Showing papers in "Metrologia in 2014"

Acoustic gas thermometry

Abstract: We review the principles, techniques and results from primary acoustic gas thermometry (AGT). Since the establishment of ITS-90, the International Temperature Scale of 1990, spherical and quasi-spherical cavity resonators have been used to realize primary AGT in the temperature range 7 K to 552 K. Throughout the sub-range 90 K < T < 384 K, at least two laboratories measured (T − T90). (Here T is the thermodynamic temperature and T90 is the temperature on ITS-90.) With a minor exception, the resulting values of (T − T90) are mutually consistent within 3 × 10−6 T. These consistent measurements were obtained using helium and argon as thermometric gases inside cavities that had radii ranging from 40 mm to 90 mm and that had walls made of copper or aluminium or stainless steel. The AGT values of (T − T90) fall on a smooth curve that is outside ±u(T90), the estimated uncertainty of T90. Thus, the AGT results imply that ITS-90 has errors that could be reduced in a future temperature scale. Recently developed techniques imply that low-uncertainty AGT can be realized at temperatures up to 1350 K or higher and also at temperatures in the liquid-helium range.

First accuracy evaluation of NIST-F2

Abstract: We report the first accuracy evaluation of NIST-F2, a second-generation laser-cooled caesium fountain primary standard developed at the National Institute of Standards and Technology (NIST) with a cryogenic (liquid nitrogen) microwave cavity and flight region. The 80?K atom interrogation environment reduces the uncertainty due to the blackbody radiation shift by more than a factor of 50. Also, the Ramsey microwave cavity exhibits a high quality factor (>50?000) at this low temperature, resulting in a reduced distributed cavity phase shift. NIST-F2 has undergone many tests and improvements since we first began operation in 2008. In the last few years NIST-F2 has been compared against a NIST maser time scale and NIST-F1 (the US primary frequency standard) as part of in-house accuracy evaluations. We report the results of nine in-house comparisons since 2010 with a focus on the most recent accuracy evaluation. This paper discusses the design of the physics package, the laser and optics systems and the accuracy evaluation methods. The type B fractional uncertainty of NIST-F2 is shown to be 0.11???10?15 and is dominated by microwave amplitude dependent effects. The most recent evaluation (August 2013) had a statistical (type A) fractional uncertainty of 0.44???10?15.

Stability comparison of two absolute gravimeters: optical versus atomic interferometers

Abstract: We report the direct comparison between the stabilities of two mobile absolute gravimeters of different technology: the LNE-SYRTE Cold Atom Gravimeter and FG5X#216 of the Universite du Luxembourg. These instruments rely on two different principles of operation: atomic and optical interferometry. The comparison took place in the Walferdange Underground Laboratory for Geodynamics in Luxembourg, at the beginning of the last International Comparison of Absolute Gravimeters, ICAG-2013. We analyse a 2h10 duration common measurement, and find that the CAG shows better immunity with respect to changes in the level of vibration noise, as well as a slightly better short term stability.

Accuracy evaluation of ITCsF2: a nitrogen cooled caesium fountain

Abstract: For almost two decades, caesium fountain primary frequency standards (PFS) have represented the best realization of the definition of the second in the International System of units. Their accuracy has progressively improved with time, reaching a few parts in 10 16 .I n this paper, we present the accuracy evaluation of ITCsF2, the new Cs fountain PFS developed at the Italian National Metrological Institute, designed to be operated at cryogenic temperature to reduce the blackbody radiation shift. The short-term stability of the ITCsF2 fountain is 2 × 10 −13 τ −1/2 when operated at high atomic density, and the relative inaccuracy reaches 2.3 × 10 −16 . We also report four calibrations of International Atomic Time with a relative frequency agreement of (−1.7 ± 3.2) × 10 −16 , between ITCsF2 and the average of the other

A determination of Planck's constant using the NRC watt balance

Abstract: We have measured Planck's constant and have obtained a value of 6.626?070?34(12)???10?34?J?s. To our knowledge this measurement of h has the lowest uncertainty reported to date. This result has been obtained from measurements of four masses of different material and nominal values varying from 1?kg to 250?g. The experimental procedures and the measurement uncertainties are described in detail.

Using measurement uncertainty in decision-making and conformity assessment

Abstract: Measurements often provide an objective basis for making decisions, perhaps when assessing whether a product conforms to requirements or whether one set of measurements differs significantly from another. There is increasing appreciation of the need to account for the role of measurement uncertainty when making decisions, so that a 'fit-for-purpose' level of measurement effort can be set prior to performing a given task. Better mutual understanding between the metrologist and those ordering such tasks about the significance and limitations of the measurements when making decisions of conformance will be especially useful. Decisions of conformity are, however, currently made in many important application areas, such as when addressing the grand challenges (energy, health, etc), without a clear and harmonized basis for sharing the risks that arise from measurement uncertainty between the consumer, supplier and third parties.In reviewing, in this paper, the state of the art of the use of uncertainty evaluation in conformity assessment and decision-making, two aspects in particular—the handling of qualitative observations and of impact—are considered key to bringing more order to the present diverse rules of thumb of more or less arbitrary limits on measurement uncertainty and percentage risk in the field. (i) Decisions of conformity can be made on a more or less quantitative basis—referred in statistical acceptance sampling as by 'variable' or by 'attribute' (i.e. go/no-go decisions)—depending on the resources available or indeed whether a full quantitative judgment is needed or not. There is, therefore, an intimate relation between decision-making, relating objects to each other in terms of comparative or merely qualitative concepts, and nominal and ordinal properties. (ii) Adding measures of impact, such as the costs of incorrect decisions, can give more objective and more readily appreciated bases for decisions for all parties concerned. Such costs are associated with a variety of consequences, such as unnecessary re-manufacturing by the supplier as well as various consequences for the customer, arising from incorrect measures of quantity, poor product performance and so on.

Determination of the Planck constant using a watt balance with a superconducting magnet system at the National Institute of Standards and Technology

Abstract: For the past two years, measurements have been performed with a watt balance at the National Institute of Standards and Technology (NIST) to determine the Planck constant. A detailed analysis of these measurements and their uncertainties has led to the value h = 6.626 069 79(30) × 10−34 J s. The relative standard uncertainty is 45 × 10−9. This result is 141 × 10−9 fractionally higher than h90. Here h90 is the conventional value of the Planck constant given by , where KJ-90 and RK-90 denote the conventional values of the Josephson and von Klitzing constants, respectively.

Contributing to TAI with a secondary representation of the SI second

Abstract: We report the first contribution to international atomic time (TAI) based on a secondary representation of the SI second. This work is done with the LNE-SYRTE FO2-Rb fountain frequency standard using the 87Rb ground-state hyperfine transition. We describe FO2-Rb and how it is connected to local and international time scales. We report on local measurements of this frequency standard in the SI system, i.e. against primary frequency standards (PFSs), down to a fractional uncertainty of 4.4 × 10−16, and on the establishment of the recommended value for the 87Rb hyperfine transition by the CIPM. We also report on the process that led to the participation of the FO2-Rb frequency standard in Circular T and to the elaboration of TAI. This participation enables us to demonstrate absolute frequency measurement directly in terms of the SI second realized by the TAI ensemble with a statistical uncertainty of 1.1 × 10−16, therefore at the limit allowed by the accuracy of PFSs. This work constitutes a demonstration of how other secondary representations, based on optical transitions, could also be used for TAI, and an investigation of a number of issues relevant to a future redefinition of the SI second.

Carrier-phase Two-Way Satellite Frequency Transfer over a Very Long Baseline

Abstract: In this paper we report that carrier-phase two-way satellite time and frequency transfer (TWSTFT) was successfully demonstrated over a very long baseline of 9000 km, established between the National Institute of Information and Communications Technology (NICT) and the Physikalisch-Technische Bundesanstalt (PTB). We verified that the carrier-phase TWSTFT (TWCP) result agreed with those obtained by conventional TWSTFT and GPS carrier-phase (GPSCP) techniques. Moreover, a much improved short-term instability for frequency transfer of 2 × 10−13 at 1 s was achieved, which is at the same level as previously confirmed over a shorter baseline within Japan. The precision achieved was so high that the effects of ionospheric delay became significant; they are ignored in conventional TWSTFT even over a long link. We compensated for these effects using ionospheric delays computed from regional vertical total electron content maps. The agreement between the TWCP and GPSCP results was improved because of this compensation.

Towards a new SI: a review of progress made since 2011

Abstract: In 2011, the General Conference on Weights and Measures (CGPM) confirmed its intention to adopt new definitions for four of the base units of the SI based on fixed numerical values of selected constants These will be the kilogram, the ampere, the kelvin and the mole The CGPM was not able to adopt the new definitions at that time because certain experimental and coordination work was not complete This paper reviews criteria proposed by the Consultative Committees of the CIPM for such a ?new SI? to be adopted and reports on recent progress with work to address them We also report on work being undertaken to demonstrate that the most important technical aspects of realizing such a new system are practicable The progress reported here confirms the consensus developing amongst the Consultative Committees and the National Metrology Institutes that it will be possible for the CGPM to adopt these new definitions in 2018

On a monte Carlo method for measurement uncertainty evaluation and its implementation.

Abstract: The 'Guide to the Expression of Uncertainty in Measurement' (GUM) provides a framework and procedure for evaluating and expressing measurement uncertainty. The procedure has two main limitations. Firstly, the way a coverage interval is constructed to contain values of the measurand with a stipulated coverage probability is approximate. Secondly, insufficient guidance is given for the multivariate case in which there is more than one measurand. In order to address these limitations, two specific guidance documents (or 'Supplements to the GUM') on, respectively, a Monte Carlo method for uncertainty evaluation (Supplement 1) and extensions to any number of measurands (Supplement 2) have been published. A further document on developing and using measurement models in the context of uncertainty evaluation (Supplement 3) is also planned, but not considered in this paper.An overview is given of these guidance documents. In particular, a Monte Carlo method, which is the focus of Supplements 1 and 2, is described as a numerical approach to implement the 'propagation of distributions' formulated using the 'change of variables formula'. Although applying a Monte Carlo method is conceptually straightforward, some of the practical aspects of using the method are considered, such as the choice of the number of trials and ensuring an implementation is memory-efficient. General comments about the implications of using the method in measurement and calibration services, such as the need to achieve transferability of measurement results, are made.

Design of the KRISS watt balance

Abstract: We report the design of the KRISS watt balance, which includes a magnet, a guiding stage and a coil position measurement system. The KRISS watt balance incorporates a closed-type cylindrical permanent magnet and a motion guiding stage. For the magnet, a flux shunt is used to reduce flux changes due to temperature variations. A piston gauge is used to achieve linearity in the motion guiding stage. In the weighing mode, the residual force between the weight of the test mass and the Lorentz force generated in a coil is measured by a commercial weighing cell. In the dynamic mode, a linear motor in the motion guiding stage vertically translates the coil and the weighing cell. The in-plane motion of the coil is measured by position sensors, and the out-of-plane motion is measured by single-pass homodyne interferometers.

Mathematical modelling to support traceable dynamic calibration of pressure sensors

Abstract: This paper focuses on the mathematical modelling required to support the development of new primary standard systems for traceable calibration of dynamic pressure sensors. We address two fundamentally different approaches to realizing primary standards, specifically the shock tube method and the drop-weight method. Focusing on the shock tube method, the paper presents first results of system identification and discusses future experimental work that is required to improve the mathematical and statistical models. We use simulations to identify differences between the shock tube and drop-weight methods, to investigate sources of uncertainty in the system identification process and to assist experimentalists in designing the required measuring systems. We demonstrate the identification method on experimental results and draw conclusions.

A magnet system for the MSL watt balance

Abstract: A novel design for the magnet system of a watt balance is presented. While primarily intended for the watt balance being developed by MSL, this design could be used for other watt balances. It has a radial magnetic field in an annular gap that is screened from external fields. The magnetic field is generated by a single ring-shaped permanent magnet and the geometry is such that the field induced in the permanent magnet by the current in the coil in the weighing mode is zero on average. Hence the magnetic field in the gap is expected to be the same for both the weighing and dynamic modes of a watt balance. From finite element modelling, the magnetic field strength can be uniform to within 40 parts in 106 over more than 50 mm (50% of the gap height) and the usable working range for the coil is about 40 mm. Further, the diameter of the annular gap can be relatively small, allowing the coil and its support structure to be kept relatively stiff and light.

Revision of the ?Guide to the Expression of Uncertainty in Measurement?. Why and how

Abstract: The ‘Guide to the Expression of Uncertainty in Measurement’ has now served for more than twenty years. In this communication, after attempting a balance over this period, the logical reasons are given that, on the one hand, led to the decision to update such a successful document, and, on the other hand, dictated the modifications that are being carried out with respect to the current 2008 edition.The author is convener of the Joint Committee for Guides in Metrology (JCGM) Working Group 1 (Guide to the expression of uncertainty in measurement, or GUM). The opinion expressed in this paper does not necessarily represent the view of this Working Group.

The investigation of a μGal-level cold atom gravimeter for field applications

Abstract: Typical atomic gravimeters have a heavy magnetic shielding to avoid the quadratic Zeeman effect, and also a complicated active vibration isolation system to suppress the vibration noise. Both of them make it difficult to build a mobile and compact gravimeter. In this paper, we present the implementation of an atomic gravimeter aiming at field applications. Our gravimeter uses improved magnetic coils instead of the expensive mu-metal for magnetic shielding. The quadratic Zeeman shift is evaluated with high accuracy. Moreover, a portable platform of relatively small size is applied for vibration isolation. The total interrogation time is optimized to 120 ms and the repetition rate is 2.2 Hz. A sensitivity of 1.0 × 10−7 g Hz−1/2 and a resolution of 5.7 × 10−9 g within 1000 s integration time are reached. A continuous g measurement over 128 h is carried out. Moreover, a whole seismic wave of about 1 h that occurred in Pakistan on 28 September 2013 is recorded by our atomic gravimeter. The results coincide with that recorded by a traditional seismic detector very well.

Principles of a new generation of simplified and accurate watt balances

Abstract: It has been known for some time that the watt balance, which is based on the virtual work principle, is insensitive to some misalignments which would, at first sight, be expected to produce significant errors. In this paper we show that, in particular circumstances, this insensitivity applies to a whole range of misalignments. This effect can be exploited to design a watt balance, with a constrained coil motion, which offers advantages over conventional designs. We present three such new designs: one is based on a conventional balance and the other two are based on strip hinges, which are known to have excellent properties when measuring small force differences and with the production of precise linear movements having little motion in the other five degrees of freedom. A constrained design of this kind would have advantages for reproducing the forthcoming SI definition of mass in the range from milligrams to kilograms, whenever and wherever desired, and might well do so with improved accuracy.

Absolute silicon molar mass measurements, the Avogadro constant and the redefinition of the kilogram

Abstract: The results of an absolute silicon molar mass determination of two independent sets of samples from the highly 28Si-enriched crystal (AVO28) produced by the International Avogadro Coordination are presented and compared with results published by the Physikalisch-Technische Bundesanstalt (PTB, Germany), the National Research Council (NRC, Canada) and the National Metrology Institute of Japan (NMIJ, Japan). This study developed and describes significant changes to the published protocols for producing absolute silicon isotope ratios. The measurements were made at very high resolution on a multi-collector inductively coupled plasma mass spectrometer using tetramethylammonium hydroxide (TMAH) to dissolve and dilute all samples. The various changes in the measurement protocol and the use of TMAH resulted in significant improvements to the silicon isotope ratio precision over previously reported measurements and in particular, the robustness of the 29Si/30Si ratio of the AVO28 material. These new results suggest that a limited isotopic variability is present in the AVO28 material. The presence of this variability is at present singular and therefore its significance is not well understood. Fortunately, its magnitude is small enough so as to have an insignificant effect on the overall uncertainty of an Avogadro constant derived from the average molar mass of all four AVO28 silicon samples measured in this study. The NIST results confirm the AVO28 molar mass values reported by PTB and NMIJ and confirm that the virtual element–isotope dilution mass spectrometry approach to calibrated absolute isotope ratio measurements developed by PTB is capable of very high precision as well as accuracy. The Avogadro constant NA and derived Planck constant h based on these measurements, together with their associated standard uncertainties, are 6.02214076(19) × 1023 mol−1 and 6.62607017(21) × 10−34 Js, respectively.

UTCr: a rapid realization of UTC

Abstract: Considering the evolving needs of time metrology and the convenience of allowing the contributing laboratories access to a realization of UTC more frequently than through the monthly Circular T, the BIPM Time Department started in 2012 to implement the computation of UTCr, a rapid realization of UTC published every week and based on daily data After 18 months of pilot experiment, this new product has been declared operational and is now an official publication of the BIPM This paper presents the main characteristics and properties of UTCr

The joule balance in NIM of China

Abstract: The advantage of the joule balance over the classic watt balance is that the dynamic measurement in the watt balance is replaced by a static measurement, which makes the whole measurement procedure easier. The main problems in the joule balance are the precise measurement of mutual inductance and coil heating. These problems and recent progress in the development of the joule balance are described and discussed in this paper. The whole system is at the stage of being adjusted and improved. The principle of the joule balance has been demonstrated by a measurement of the Planck constant, h = 6.626 104(59) × 10−34 J s with an 8.9 ppm measurement uncertainty.

Evaluation of Long Term Performance of Continuously Running Atomic Fountains

Abstract: An ensemble of rubidium atomic fountain clocks has been put into operation at the US Naval Observatory (USNO). These fountains are used as continuous clocks in the manner of commercial caesium beams and hydrogen masers for the purpose of improved timing applications. Four fountains have been in operation for more than two years and are included in the ensemble used to generate the USNO master clock. Individual fountain performance is characterized by a white-frequency noise level below 2 × 10−13 and fractional-frequency stability routinely reaching the low 10−16 s. The highest performing pair of fountains exhibits stability consistent with each fountain integrating as white frequency noise, with Allan deviation surpassing 6 × 10−17 at 107 s, and with no relative drift between the fountains at the level of 7.5 × 10−19/day. As an ensemble, the fountains generate a timescale with white-frequency noise level of 1 × 10−13 and long-term frequency stability consistent with zero drift relative to the world's primary standards at 1 × 10−18/day. The rubidium fountains are reported to the BIPM as continuously running clocks, as opposed to secondary standards, the only cold-atom clocks so reported. Here we further characterize the performance of the individual fountains and the ensemble during the first two years in an operational environment, presenting the first look at long-term continuous behavior of fountain clocks.

Link calibration against receiver calibration: an assessment of GPS time transfer uncertainties

Abstract: We present a direct comparison between two different techniques for the relative calibration of time transfer between remote time scales when using the signals transmitted by the Global Positioning System (GPS). Relative calibration estimates the delay of equipment or the delay of a time transfer link with respect to reference equipment. It is based on the circulation of some travelling GPS equipment between the stations in the network, against which the local equipment is measured. Two techniques can be considered: first a station calibration by the computation of the hardware delays of the local GPS equipment; second the computation of a global hardware delay offset for the time transfer between the reference points of two remote time scales. This last technique is called a 'link' calibration, with respect to the other one, which is a 'receiver' calibration. The two techniques require different measurements on site, which change the uncertainty budgets, and we discuss this and related issues. We report on one calibration campaign organized during Autumn 2013 between Observatoire de Paris (OP), Paris, France, Observatoire de la Cote d'Azur (OCA), Calern, France, and NERC Space Geodesy Facility (SGF), Herstmonceux, United Kingdom. The travelling equipment comprised two GPS receivers of different types, along with the required signal generator and distribution amplifier, and one time interval counter. We show the different ways to compute uncertainty budgets, leading to improvement factors of 1.2 to 1.5 on the hardware delay uncertainties when comparing the relative link calibration to the relative receiver calibration.

Molar-mass measurement of a 28Si-enriched silicon crystal for determination of the Avogadro constant

Abstract: The molar mass of a 28Si-enriched crystal was measured at the National Metrology Institute of Japan to determine the Avogadro constant by the x-ray crystal density method as part of the International Avogadro Coordination project. The molar mass was determined by isotope ratio measurements using a multicollector inductively coupled plasma mass spectrometer combined with an isotope dilution technique. The 28Si-enriched crystal was dissolved in tetramethylammonium hydroxide and three different blended solutions were used to correct for mass bias in the measurement. The molar mass of the 28Si-enriched crystal was determined to be 27.976 970 09 g mol−1 with a standard uncertainty of 0.000 000 14 g mol−1. This corresponds to a relative standard uncertainty of 5.2 × 10−9. This result is consistent with measurements reported by the Physikalisch-Technische Bundesanstalt, Germany.

Time transfer by laser link (T2L2): characterization and calibration of the flight instrument,

Abstract: The T2L2 project (time transfer by laser link) allows for the synchronization of remote ultra-stable clocks over intercontinental distances (Fridelance et al 1997 Exp. Astron. 7, Samain and Fridelance 1998 Metrologia 35 151–9). The principle is derived from satellite laser ranging technology with dedicated space equipment designed to record arrival times of laser pulses at the satellite. The space segment has been launched in June 2008 as a passenger experiment on the ocean altimetry satellite Jason 2. T2L2 had been specified to yield a time stability of better than 1 ps over 1000 s integration time and an accuracy of better than 100 ps. This level of performance requires a rigorous data processing which can be performed only with a comprehensive calibration model of the whole instrumentation. For this purpose, several experimental measurements have been performed before and during the integration phase of the T2L2 space instrument. This instrument model is one of the cornerstones of the data reduction process which is carried out to translate the raw information to a usable picosecond time transfer. After providing a global synopsis of the T2L2 space instrument, the paper gives a description of the experimental setup for the instrument characterization. It then details the different contributions within the calibration model and concludes with an applied example of a space to ground time transfer.

A new weighting procedure for UTC

Abstract: In this paper we present a new weighting algorithm for the generation of Coordinated Universal Time (UTC) The new weighting procedure is based on the idea that the best clocks are the most predictable The effect of the new algorithm on the weight distribution and on the stability of UTC is presented and discussed An improvement in the frequency stability of UTC at both short and long terms is observed, as well as a better clock weight distribution in the ensemble

A watt balance based on a simultaneous measurement scheme

Abstract: The International Bureau of Weights and Measures (BIPM) is developing a novel watt balance based on a simultaneous measurement scheme for the forthcoming redefinition of the kilogram The two distinct measurement phases in a conventional watt balance are carried out in a single phase where all quantities are measured simultaneously The main characteristics of this simultaneous measurement approach are described An analysis of the advantages and the drawbacks is carried out

On the adjustment of inconsistent data using the Birge ratio

Abstract: The Birge ratio is applied in metrology to enlarge quoted uncertainties when combining inconsistent measurement results on the same measurand. We discuss the statistical model underlying such a procedure and argue that the resulting uncertainty associated with the adjusted value is underrated. We provide a simple modification of this uncertainty on the basis of an objective Bayesian inference. While the proposed uncertainty approaches that obtained by the conventional procedure for a large number n of combined measurement results, differences are significant for small n. For example, for n = 4 we get an increase of 73% in the standard uncertainty associated with the adjusted value, and for n = 10 the increase is still 13%. We derive the posterior distribution for the adjusted value in closed form, including a 95% credible interval. In addition, we show that our results do not only hold when the distribution of the measurement results is assumed to be Gaussian, but for a whole family of (elliptically contoured) location-scale distributions. We illustrate the modified Birge method by its application to data from the 2002 adjustment of the Newtonian constant of gravitation.

High-accuracy measurement of linearity of optical detectors based on flux addition of LEDs in an integrating sphere

Abstract: We present an instrumental realization of linearity measurement of optical detectors based on flux addition of two light-emitting diodes (LEDs) in an integrating sphere. The instrument measures the linearity ratios at pre-defined photocurrent levels according to the algorithm which eliminates the drift effect of LED flux (Shin D J et al 2005 Metrologia 42 154). As a result, the non-linearity correction factors for detector responsivity are determined with their relative standard uncertainties ranging from 2 × 10−5 to 6 × 10−5 (k = 1) for photocurrent ranges from 10−8 A to 10−3 A. By changing the type of LEDs, the wavelength dependence of non-linearity can also be measured, which is demonstrated at 390 nm, 670 nm and 750 nm for a Si photodiode used as a transfer standard for spectral responsivity scale comparisons.

Controlling the non-linear intracavity dynamics of large He–Ne laser gyroscopes

Abstract: A model based on Lamb's theory of gas lasers is applied to a He–Ne ring laser (RL) gyroscope to estimate and remove the laser dynamics contribution from the rotation measurements. The intensities of the counter-propagating laser beams exiting one cavity mirror are continuously observed together with a monitor of the laser population inversion. These observables, once properly calibrated with a dedicated procedure, allow us to estimate cold cavity and active medium parameters driving the main part of the non-linearities of the system. The quantitative estimation of intrinsic non-reciprocal effects due to cavity and active medium non-linear coupling plays a key role in testing fundamental symmetries of space–time with RLs. The parameter identification and noise subtraction procedure has been verified by means of a Monte Carlo study of the system, and experimentally tested on the G-PISA RL oriented with the normal to the ring plane almost parallel to the Earth's rotation axis. In this configuration the Earth's rotation rate provides the maximum Sagnac effect while the contribution of the orientation error is reduced to a minimum. After the subtraction of laser dynamics by a Kalman filter, the relative systematic errors of G-PISA reduce from 50 to 5 parts in 103 and can be attributed to the residual uncertainties on geometrical scale factor and orientation of the ring.

Assessment of AERONET-OC LWN uncertainties

Abstract: This study presents a detailed analysis of the uncertainties affecting the normalized water-leaving radiance (LWN) from above-water measurements performed within the context of the Ocean Colour component of the Aerosol Robotic Network (AERONET-OC). The analysis, conducted in agreement with the ?Guide to the Expression of Uncertainty in Measurement? (GUM), indicates uncertainties of LWN markedly dependent on the optical properties of seawater for a number of AERONET-OC sites located in different marine regions. Results obtained for the Adriatic Sea site, characterized by a large variety of measurement conditions, confirm previous uncertainties from an independent study indicating median values of relative combined uncertainties of 5% in the blue?green part of the spectrum and of approximately 7% in the red. Additional investigations show that the former uncertainties can be reduced by ?1% when restricting the determination of AERONET-OC LWN to measurements performed at low sun zenith angle and low aerosol optical thickness.