogadro project

Abstract: For the accurate determination of the Avogadro constant, two 28Si spheres were produced, whose macroscopic density, in addition to other values, must be determined. To make a contribution to the new definition of the kilogram, a relative standard uncertainty of less than 2 ? 10?8 has to be achieved. Each silicon surface is covered by a surface layer (SL). Consequently, correction parameters for the SL are determined to be applied to the mass and volume determination of the enriched spheres. With the use of a large set of surface analysing techniques, the structure of the SL is investigated. An unexpected metallic contamination existing on the sphere surface enlarges the uncertainty contribution of the correction parameters above the originally targeted value of 1 ? 10?8. In the framework of this investigation this new obstacle is resolved in two ways. A new combination of analytical methods is applied to measure the SL mass mSL and the thickness dSL, including this new contamination, with an uncertainty of u(mSL) = 14.5??g and 14.4??g, respectively, and u(dSL) = 0.33?nm and 0.32?nm for the 28Si spheres AVO28-S5 and AVO28-S8, respectively.In the second part of the work, the chemical composition of these metallic contaminations is found to be Cu, Ni and Zn silicide compounds. For the removal of this contamination, a special procedure is developed, tested and applied to the spheres to produce the originally expected surface structure on the spheres. After the application of this new procedure the use of x-ray reflectometry directly at the spheres will be possible. It is expected to reduce the uncertainty contribution due to the SL down to 1 ? 10?8.

Volume determination of the Avogadro spheres of highly enriched 28Si with a spherical Fizeau interferometer

Abstract: Within the scope of the efforts concerning the redefinition of the SI base unit 'kilogram' the final results of the international research project aiming at the redetermination of the Avogadro constant are ready to be announced. Among other quantities the volume of two spheres which are made of a highly enriched 28Si single crystal had to be determined. For this purpose a special Fizeau interferometer for the measurement of the spheres' diameters has been developed at PTB. This paper reports the final results of the volumes, the uncertainties and also the latest findings regarding systematic corrections including the effects of surface layers on the pure silicon core. The results are confirmed by density comparison measurements.

Gravimetric methods for the preparation of standard gas mixtures

Abstract: The most widely used method for the preparation of primary standard gas mixtures involves weighing the individual components into a cylinder. We present a new mathematical description of the method and its uncertainties. We use this to demonstrate how strategies for serial dilution can be identified that minimize the uncertainty in the final mixture and show how they can be implemented practically. We review published reports of high accuracy gravimetry and give examples of relative uncertainties in the composition of standards approaching 1 part-per-million in the best cases and in the range of 100 to 1000 parts-per-million more typically.

Volume measurements of 28Si spheres using an interferometer with a flat etalon to determine the Avogadro constant

Abstract: The volumes of two 1 kg silicon spheres, AVO28-S5 and AVO28-S8, fabricated from a 28Si-enriched crystal were measured to determine the Avogadro constant by the x-ray crystal density method in the International Avogadro Coordination Project. The volumes were determined from diameter measurements of the two spheres using a laser interferometer with a flat etalon. In the diameter measurement, the sphere was placed between the two flat etalon plates. The gaps between the sphere and the etalon plates and the distance between the etalon plates were measured by phase-shifting interferometry with optical frequency tuning. The apparent volumes of the 28Si spheres were determined from the diameter measurement in many directions with relative combined standard uncertainties of 5.0 × 10−8 and 4.4 × 10−8 for AVO28-S5 and AVO28-S8, respectively. The effect of the surface layer on the diameter measurement was evaluated on the basis of the results of characterizing the sphere surface. By taking into account the effect of the surface layer, the silicon core volume excluding the surface layer and the actual volume including the surface layer were also determined. Details of the interferometer, data analysis and the uncertainty in the measurement are described.

Accurate rubidium atomic fountain frequency standard

Abstract: The design, operating parameters and the accuracy evaluation of the NPL Rb atomic fountain are described The atomic fountain employs a double magneto-optical arrangement that allows a large number of 87Rb atoms to be trapped, a water-cooled temperature-stabilized interrogation region and a high quality factor interrogation cavity From the uncertainties of measured and calculated systematic frequency shifts, the fractional frequency accuracy is estimated to be 37 ? 10?16 The fractional frequency stability, limited predominantly by noise in the local oscillator, is measured to be 7 ? 10?16 after one day of averaging Based on the proposed quasi-continuous regime of operation of the fountain, the accuracy of the Rb standard of 5 ? 10?17 reachable in two days of averaging is predicted

Timescales at the BIPM

Abstract: This paper reviews the present status of the timescales established at the International Bureau of Weights and Measures (BIPM). We focus our attention on the calculation and the characteristics of Coordinated Universal Time (UTC) and present its applications.

Comparison of the Micro-g LaCoste gPhone-054 spring gravimeter and the GWR-C026 superconducting gravimeter in Strasbourg (France) using a 300-day time series

Abstract: We report on the results of a comparative analysis carried out on more than ten months of co-located records collected at the J9 gravity station in Strasbourg (France) with a new generation spring gravimeter, the Micro-g LaCoste, Inc. gPhone-054 and the GWR-C026 superconducting gravimeter (SG-C026). The gPhone is essentially a LaCoste & Romberg, model G meter, but with an improved thermal system (a double oven) for increased temperature stability, which should result in an unprecedented improvement in noise and drift of this mechanical gravimeter. We test the performances of the gPhone-054 in terms of resolution, accuracy, noise level and long-term stability (drift) with respect to the SG-C026. Our comparative analysis is performed in a wide spectral domain, ranging from the body tides to the seismic band. This study confirms that the SG has better performances over the whole analysed spectral band. The gPhone-054 instrumental drift observed during this study still remains a critical point preventing the study of long-term gravity changes. In fact, compared with the SG, the drift is large and even non-linear. We observed a drift rate evolution characterized by a decrease from 50 µGal/day to 15 µGal/day, after about 1 month of operation. This makes it hard to distinguish real-time gravity changes from the instrumental drift. We tried to improve the drift modelling by using frequent absolute gravity (AG) measurements, but unfortunately during that time no significant gravity changes have been detected which would have helped us to discriminate short-term drift excursions from real gravity changes. In terms of noise levels, the gPhone-054 turns out to be about 10 times noisier than the SG-C026 at seismic frequencies, while in the tidal band, it is twice as noisy. In between, at periods ranging from 1 h to 6 h, the gPhone-054 is about 3 times noisier than the SG-C026 but performs slightly better than the Scintrex CG5 in terms of noise level and precision.

Measurement of the {2?2?0} lattice-plane spacing of a 28Si x-ray interferometer

Abstract: The spacing of the {2?2?0} lattice planes of a 28Si crystal, used to determine the Avogadro constant by counting silicon atoms, was measured by combined x-ray and optical interferometry to a relative accuracy of 3.5 ? 10?9. The result is d2?2?0 = (192?014?712.67 ? 0.67)?am, at 20.0??C and 0?Pa. This value is greater by (1.9464 ? 0.0067) ? 10?6d2?2?0 than the spacing in natural Si, a difference which confirms quantum-mechanics calculations. This result is a key step towards a realization of the mass unit based on a conventional value of the Planck or the Avogadro constant.

Relativistic time transfer in the vicinity of the Earth and in the solar system

Abstract: The algorithms for relativistic time transfer in the vicinity of the Earth and in the solar system are derived. The concepts of proper time and coordinate time are distinguished. The coordinate time elapsed during the transport of a clock and the propagation of an electromagnetic signal is analysed in three coordinate systems: an Earth-Centred Inertial (ECI) coordinate system, an Earth-Centred Earth-Fixed (ECEF) coordinate system and a barycentric coordinate system. The timescales of Geocentric Coordinate Time (TCG), Terrestrial Time (TT) and Barycentric Coordinate Time (TCB) are defined and their relationships are discussed. Some numerical examples are provided to illustrate the magnitudes of the effects.

State-of-the-art mass determination of 28Si spheres for the Avogadro project

Abstract: For a new determination of the Avogadro constant to play a role in the future redefinition and realization of the mass unit, the mass measurements involved need to be carried out at a very high level of accuracy. From a mass comparison among two 1 kg 28Si spheres and the 1 kg platinum–iridium (Pt/Ir) mass standards, the mass of the spheres must be determined with a combined standard uncertainty of less than 5 µg. The BIPM, the PTB and the NMIJ have carried out such a state-of-the-art mass comparison in air and under vacuum in order to reach this target set by the International Avogadro Coordination (IAC). The results obtained for the spheres AVO28-S5 and AVO28-S8 involved in the comparison have demonstrated that by using air buoyancy artefacts and sorption artefacts it is possible to achieve a relative uncertainty of 4.1 × 10−9. The reference value for each sphere has been determined, taking into account the traceability of the masses to the International Prototype of the kilogram, , in due consideration of the correlations among 17 standards used directly or indirectly in this comparison.

Molar mass of silicon highly enriched in 28Si determined by IDMS

Abstract: The molar mass of a new silicon crystal material highly enriched in 28Si ('Si28', x(28Si) >99.99%) has been measured for the first time using a combination of a modified isotope dilution mass spectrometry (IDMS) technique and a high resolution multicollector-ICP-mass spectrometer. This work is related to the redetermination of the Avogadro constant NA with an intended relative measurement uncertainty urel(NA) ≤ 2 × 10−8. The corresponding experimental investigations of the International Avogadro Coordination (IAC) were performed using this novel 'Si28' material. One prerequisite of the redetermination of NA is the determination of the isotopic composition and thus molar mass of 'Si28' with urel(M('Si28')) ≤ 1 × 10−8. At PTB, a molar mass M('Si28') = 27.976 970 27(23) g mol−1 has been determined with an associated relative uncertainty urel(M('Si28')) = 8.2 × 10−9, opening the opportunity to reach the target uncertainty of NA.

Dimensional characterization of a quasispherical resonator by microwave and coordinate measurement techniques

Abstract: We describe the dimensional characterization of copper quasisphere NPL-Cranfield 2. The quasisphere is assembled from two hemispheres such that the internal shape is a triaxial ellipsoid, the major axes of which have nominal radii 62.000 mm, 62.031 mm and 62.062 mm. The artefact has been manufactured using diamond-turning technology and shows a deviation from design form of less than ±1 µm over most of its surface. Our characterization involves both coordinate measuring machine (CMM) experiments and microwave resonance spectroscopy.We have sought to reduce the dimensional uncertainty below the maximum permissible error of the CMM by comparative measurements with silicon and Zerodur spheres of known volume. Using this technique we determined the equivalent radius with an uncertainty of u(k = 1) = 114 nm, a fractional uncertainty of 1.8 parts in 106. Due to anisotropy of the probe response, we could only determine the eccentricities of the quasihemispheres with a fractional uncertainty of approximately 2%.Our microwave characterization uses the TM11 to TM18 resonances. We find the equivalent radius inferred from analysis of these modes to be consistent within ±4 nm with an overall uncertainty u(k = 1) = 11 nm. We discuss corrections for surface conductivity, waveguide perturbations and dielectric surface layers.We find that the CMM radius estimates derived from each hemisphere cannot be used to accurately predict the equivalent radius of the assembled resonator for two reasons. Firstly, the equatorial flanges are flat only to within ±1 µm, leading to an equatorial 'gap' whose dimension cannot be reliably estimated. Secondly, the resonator undergoes significant elastic distortion when the bolts connecting the hemispheres are tightened. We provide CMM and microwave measurements to support these conclusions in addition to finite-element modelling.Finally, we consider the implications of this work on a forthcoming experiment to determine the Boltzmann constant with a relative uncertainty below 1 part in 106.

Earth rotation monitoring, UT1 determination and prediction

Abstract: Monitoring the Earth's rotation angle is essential in various domains linked to reference systems such as space navigation, precise orbit determinations of artificial Earth satellites including the Global Navigation Satellite Systems (GNSS), positional astronomy and for geophysical studies on time scales ranging from a few hours to decades.Universal Time UT1 is based on the rotation of the Earth on its axis. Historically it was related to mean solar time on the meridian of Greenwich, sometimes known as Greenwich Mean Time. Monitoring Earth orientation, and in particular UT1, is the primary task of the International Earth Rotation and Reference Systems Service (IERS). The Earth Orientation Center is responsible for monitoring Earth orientation parameters (EOPs) including long-term consistency and leap second announcements. The Rapid Service/Prediction Center is in charge of the rapid, near real-time solution and predictions. These two complementary services of the IERS provide Earth orientation information from results derived predominantly from Very Long Baseline Interferometry with valuable input from GNSS observations and global atmospheric angular momentum for both the combination and prediction of EOPs.

Quantifying the predictive uncertainty of complex numerical models

Abstract: The overall uncertainty of a model prediction is a combination of the uncertainty of the input parameters and the uncertainty of the model assumptions. The former is referred to as parameter uncertainty; the latter model uncertainty. A method for quantifying model uncertainty is proposed for complicated numerical models that are not amenable to more traditional approaches. The method is based solely on comparisons of model predictions with experimental measurements, with the difference reported in terms of only two metrics, a bias factor and a relative standard deviation. The simplicity of the approach makes it ideal for models used for regulatory compliance because approving authorities often lack detailed training in modelling and uncertainty analysis.

Spectrometer bandwidth correction for generalized bandpass functions

Abstract: The bandpass of spectrometers can cause appreciable errors when making radiometric measurements. This paper describes a practical method for correcting a set of equispaced measured values provided by a spectrometer with a finite bandwidth, an arbitrary bandpass function and at an arbitrary wavelength step. The paper reviews the limits of the approach for real spectra in the presence of measurement noise and suggests ways of reducing the effect of noise.

Perturbations of the local gravity field due to mass distribution on precise measuring instruments: a numerical method applied to a cold atom gravimeter

Abstract: We present a numerical method, based on a FEM simulation, for the determination of the gravitational field generated by massive objects, whatever geometry and space mass density they have. The method was applied for the determination of the self-gravity effect of an absolute cold atom gravimeter which aims at a relative uncertainty of 10−9. The deduced bias, calculated with a perturbative treatment, is finally presented. The perturbation reaches (1.3 ± 0.1) × 10−9 of the Earth's gravitational field.

Final report on the Seventh International Comparison of Absolute Gravimeters (ICAG 2005)

Abstract: The Bureau International des Poids et Mesures (BIPM), S?vres, France, hosted the 7th International Comparison of Absolute Gravimeters (ICAG) and the associated Relative Gravity Campaign (RGC) from August to September 2005.ICAG 2005 was prepared and performed as a metrological pilot study, which aimed: To determine the gravity comparison reference values; To determine the offsets of the absolute gravimeters; and As a pilot study to accumulate experience for the CIPM Key Comparisons. This document presents a complete and extensive review of the technical protocol and data processing procedures. The 1st ICAG?RGC comparison was held at the BIPM in 1980?1981 and since then meetings have been organized every 4 years.In this paper, we present an overview of how the meeting was organized, the conditions of BIPM gravimetric sites, technical specifications, data processing strategy and an analysis of the final results. This 7th ICAG final report supersedes all previously published reports.Readings were obtained from participating instruments, 19 absolute gravimeters and 15 relative gravimeters. Precise levelling measurements were carried out and all measurements were performed on the BIPM micro-gravity network which was specifically designed for the comparison.

Phase corrections in the optical interferometer for Si sphere volume measurements at NMIJ

Abstract: To determine the volume of solid density standards, manufactured as Si-crystal spheres, an optical interferometer is used to measure their diameter at the NMIJ. To support and complement these measurements, the effect of the Gouy phase has been studied analytically and numerically. In measurement, the sphere is placed between the end-mirrors of a Fizeau cavity and the distances between the cavity mirrors and the sphere are measured, as well as the cavity length. The present analysis outlines a model of the interferometer operation and quantifies the Gouy-phase correction in the diameter measurement.

Simultaneous gravity and gradient measurements from a recoil-compensated absolute gravimeter

Abstract: This paper discusses simultaneous gravity and vertical gravity gradient measurements obtained with a newly designed recoil-compensated dropping chamber adapted to an FG5 absolute gravimeter. The new dropping chamber incorporates counterweights to compensate recoil effects. It has the same physical length as the standard FG5 dropping chamber but the free-fall distance was increased from 20 cm to 25 cm. The new drive train pulls on the centre of the system to reduce unwanted horizontal velocity and rotation of the free-falling test mass. The test-mass material was chosen to reduce possible magnetic eddy-current damping caused by external magnetic field gradients. External lead masses were used to change the gravity and vertical gravity gradient. The measurements agree well with the theoretical gravity field changes derived from the position of the external weights. The experiment clearly demonstrates the efficacy of using an absolute gravity meter to measure both the gravity and the gravity gradient signals caused by variations in the external gravity field. This technique shows promise for passive gravity-monitoring applications.

Infrared spectrometric measurement of impurities in highly enriched 'Si28'

Abstract: Within the scope of re-determining the Avogadro constant, the content of impurities in ultra-pure, mono-isotopically enriched silicon 'Si28' was measured by infrared spectrometry. The calibration factors required for the calculation of the concentrations from the maximum absorption coefficients had to be determined in this context for the isotopically enriched material. For this purpose a model was prepared, which enables conversion of the factors for Si with natural isotopic abundance. On the basis of these results, the concentrations of the three most decisive impurities in the enriched material were determined. These are oxygen atoms on interstitial sites and substituting carbon and boron atoms which occupy silicon sites. Whereas carbon and oxygen were measured in accordance with standardized procedures modified for the particular metrological use, the absorption measurement of the boron was carried out differently to the conventional standard. The concentrations determined for the three types of crystal impurities are high enough to make a correction of the lattice parameter for the 'Si28' material necessary.

Local representations of UTC in national laboratories

Abstract: Local representations of the international time scale, Coordinated Universal Time (UTC), are maintained by approximately 69 national measurement institutes and other time laboratories. These laboratories contribute their clock and time transfer measurements for use in the computation of UTC. Although local representations of UTC, commonly called UTC(k) time scales, vary considerably, for example in the numbers of atomic clocks available, they also share many characteristics. In this paper, we examine the rationale and requirements for maintaining a local representation of UTC. Its applications might range from underpinning the reference time scale of a Global Navigation Satellite System to providing traceability for frequency and time dissemination services. We address the practical aspects of setting up and operating a UTC(k) time scale, including the equipment and algorithms that generate the time scale, optimize its performance and measure its offset from the similar time scales maintained by other laboratories. We conclude by considering briefly some future developments that may have an impact on the laboratories operating local representations of UTC.

The neutron cross section standards, evaluations and applications

Abstract: The measurement of a neutron cross section can be simplified if it is measured relative to a neutron cross section standard. Then it is not necessary to measure the neutron fluence. The standards in effect provide determinations of the neutron fluence. These standards are not defined standards but instead working standards. As such it is important to make measurements to improve the quality of the standards. Also better techniques for evaluating the standards must be promoted. The history of the evaluation process for the standards is discussed from the first rather primitive one to the latest very sophisticated evaluation that was an international cooperative effort. An overview of the main detectors used for applications of the standards is given.

Quasi-monoenergetic high-energy neutron standards above 20?MeV

Abstract: This paper provides an overview of high-energy quasi-monoenergetic neutron sources and facilities above 20 MeV around the world. Various technical matters are discussed which are required in characterizing the neutron fields by spectrometry, fluence and beam profile measurements. Important topics regarding the calibration of neutron detectors are also introduced with emphasis on beam monitoring, tail correction, background subtraction and fluence-to-dose conversion. Efforts to standardize the high-energy neutron fluence in Japan and by the German national metrology institute in collaboration with Belgian and South African institutions are also presented.

GNSS times and UTC

Abstract: Global Navigation Satellite Systems (GNSSs) use internal reference time scales: GPS Time, GLONASS Time, Galileo System Time and BeiDou System Time. Constructed from a clock ensemble, they are designed for internal system synchronization, necessary to produce a navigation solution. They are usually steered to an external stable reference time scale, for example UTC(USNO), modulo 1 s, for GPS time. To achieve safe operation of a GNSS, a system time should preferably be a uniform time scale not affected by the leap seconds of Coordinated Universal Time (UTC). But this is not compatible with international recommendations that radio broadcast time signals should conform as closely as possible to UTC. This paper describes the various approaches chosen by GNSS providers and the relation between GNSS system times and UTC in terms of numbering of seconds. Different solutions for numbering seconds do not help the GNSS interoperability. This paper also explains why, on some occasions, GNSS system times play a role of alternative time scales with the consequent risk of confusion.