1,720,981 research outputs found
Improved modelling of surface forces in the orbit determination of ERS and ENVISAT
No full textPrecise orbit determination of the European remote sensing satellites ERS-2 and ENVISAT, a prerequisite for the processing of their altimeter data, requires highly accurate force models. Inaccuracies in the modelling of non-gravitational forces, caused by interaction of photons, molecules, and atoms with the spacecraft surfaces, form a major error source. A highly detailed new non-gravitational force modelling package named ANGARA was implemented and tested for ERS-2 and ENVISAT. The resulting orbits for ERS-2 were compared with those from an existing box-wing panel model and a simple constant-area model. In addition, the effects on orbit accuracy of three thermospheric density models (DTM94, MSIS-86, and NRLMSISE-00) and a horizontal wind model (HWM-93) were evaluated. Force model parameters, estimated from tracking data during the precise orbit determination process, absorb most of the differences between the considered models. At the same time, an analysis of these parameters, and orbit determination runs with a limited parameterization, can give important clues about modelling accuracy. Atmospheric density is the dominant error source at high solar activity, degrading the radial orbit accuracy. Radiation pressure forces, for which uncertainties in material properties are the largest error source, play an equally important role at low solar activity
Improved modelling of surface forces in the orbit determination of ERS and ENVISAT
No full textPrecise orbit determination of the European remote sensing satellites ERS-2 and ENVISAT, a prerequisite for the processing of their altimeter data, requires highly accurate force models. Inaccuracies in the modelling of non-gravitational forces, caused by interaction of photons, molecules, and atoms with the spacecraft surfaces, form a major error source. A highly detailed new non-gravitational force modelling package named ANGARA was implemented and tested for ERS-2 and ENVISAT. The resulting orbits for ERS-2 were compared with those from an existing box-wing panel model and a simple constant-area model. In addition, the effects on orbit accuracy of three thermospheric density models (DTM94, MSIS-86, and NRLMSISE-00) and a horizontal wind model (HWM-93) were evaluated. Force model parameters, estimated from tracking data during the precise orbit determination process, absorb most of the differences between the considered models. At the same time, an analysis of these parameters, and orbit determination runs with a limited parameterization, can give important clues about modelling accuracy. Atmospheric density is the dominant error source at high solar activity, degrading the radial orbit accuracy. Radiation pressure forces, for which uncertainties in material properties are the largest error source, play an equally important role at low solar activity
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Impacts on HST and EuReCa solar arrays compared with LDEF using a new glass-to-aluminium conversion
No full textIn order to compare impact fluxes measured on solar arrays in Low Earth Orbit (LEO) to those on aluminium surfaces, as measured by the Long Duration Exposure Facility (LDEF) for example, a conversion between the conchoidal diameter impact feature (D-co) (as observed on solar array surfaces) and the,ballistic limit in aluminium (Fmax) has been developed. We find good agreement between the converted HST and EuReCa solar array fluxes and LDEF fluxes at large sizes (F-max > 30 mu m) where meteoroids dominate. We also find the converted HST and EuReCa fluxes are in broad agreement with the microdebris enhancement (F-max < 30 mu m) apparent on LDEF, indicating that the microdebris flux has not changed significantly between the LDEF epoch (1984-1990), and the EuReCa (1992-1993) and HST (1990-1993) epochs
Hypervelocity impacts in low Earth orbit: Cosmic dust versus space debris
No full textThe understanding of the micron-sized populations of natural micrometeoroids and artificial space debris in low Earth orbit has benefited considerably from the post-flight investigations of retrieved surfaces from spacecraft, such as the Long Duration Exposure Facility. The returned solar array from the Hubble Space Telescope has added to this repository and has offered a further opportunity to document these particles. 25 individual solar cells were specially selected on the basis that they contained impact craters (diameter 100-1000 mu m) which had the most potential to retain impactor residue chemistry. The solar cells were subject to a detailed investigation using analytical scanning electron microscopy which identified 29 impact craters, the analysis of which identified 3 residues as artificial in origin, 6 unclassified and 20 as natural in origin. The limited number of unclassified residues identified indicates that the methods of analysis employed in this investigation are a significant step forward for such studies and, if employed on a greater number of samples, will improve the calculations of the time-integrated flux rates for micrometeoroids and space debris in the low Earth orbit environment. Notwithstanding the small sample set examined, the observed chemical classification of the impact residues in terms of micrometeoroid to space debris (in the particle size range 8-80 mu m) corresponds well to the flux model that predicts the dominance of natural particles
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