604 research outputs found
The infrared optical constants of limestone particles and implications for the search of carbonates on Mars
No full textTheoretical studies and numerical models of the atmosphere and surface of Mars need the knowledge of the optical constants of candidate materials for Martian dust, Limestone, as a carbonate bearing material, is also sommonly considered a likely component particularly important for its links with the climate evolution and water resources on Mars. In this work we present the complex indices of refraction of submicron particles of limestone, derived from laboratory transmission spectra in the wavelength range 3,5-20 Î1⁄4m by means of dispersion theory. The results are discussed and compared with the bulk optical constants of the same material already available in the literature. Since a comparison between the infrared spectrum of our dust sample of limestone (composed of more than 98% of calcium carbonate) and that of pure calcite particles shows no differences, we can consider the optical properties presented in this paper valid also for a collection of randomly oriented submicron calcite grains. The extinction and scattering efficiencies of limestone grains, calculated using our particulate optical constants, have been used as input parameters in a simple model of radiative transfer to compute synthetic spectra of the surface and the aerosol of Mars. The results clearly show that, for quantitative anlyses concerning the abundance and composition of the different solid materials present on the surface and atmosphere, the physical status of the various dust components has to be taken into account. This means that any extrapolation in the use of the bulk optical constants to describe the optical behavior of small particles and vice versa has to be checked a priori. © 1998 Elsevier Science Ltd. All rights reserved
RELEVANCE OF POLICY ANALYSIS: NEEDS FOR DESIGN, IMPLEMENTATION AND PACKAGING
No full textThis article challenges the traditional model of the economist as a humble technocrat who simply provides analysis given the preferences of policy decision-makers. Since decision-makers rarely reveal their preferences, it is important that the would-be policy research/analyst know the political economy and be willing to identify potential performance goals for society. Researchers who are willing to incur the transaction cost associated with becoming involved in useful policy research must learn to work within the imperfect policy process. Policy research that considers the importance of implementation and that acknowledges the institutions and the history will have the highest chance of being useful to policy-makers.Implementation, Institutions, Policy research, Political economy, Agricultural and Food Policy,
Reducing the effects of intersymbol interference in diffuse DPIM optical wireless systems
No full textThe paper investigates the performance of digital pulse interval modulation (DPIM) in the presence of multipath propagation and additive white Gaussian noise. To combat intersymbol interference (ISI), guard slots and a nonlinear equaliser have been introduced. The average optical power requirements (AOPR) due to ISI for cases with/without guard slots and with equaliser are analysed using a ceilingbounce model. Results obtained show that in the absence of equalisation, DPIM without guard slot offers a lower AOPR compared with on–off keying (OOK). Introducing guard slots gives a further reduction in AOPR by up to 4 dB due to the reduced duty cycle of the DPIM signal. The performance of DPIM without guard slot but using an equaliser is found to be significantly better than DPIM with guard slots on a channel with severe ISI
A Biaxial Fatigue Specimen for Uniaxial Loading
No full textThe aim of this paper is to present a novel un-notched fatigue test specimen in which a biaxial stress state is achieved using a uniaxial loading condition. This allows the problem of multi-axial fatigue to be studied using relatively common one-axis servo-hydraulic testing machines. In addition the specimen presented here is very compact and can be made using a small volume of material (100x40x4.5mm). For this specimen, the degree of biaxiality, defined by the parameter is equal to approximately 0.45. The specimen geometry was optimised using the Dang Van multi-axial fatigue criterion. In addition to use as a fatigue specimen, it has been demonstrated that the biaxial specimen presented here is also suitable for biaxial tensile tests, to determine the rupture strength of a material in a biaxial stress state. Two different materials have been investigated: The first was wrought aluminium 2024-O in the form of 5mm sheets. The second was a cast aluminium-silicon alloy AlSi7Cu0.5Mg0.3, commonly used in automotive and aeronautical applications. The fatigue strengths were determined at 2x106 cycles and at various R-ratios using a staircase procedure. For the aluminium 2024, it is shown that the biaxial stress state increases the maximum permissible first principal stress when compared to the uniaxial condition. However, in terms of the cast aluminium alloy, it has been demonstrated that this type of fatigue specimen is not suitable for materials containing casting defects, in particular micro-shrinkage pores, because the volume of material, in which the stress state is biaxial, is not large enough.The authors gratefully acknowledge the financial support of PSA – Peugeot Citroën and also that of the Conseil Général du Département de Maine-et-Loire, France
Descriptive Analysis of Matrix-Valued Time-Series
No full textIn this article we present a technique of data analysis applied to three-dimensional tables as, for instance, matrix-valued time-series. The main goal of the method is to describe the evolution of the statistical units with respect to time in a space summarizing the set of matrices. Moreover, our technique points out similar statistical units provided by a classification of their trajectories.matrix-valued time-series; data analysis; statistical units
Click to download data: an event study of Internet access to economic statistics
No full textThis study examines the online access statistics of the Central Bank of Turkey’s Electronic Data Delivery System within an event study framework. The comparisons of pre-event and post-event statistics suggest that announcements of both the policy interest rates and the consumer price data considerably affect society’s data access behavior. The timing and amplitude of these effects are further studied with respect to inflation expectations and surprise content of events; yet no solid pattern was revealed.Data access; Macroeconomic data; Market efficiency; Event study
Click to Download Data : An Event Study of Internet Access to Economic Statistics
No full textThis study examines the online access statistics of the Central Bank of Turkey's Electronic Data Delivery System within an event study framework. The comparisons of pre-event and post-event statistics suggest that announcements of both the policy interest rates and the consumer price data considerably affect society's data access behavior. The timing and amplitude of these effects are further studied with respect to inflation expectations and surprise content of events; yet no solid pattern was revealed.Data access, Macroeconomic data, Market efficiency, Event study
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