1,720,987 research outputs found
Combined System-Trajectory Design for Geostationary Orbit Platforms on Hybrid Transfer
Full text linkA novel methodology for a combined systems–trajectory optimization for a geostationary equatorial orbit (GEO) platform is proposed to obtain comprehensive design solutions. A combined chemical–electric propulsion system is used to execute hybrid high-thrust/low-thrust trajectory transfer to GEO, thereby balancing the overall system mass and transfer time. A systematic and payload-centric mission design provides a new set of design options to deliver tailored solutions to customized payloads. The hybrid trajectory characterization and spacecraft systems design find the required platform launch mass to deliver a GEO platform with a defined final mass and operational power. Elements of the system design are combined with those of multispiral low-thrust trajectory optimization as well as radiation absorption and solar array degradation to provide a comprehensive design solution. The result is a wide set of solutions to reach GEO, where fully chemical and fully electric transfers represent the boundaries of the hybrid transfer trade space. A payload throughput power of 20 kW entails a spacecraft mass in GEO between 4000 and 4550 kg, an initial thrust-to-mass ratio range of 1.7−2.3×10−4 m/s2, and a cover-glass thickness between 4 and 24 mils to guarantee a minimum end-of-life/beginning-of-life power ratio of 85%. In addition, all-electric solutions from different injection orbits yield transfers to GEO with a time of flight of 60–150 days and an initial mass for the platform of 4400–5500 kg
Assessment of Hybrid Propulsion for Geostationary Transfer Orbits: a Mission Design Approach
No full textThe classic strategy to place a satellite in the geostationary orbit (GEO) relies on chemical propulsion. Starting from a low-Earth orbit, this solution allows acquiring the final GEO by applying, ideally, only two impulses. This option has proven to be effective, and grants short transfer times. On the other hand, the increase of power levels required by payloads in GEO satellites has paved the way to the all-electric solutions. In these satellites, the GEO is achieved by a low-thrust transfer, typically starting from a geostationary transfer o rbit. This strategy involves designing more efficient satellites (that is, less propellant is used in the transfer phase) at the cost of accounting for longer transfer times, which can be on the order of up to six months, depending on the thrust-to-mass authority [Kluever, 2010]. The two options available give rise to platforms having divergent features. On the one hand we have the fully chemical satellite, with short transfer times but large propellant masses, on the other hand, the fully electric satellite, with low propellant mass fractions but long transfer times. This dichotomy is too firm, and forbids widening the trade space in preliminary design of GEO satellites. A way to account for intermediate design solutions consists in allowing the two propulsion systems to coexist on the GEO platform [Kluever, 2015]. In principle, a hybrid propulsion system may lead to a family of design solutions that fill the gap between the two boundary solutions. Recent studies have shown that hybrid spacecraft have peculiar features when applied to missions to Mars, Moon, and NEOs [Mingotti, Topputo, Massari 2013]. However, a methodology to preliminary assess this kind of solutions is not established yet, and requires non-trivial procedure. In this paper we elaborate on the concept of hybrid propelled satellite for GEO applications. A preliminary design procedure is derived, which allows evaluating the usefulness of hybrid platforms for given payload. Elements of preliminary system design are combined to those of preliminary trajectory optimization. These involve power subsystem sizing, electric and chemical propulsion modeling, and multispiral, long-duration, low-thrust trajectory optimization. The overall benefits of hybrid GEO satellites are evaluated by using economical models as well. The results show that hybrid platforms may be conceived as a viable option to widen the trade space for the next generation of GEO satellites
System-Trajectory Optimization of Hybrid Transfers to the Geostationary Orbit
No full textThis paper proposes a methodology for deriving system-trajectory optimization to Geostationary orbit. The features of chemical and electric propulsion are exploited to deliver the payload onto the final orbit. Hybrid transfers may fill the gap between classical high-thrust transfer, which are fast but mass penalizing, and long-duration, all-electric new solution with lighter platforms. Low-thrust increases the transfer time, thus the radiation absorption through the Van Allen Belts has to be estimated; moreover, elements of system design are combined with those of trajectory optimization. These involve power subsystem design with solar array degradation analysis, electric and chemical propulsion modeling, and multi-spiral, low-thrust trajectory optimization. The system-trajectory design becomes payload-centric to provide the widest range of options, in order to deliver the most tailored solutions to customers. Hybrid transfers may be conceived as a viable option to reach the Geostationary orbit providing attractive total trip times and, at the same time, feasible launch masses
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
Comparison of measurements made with two different instruments of the same atmospheric vertical profile
No full textVariations 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
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