1,721,078 research outputs found
Impact of tropospheric scintillation in the Ku/K bands on the communications between two LEO satellites in a radio occultation geometry
No full textA theoretical analysis of the impact of clear-air tropospheric scintillation on a radio occultation link between two low Earth orbit satellites in K- and Ku-bands is presented, with particular reference to differential approaches for the measure of the total content of water vapor. The troposphere is described as a spherically symmetric turbulent medium satisfying Kolmogorov theory. Rytov's first iteration solution for weak fluctuations is used to derive an expression for the variance of amplitude fluctuations of the wave as well as their spectrum and the correlation between fluctuations at different frequencies. The validity of the assumptions made and the influence of atmospheric parameters on the quantities of interest are also investigated and discussed. Finally, numerical results are presented to provide an estimate of the level of scintillation-induced disturbances
The ESA-ANISAP study: estimate of tropospheric scintillation along a LEO-LEO link through high resolution radiosonde data
No full textThis work describes the procedure developed in the framework of the ESA-ANISAP study to obtain, for a given atmospheric profile, an estimate of scintillation effects in a LEO-LEO link. In this procedure, the refractive-index structure constant profiles describing the intensity of turbulence, are derived by applying the vertical gradient approach to high resolution radiosonde data. The fact that turbulence in the free atmosphere is confined to vertically thin layers is accounted for by identifying the turbulent layers through the analysis of the Richardson number profiles. Then, the derived structure constant profiles are inserted into a parametric scintillation model to create a scintillation disturbance estimate consistent with the considered atmospheric profile. In the parametric model, the troposphere is described as a spherically symmetric turbulent medium. Rytov‟s first iteration solution for weak fluctuations is used to derive an expression for two quantities of interest to evaluate the performances of the Normalized Differential Spectral Attenuation for the estimate of the total content of water vapour along the propagation path between two LEO satellite, namely: the variance of amplitude fluctuations of the wave and the correlation between the fluctuations at different frequencies. The influence on these quantities of some turbulence characteristics which are not known with confidence, like the outer scale length and the behaviour of the spectrum in the input range, is also investigated
Ndsa measurements between two counter rotating leo satellites: Performance evaluation at global scale in Ku, K and M bands
No full textInternational Geoscience and Remote Sensing Symposium (IGARSS)
2013, Article number 6723551, Pages 3371-3374
2013 33rd IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2013; Melbourne, VIC; Australia; 21 July 2013 through 26 July 2013; Category numberCFP13IGA-ART; Code 102722
Ndsa measurements between two counter rotating leo satellites: Performance evaluation at global scale in Ku, K and M bands (Conference Paper)
Cuccoli, F.a,
Facheris, L.a,
Garzelli, A.c,
Zoppetti, C.b
a CNIT RaSS C/o, Department of Electronics and Telecommunications, University of Florence, Via di Santa Marta 3, 50139 Florence, Italy
b Dept. Information Engineering and Mathematical Sciences, Univ. Siena, Via Roma 56, 53100 Siena, Italy
View references (7)
Abstract
In this work we present and discuss the effects of different scintillation models on the Normalize Differential Spectral Attenuation (NDSA) measurements in a counter-rotating Low Earth Orbiting (LEO) set event at 17, 19, 21, 32, 179 and 182 GHz. Some parameters of the scintillation models used here are computed at global scale using an European Centre for Medium-Range Weather Forecasts (ECMWF) global dataset. The scintillation effects on the NDSA measurements are analyzed in terms of relative error of the their estimation
Tropospheric IWV profiles estimation through multifrequency signal attenuation measurements between two counter-rotating LEO satellites: performance analysis
No full textThe exploitation of multifrequency differential attenuation measurements at microwaves made between two LEO satellites in limb mode is the ground of the NDSA (Normalized Differential Spectral Attenuation) approach for estimating integrated tropospheric water vapor profiles through multifrequency measurements at 17, 19, 21, 179 and 182 GHz, plus 32 GHz for liquid water detection and correction (whenever possible). Such measurements are affected by two kinds of impairments, the first generated by thermal noise at the receiver, the second generated by the signals’ fluctuations due to the variations of the tropospheric refraction index and referred to as scintillation disturbance. Characterizing scintillation for simulating its effects to evaluate NDSA performance is not easy in general: in particular, it is quite hard (and also rather questionable so some extent) to relate the scintillation parameters to a given simulated atmospheric situation. For this reason, in the past years we limited ourselves to evaluate the NDSA performance by accounting for scintillation in a parametric way, independently of the atmospheric context in which simulations were carried out. In this paper, instead, we show the first results of the NDSA performance analysis based on a completely different approach, where the scintillation profiles and parameters are directly derived from the simulated atmospheric context, based on a procedure that starts from high resolution radiosonde data. A brief critical analysis of such an approach is proposed, evidencing some aspects related to the current knowledge of the scintillation spectra and parameters. The NDSA performance analysis based on certain hypotheses for the scintillation characteristics is then shown for some selected simulated atmospheric condition
A Support Vector Machine Hydrometeor Classification Algorithm for Dual-Polarization Radar
Full text linkAn algorithm based on a support vector machine (SVM) is proposed for hydrometeor classification. The training phase is driven by the output of a fuzzy logic hydrometeor classification algorithm, i.e., the most popular approach for hydrometer classification algorithms used for ground-based weather radar. The performance of SVM is evaluated by resorting to a weather scenario, generated by a weather model; the corresponding radar measurements are obtained by simulation and by comparing results of SVM classification with those obtained by a fuzzy logic classifier. Results based on the weather model and simulations show a higher accuracy of the SVM classification. Objective comparison of the two classifiers applied to real radar data shows that SVM classification maps are spatially more homogenous (textural indices, energy, and homogeneity increases by 21% and 12% respectively) and do not present non-classified data. The improvements found by SVM classifier, even though it is applied pixel-by-pixel, can be attributed to its ability to learn from the entire hyperspace of radar measurements and to the accurate training. The reliability of results and higher computing performance make SVM attractive for some challenging tasks such as its implementation in Decision Support Systems for helping pilots to make optimal decisions about changes inthe flight route caused by unexpected adverse weather
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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