Search

We have developed the WebTimer, an Internet service, accessible through a browser, for checking the operation of stopwatches and timers. We explain its design, operation and performance. A detailed uncertainty budget has been developed to assist the user. The WebTimer has been available as a paid subscription service since July 2020.

We have evaluated the performance of the Septentrio mosaic-T GNSS receiver for both code and carrier-phase time transfer. The receiver’s performance has been compared on short baselines with several other receivers. The data indicate satisfactory performance for all but the most demanding applications.

GPS-based time transfer can be used to establish a traceable link between a national time standard and a remote user’s standard. Equipment cost can be a consideration in such applications, and the GPS receiver used can be a significant component of this cost. We consider the use of low-cost timing receivers in time transfer, establishing the requirements for such receivers. Three suitable receivers are characterized, using software we have written for the Open Traceable Time Platform, an open source project developing a system for legally traceable time and frequency. The tested receivers were found to be suitable for all but the most demanding applications.

Some disulfide bonds perform important structural roles in proteins, but another group has functional roles via redox reactions. Forbidden disulfides are stressed disulfides found in recognizable protein contexts, which currently constitute more than 10% of all disulfides in the PDB. They likely have functional redox roles and constitute a major subset of all redox-active disulfides. The torsional strain of forbidden disulfides is typically higher than for structural disulfides, but not so high as to render them immediately susceptible to reduction under physionormal conditions. Previously we characterized the most abundant forbidden disulfide in the Protein Data Bank, the aCSDn: a canonical motif in which disulfide-bonded cysteine residues are positioned directly opposite each other on adjacent anti-parallel β-strands such that the backbone hydrogen-bonded moieties are directed away from each other. Here we perform a similar analysis for the aCSDh, a less common motif in which the opposed cysteine residues are backbone hydrogen bonded. Oxidation of two Cys in this context places significant strain on the protein system, with the β-chains tilting toward each other to allow disulfide formation. Only left-handed aCSDh conformations are compatible with the inherent right-handed twist of β-sheets. aCSDhs tend to be more highly strained than aCSDns, particularly when both hydrogen bonds are formed. We discuss characterized roles of aCSDh motifs in proteins of the dataset, which include catalytic disulfides in ribonucleotide reductase and ahpC peroxidase as well as a redox-active disulfide in P1 lysozyme, involved in a major conformation change. The dataset also includes many binding proteins.

The Open Traceable Time Platform is an open source project that is developing a complete software and low-cost hardware solution for disseminating time and frequency traceable to national standards. The origins of this project, its aims and progress to date are summarised.

New global navigation satellite systems (GNSS) such as BeiDou and Galileo are now available and potentially useable for time transfer. Low-cost receivers suitable for time transfer are also now being offered with multi-GNSS capabilities. Preliminary data on the time transfer performance of three low-cost receivers are presented and discussed.

Supplemental Digital Content is available in the text., Background and Purpose— Movement behaviors, that is, both physical activity and sedentary behavior, are independently associated with health risks. Although both behaviors have been investigated separately in people after stroke, little is known about the combined movement behavior patterns, differences in these patterns between individuals, or the factors associated with these patterns. Therefore, the objectives of this study are (1) to identify movement behavior patterns in people with first-ever stroke discharged to the home setting and (2) to explore factors associated with the identified patterns. Methods— Cross-sectional design using data from 190 people with first-ever stroke discharged to the home setting. Movement, behavior was measured over 2 weeks using an accelerometer. Ten movement behavior outcomes were calculated and compressed using principal component analysis. Movement behavior patterns were identified using a k-means clustering algorithm. Demographics, stroke, care, physical functioning, and psychological, cognitive and social factors were obtained. Differences between and factors associated with the patterns were investigated. Results— On average, the accelerometer was worn for 13.7 hours per day. The average movement behavior of the participants showed 9.3 sedentary hours, 3.8 hours of light physical activity, and 0.6 hours of moderate-vigorous physical activity. Three patterns and associated factors were identified: (1) sedentary exercisers (22.6%), with a relatively low age, few pack-years, light drinking, and high levels of physical functioning; (2) sedentary movers (45.8%), with less severe stroke symptoms, low physical functioning and high levels of self-efficacy; and (3) sedentary prolongers (31.6%), with more severe stroke symptoms, more pack-years, and low levels of self-efficacy. Conclusions— The majority of people with stroke are inactive and sedentary. Three different movement behavior patterns were identified: sedentary exercisers, sedentary movers, and sedentary prolongers. The identified movement behavior patterns confirm the hypothesis that an individually tailored approach might be warranted with movement behavior coaching by healthcare professionals.

Very-long-baseline interferometry (VLBI) for high-resolution astronomical imaging requires phase-stable frequency references at widely separated radio-telescope antennas. For the first time to our knowledge, we have disseminated a suitable radio-frequency (RF) reference for VLBI over a “real-world” telecom optical-fiber link between radio telescopes that are >100 km apart, by means of an innovative phase-conjugation technique. Bidirectional optical amplification is used in parallel with live traffic, and phase perturbations in the effective optical-fiber path length are compensated. This RF-over-fiber approach obviates the need for separate hydrogen masers at each antenna, offering significant advantages for radio-astronomy facilities such as the Square Kilometer Array.

A fiber-based frequency comb has been developed to measure the frequency of a 633-nm iodine-stabilized laser at the National Metrology Institute of Japan (NMIJ). The measured frequency was consistent with the previous results measured with a Ti:sapphire-based comb. The NMIJ comb was shipped to Australia for the validation of the measurement capability of the National Measurement Institute, Australia (NMIA) and NMIJ combs using a common microwave reference and a common optical frequency. Consequently, the frequency consistency of the two combs was approximately 8 times10-17. Furthermore, we demonstrate that an absolute mode number of the comb can easily and clearly be determined by using the two combs.

The density of a single-crystal natural silicon sphere identified as ldquoAvo#3rdquo has been redetermined at the National Measurement Institute, Australia (NMIA), as part of an international collaboration to determine the value of the Avogadro constant. A systematic review of the measurement system found that a significant bias was introduced in the measurement of the volume of the sphere, which is calculated from the interferometric measurements of the mean diameter. It was found that the dynamic response of the laser that is used to generate the frequency steps for the phase-shifting interferometry introduced a bias to the returned phase value. We determined the magnitude of the bias and then applied corrections to obtain the final measured value. The density reported here is based on some new and some recalculated measurements taken between April 2000 and May 2006 during the ongoing development of the measurement system.

The National Measurement Institute, Australia (NMIA) has been developing GPS Common View (GPSCV) time-transfer systems since the late 1990's. These provide traceability to UTC(AUS) for laboratory frequency references and network time. We describe our system, including advice on the use of low-cost receivers for GPSCV, summarise our 15 years of providing services based on it and conclude with a summary of current and future developments.

The National Measurement Institute, Australia has been developing methods for time and frequency transfer over optical fibre with an eye towards applications in radio-astronomy. We report here on a digital time-transfer system that uses pseudo-random noise-ranging and is implemented on a Field Programmable Gate Array. The system has demonstrated a time deviation of a few ps on a 100 km link.

The cross sections for quenching by H2 of the 4p´[1/2]1 and 4p´[1/2]0 states of Ar I, corresponding to emission lines at 696.5 and 750.4 nm, are obtained in a gas discharge using emission spectroscopy in conjunction with a laser-induced fluorescence measurement of the atomic hydrogen density, and found to be (1.3±0.3) × 10-19 and (1.5±0.3) × 10-19 m2, respectively. The cross sections for the quenching of H(n = 4) and H(n = 5) by H2 are obtained using emission spectroscopy and the known quenching cross section for H(n = 3) and are found to be 3.1(±0.1)(-0.9) × 10-19 and 8.3(±0.7)(-2.5) × 10-19 m2, respectively, where the first uncertainty is the random error and the second uncertainty is a possible systematic error.

Two-photon laser-induced fluorescence (LIF) is used to study the production and loss of H atoms in a pulsed microwave discharge in over the pressure range 1-50 Torr. Absolute measurements of the H atom density are made at the end of the pulse. These measurements were calibrated using a new technique based on the decay rate of the LIF signal. The temporal variation of emission during pulsing of the discharge is used to estimate the rate of dissociation of , which compares well with the predictions of a one-dimensional model for the electron energy distribution function. This measurement also gives the wall recombination probability for H atoms, which is compared with that obtained by LIF measurement of the decay of the H atom density in the pulse afterglow.

The validity of assuming a Maxwellian electron energy distribution function (EEDF) is examined for the case of a diamond depositing microwave discharge in H2 (With the effects of the carbon bearing species CH4 neglected). A Boltzmann kinetic model is developed for computation of the EEDF and this is used to predict rates for electron impact processes of interest. Particular attention is paid to the effects of superelastic and electron-electron collisions. The rate coefficients are compared with those obtained by assuming a Maxwellian EEDF of the same average energy as the calculated EEDF. Under typical reactor conditions, assumption of a Maxwellian EEDF is found to be satisfactory for predicting the H2 dissociation rate and local energy balance may be used to predict the average electron energy.

We present two optical fiber-based radio-frequency signal distribution systems based on analog and all-digital electronics. The achieved fractional frequency transfer stabilities are similar: ∼6×10−17 (over 104 s) for analog using a 20km fiber spool.

We present a novel optical fiber-based radio-frequency distribution system that incorporates low-cost commercially available components. It has a fractional frequency stability of 7×10−17 (averaged over 104 s) for distribution of an 80-MHz signal.

An optical-fibre network is being planned for distribution of optical and microwave reference frequencies in Australia. This network would connect a number of research facilities and the National Measurement Institute (NMI), allowing long-range comparison of frequency standards to high precision and providing reference signals for a wide range of applications. First results for transfer in the laboratory achieve a fractional frequency stability of 7×10−17 at an averaging time of 104 s for the distribution of an 80 MHz radio frequency signal. We present details of the technique used to cancel noise introduced by the fibre, describe the all-digital radio-frequency phasemeter used to characterise performance, and outline plans for the further development of the network.

Microwave frequency standards based on the 12.6 GHz ground state hyperfine transition in 171Yb+ have been under development at the National Measurement Institute, Australia, for many years. Using a laser-cooled ion cloud, the transition frequency was measured in 2001 to an accuracy of 8 parts in 1014 , limited by the homogeneity of the magnetic field. Uncertainties associated with field inhomogeneity in the new vacuum system are now below 1 part in 1015. We demonstrate that other systematic uncertainties such as AC Zeeman shift, microwave imperfections and pressure shifts permit operation in the 10-15 accuracy range. The performance of the 171Yb+ microwave standard can therefore be comparable to that of a caesium fountain.

Cryogenic Sapphire Oscillators (CSO) developed at UWA have now been in operation around the world continuously for many years. Such oscillators, due to their excellent spectral purity are essential for interrogating atomic frequency standards at the limit of quantum projection noise; otherwise aliasing effects will dominate the frequency stability due to the periodic sampling between successive interrogations of the atomic transition. For this reason, UWA oscillators are now operational at NMI (Sydney), LNE-SYRTE (Paris), the French Space Agency (CNES, Toulouse) and at UWA (Perth). Other applications, which have attracted attention in recent years, include tests on fundamental principles of physics, such as tests of Lorentz invariance. This paper reports on the long-term operation and performance of such oscillators. We compare the long-term drift of some different CSOs. The drift rates turn out to be linear over many years and in the same direction. However, the magnitude seems to vary by more than one order of magnitude between the oscillators, ranging from 10^14 per day to a few parts in 10^13 per day., Accepted for publication in IEEE Trans. UFFC 21st June 2006

Microwave frequency standards based on the 12.6 GHz ground state hyperfine transition in 171Yb+ have been under development at the National Measurement Institute, Australia, for many years. Using a laser-cooled ion cloud, the transition frequency was measured in 2001 to an accuracy of 8 parts in 1014, limited by the homogeneity of the magnetic field due to the stainless-steel vacuum chamber. We have designed and commissioned a new chamber in the alloy CrCu, which is non-magnetic and has good vacuum properties. The design incorporates large viewports and high-quality quartz windows for optical access. Uncertainties associated with field inhomogeneity in the new vacuum system are now below 1 part in 1015, a significant reduction which permits operation in the 10-15 accuracy range. We have also demonstrated the use of photoionization to load the trap in a preliminary experiment, including isotope-selective loading

The National Measurement Institute, Australia, (NMIA) is continuing the development of reliable, high-integrity and remotely-operable GPS-based systems for precise time and frequency transfer. These systems combine an OEM receiver with a PC and support a variety of applications, of which we describe three: the intercomparison of receiver internal delays among laboratories in the Asia-Pacific, using a portable time-transfer system; a geodetic monitoring station, contributing data to Australian and international geodetic reference networks; and delivery of traceable time and frequency to a distant location, or equivalently continuous remote calibration of a frequency standard at a client's premises

The time and frequency group of the National Measurement Institute, Australia, (NMIA) maintains the Australian national standards for time of day and for frequency. Research and development activities of the group include: development of microwave frequency standards in the 10-15 accuracy range based on trapped Yb+ ions; development of GPS common-view time transfer systems, and the use of these systems to deliver traceable time and frequency to a client's premises by continuous remote calibration; two-way satellite time and frequency transfer; and time and frequency dissemination around Australia by means including the recently developed NMIA 'speaking clock' telephone service

We report operation of a microwave frequency standard based on the 12.6 GHz ground-state hyperfine transition in a cloud of laser-cooled /sup 171/Yb/sup +/ ions. The projected performance of this standard (frequency uncertainty /spl les/ 4 /spl times/ 10/sup -15/ and stability 5 /spl times/ 10/sup -14/ /spl tau//sup -1/2/ for a 10 s Ramsey time) is comparable to that of a cesium fountain.

The microwave frequency standard based on buffer gas-cooled /sup 171/Yb/sup +/ ions at the CSIRO National Measurement Laboratory has demonstrated high stability, but has an accuracy limited by the second-order Doppler shift due to the thermal motion of the ions. We report measurements obtained with a cloud of approximately 10/sup 4/ ions laser-cooled to below 1 K, including near shot noise-limited Ramsey fringes with a pulse separation up to 10 s. Estimates indicate that a potential stability of /spl sigma//sub y/(/spl tau/)=5/spl times/10/sup -14/ /spl tau//sup -1/2/ and a total uncertainty of 5 parts in 10/sup 15/ or better are achievable for this system.

Two fiber-based frequency combs were compared at 633 nm. An offset laser was locked to a frequency comb, and the locked laser was measured with another comb. Consequently, the frequency uncertainty of the combs was less than 3 times 10-16 at 10 000 s averaging, which is sufficiently lower than the uncertainty of UTC.