8,860 research outputs found
Ionospheric limitations to time transfer by satellite
The ionosphere can contribute appreciable group delay and phase change to radio signals traversing it; this can constitute a fundamental limitation to the accuracy of time and frequency measurements using satellites. Because of the dispersive nature of the ionosphere, the amount of delay is strongly frequency-dependent. Ionospheric compensation is necessary for the most precise time transfer and frequency measurements, with a group delay accuracy better than 10 nanoseconds. A priori modeling is not accurate to better than 25%. The dual-frequency compensation method holds promise, but has not been rigorously experimentally tested. Irregularities in the ionosphere must be included in the compensation process
Applications of radio interferometry to navigation
Radio astronomy experiments have demonstrated the feasibility of making precise position measurements using interferometry techniques. The application of this method to navigation and marine geodesy is discussed, and comparisons are made with existing navigation systems. The very long baseline technique, with a master station, can use either an artificial satellite or natural sources as position references; a high-speed data link is required. A completely ship-borne system is shown to be feasible, at the cost of poorer sensitivity for natural sources. A comparison of Doppler, delay and phase-track modes of operating a very long baseline configuration is made, as that between instantaneous measurements and those where a source can be tracked from horizon to transit. Geometric limitations in latitude and longitude coverage are discussed. The characteristics of natural radio sources, their flux, distribution on the sky, and apparent size are shown to provide a limit on position measurements precision. The atmosphere and frequency standard used both contribute to position measurement uncertainty by affecting interferometric phase
Probing quasiparticle excitations in a hybrid single electron transistor
We investigate the behavior of quasiparticles in a hybrid electron turnstile
with the aim of improving its performance as a metrological current source. The
device is used to directly probe the density of quasiparticles and monitor
their relaxation into normal metal traps. We compare different trap geometries
and reach quasiparticle densities below 3um^-3 for pumping frequencies of 20
MHz. Our data show that quasiparticles are excited both by the device operation
itself and by the electromagnetic environment of the sample. Our observations
can be modelled on a quantitative level with a sequential tunneling model and a
simple diffusion equation
Magnetodielectric coupling of infrared phonons in single crystal CuOSeO
Reflection and transmission as a function of temperature have been measured
on a single crystal of the magnetoelectric ferrimagnetic compound
CuOSeO utilizing light spanning the far infrared to the visible
portions of the electromagnetic spectrum. The complex dielectric function and
optical properties were obtained via Kramers-Kronig analysis and by fits to a
Drude-Lortentz model. The fits of the infrared phonons show a magnetodielectric
effect near the transition temperature (~K). Assignments to
strong far infrared phonon modes have been made, especially those exhibiting
anomalous behavior around the transition temperature
Digital image correlation in dental materials and related research: A review
OBJECTIVE: Digital image correlation (DIC) is a non-contact image processing technique for full-field strain measurement. Although DIC has been widely used in engineering and biomechanical fields, it is in the spotlight only recently in dental materials. Therefore, the purpose of this review paper is introducing the working principle of the DIC technique with some modifications and providing further potential applications in various dental materials and related fields. METHODS: The accuracy of the algorithm depending on the environmental characteristics of the DIC technique, as well as the advantages and disadvantages of strain measurement using optical measurements, have been elaborated in dental materials and related fields. Applications to those researches have been classified into the following categories: shrinkage behavior of light-cured resin composite, resin-tooth interface, mechanical properties of tooth structure, crack extension and elastic properties of dental materials, and deformation of dental restoration and prosthesis. This classification and discussion were performed using literature survey and review based on numerous papers in the international journals published over the past 20 years. The future directions for predicting the precise deformation of dental materials under various environments, as well as limitations of the DIC technique, was presented in this review. RESULTS: The DIC technique was demonstrated as a more effective tool to measure full-field polymerization shrinkage of composite resin, even in a simulated clinical condition over the existing methods. Moreover, the DIC combined with other technologies can be useful to evaluate the mechanical behavior of material-tooth interface, dentine structure and restorative and prosthetic materials with high accuracy. Three-dimensional DIC using two cameras extended the measurement range in-plane to out-of-plane, enabling measure of the strain directly on the surface of dental restorations or prosthesis. SIGNIFICANCE: DIC technique is a potential tool for measuring and predicting the full-field deformation/strain of dental materials and actual prostheses in diverse clinical conditions. The versatility of DIC can replace the existing complex sensor devices in those studies
A New Technique for Finding Needles in Haystacks: A Geometric Approach to Distinguishing Between a New Source and Random Fluctuations
We propose a new test statistic based on a score process for determining the
statistical significance of a putative signal that may be a small perturbation
to a noisy experimental background. We derive the reference distribution for
this score test statistic; it has an elegant geometrical interpretation as well
as broad applicability. We illustrate the technique in the context of a model
problem from high-energy particle physics. Monte Carlo experimental results
confirm that the score test results in a significantly improved rate of signal
detection.Comment: 5 pages, 4 figure
Application of high-strength biodegradable polyurethanes containing different ratios of biobased isomannide and poly (E-caprolactone) diol
Biodegradable–biocompatible polyurethanes were prepared with fixed hexamethylene diisocyanate and varying ratios of isomannide and poly(ϵ-caprolactone) diol using a simple one-step polymerization without a catalyst. The polyurethane structures were confirmed by 1H-nuclear magnetic resonance, Fourier transform infrared spectroscopy, and gel permeation chromatography. The glass transition temperatures were determined by thermal analysis to be between 25°C and 30°C. Degradation tests performed at 37°C in phosphate buffer produced mass losses of 5%–10% after 8 weeks. After 5 days of culture, using osteoblastic cells, the relative cell number on all the polyurethane films was only slightly lower than that of an optimized tissue culture plastic. These polymers offer significant promise with a simplistic synthesis and controlled degradation
Population and Harvest Trends of Big Game and Small Game Species
This technical document supports the Forest Service’s requirement to assess the status of renewable natural resources as mandated by the Forest and Rangeland Renewable Resources Planning Act of 1974 (RPA). It updates past reports on national and regional trends in population and harvest estimates for species classified as big game and small game. The trends reported here were derived from State Wildlife Agency biologists and supplemented with data from the North American Breeding Bird Survey for those bird species that are commonly sought by upland game hunters. Big game populations and harvests have generally increased over the 1975-2000 period. Small game populations and harvests, particularly those associated with grassland and agricultural systems, show strong patterns of decline. However, population and harvest trends for both groups need to be interpreted with caution because: (1) not all state agencies reported both population and harvest statistics for all species that are commonly sought by recreational hunters, and (2) there were cases of inconsistent reporting at the species level within RPA reporting regions that necessitated aggregating across species. The trends documented here are consistent with trends documented in past RPA reports completed in 1989 and 1999, although those data were also qualified by the same interpretational caveats that apply to the current report. Trends observed generally among big game species were encouraging, but the continual decline in small game populations and harvest remains an important wildlife resource management issue. Until population and harvest monitoring is improved among institutions that share the stewardship responsibility for recreationally important wildlife, national and regional trends will have to be interpreted carefully
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