1,720,982 research outputs found
Optimal blade pitch profile for an autorotative entry vehicle
No full textWe consider the Entry Descent and Landing problem for a vehicle equipped with an unpowered rotary decelerator, having Mars as planetary target. We aim at computing an optimal blade pitch profile to maximize the overall decelerating effect exerted by the rotor. To this end, we set up an optimization problem with one state variable (the altitude) specified at an unknown terminal time, with the landing speed as the objective function to be minimized. Results show the effectiveness of the proposed approach in reducing the terminal velocity with respect to what achieved when using simple constant pitch settings, by more than 10 m/s. © 2018 Univelt Inc. All rights reserved
A DYNAMIC ATTITUDE TESTBED FOR CUBESATS
No full textThis work describes the development of a three degrees of freedom CubeSat attitude simulator testbed. To enable a reliable verification of attitude determination and control systems for nanosatellites, the environment of low Earth orbits with an almost disturbance-free rotational dynamics must be simulated. To this end, several subsystems have been developed and integrated: i) an air-bearing based, three degree of freedom platform with automatic balancing system ii) a Helmholtz cage for geomagnetic field simulation, iii) a Sun simulator, and iv) a metrology vision system for ground-truth attitude measurement. Apart of the Helmholtz cage, which is a COTS equipment, the other subsystems required substantial development effort, whose main outcome are: residual disturbance in the order of 10-5 Nm, attitude determination accuracy of the metrology system equal to 0.15° rms; divergence of the sun simulator light less than 0.2° in a 30cm diameter area. The commissioning phase of the facility is currently ongoing
Preliminary Mission Analysis and Design of a Cubesat Mission to the Didymos Binary Asteroid System
No full textNanosatellite missions represent a promising option for the exploration of the near-Earth asteroid population since they provide low-cost versatile platforms for scientific observations. This paper describes the orbital analysis for the DustCube mission, which was preliminarily selected to reach the binary asteroid system Didymos on-board ESA’s AIM spacecraft.
Possible candidate orbits that exploit the binary nature of the system and satisfying the mission requirements and constraints were identified. The overall feasibility of the proposed Concept of Operations was then addressed by integrating the spacecraft trajectories in a realistic dynamical environment, evaluating their sensitivity to state errors, and estimating the accuracy requirements of the Orbit Determination and Control Subsystem (ODCS).
The proposed operational concept proved consistent with the maneuvering capabilities of a CubeSat platform in terms of frequency of the station-keeping maneuvers and total delta-v budget. The proposed solution combines an initial parking orbit at the L4 equilibrium point with a Distant Retrograde Orbit for proximity operations, allowing to fulfil all mission requirements. However, this result is strictly coupled with the overall performance of the orbit determination system, which is required to allow for safe operations
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
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