1,721,053 research outputs found
Rendezvous Maneuvers of Multiple Spacecraft Using Differential Drag Under J2 Perturbation
Full text linkIn this work, the residual atmospheric drag is exploited to perform rendezvous maneuvers among multiple
spacecraft in low Earth orbits. These maneuvers are required, for instance, for autonomous on-orbit assembly. By
varying the level of aerodynamic drag of each spacecraft, relative differential accelerations are generated among the
spacecraft of the group and therefore their relative orbits are controlled. Each of the spacecraft is assumed to include
a drag plate, which can be actively opened or closed, to vary the atmospheric drag. The recently developed
Schweighart–Sedwick model is used to describe the relative dynamics of different spacecraft with respect to a
circular orbit with the inclusion of J2 effects. Furthermore, the natural relative dynamics of each chaser with respect
to the target is decoupled into a secular motion and a periodic oscillation. In particular, the following two-phase
control method is proposed. First, the secular motion of each chaser is controlled via differential drag in order for the
spacecraft to sequentially move from an arbitrary initial condition to a closed stable relative orbit around the target
spacecraft. After the relative orbit stabilization, a relative eccentricity control is applied to each spacecraft to zero-out
the semi-axis of the relative orbit around the target and to achieve the rendezvous condition. The control algorithm
considers mutual constraints among the values of differential drag that the different spacecraft can experience.
Potential collisions are avoided by changing the maneuvering initial time. The main advantage of the proposed
technique is that it enables a fleet of spacecraft to rendezvous without propellant expenditure. Furthermore, no
numerical optimization is needed, because the control policy is based on closed-form analytical solutions. The
proposed technique was validated via numerical simulations.This research was partially supported by the Defense Advanced Research Projects Agency. This research was performed while R. Bevilacqua was holding a National Research Council Research Associateship Award at the Spacecraft Robotics Laboratory of the Naval Postgraduate School
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
Lyapunov-Based Thrusters’ Selection for Spacecraft Control: Analysis and Experimentation
Full text linkThe article of record as published may be located at http://dx.doi.org/10.2514/1.47296This paper introduces a method for spacecraft rotation and translation control by on–off thrusters with
guaranteed Lyapunov-stable tracking of linear dynamic models. In particular, the proposed control method switches
on, at each time step, only those thrusters needed to maintain stability. Furthermore, the strategy allocates the
configuration so that the minimum number of actuators is used. One of the benefits of the proposed method is that it
substitutes both the thruster mapping and the pulse modulation algorithms typically used for real-time allocation of
the firing thrusters and for determining the duration of the firing. The proposed approach reduces the computational
burden of the onboard computer versus the use of classical thruster mapping algorithms, which typically involve
iterative matrix operations. The paper presents analytical demonstrations, numerical simulations on a six-degree-offreedom
spacecraft, and experimental tests on a hardware-in-the-loop three-degree-of-freedom spacecraft simulator
floating over air pads on a flat floor. The method proves to be effective and easy to implement in real time.This research was performed while R. Bevilacqua was holding a National Research Council Research Associateship Award at the Spacecraft Robotics Laboratory of the Naval Postgraduate School
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
Quasi-Optimal Control for Path Constrained Relative Spacecraft Maneuvers Based on Dynamic Programming
Full text linkThe article of record may be found at http://www.e-ndst.kiev.uaAutonomous close flight and docking of a chaser spacecraft to a target are still challenging problems. In this paper the Hill–Clohessy–Wiltshire equations are taken as dynamic model and inverted, after a variable change, in order to be used by a control algorithm to drive the chaser spacecraft along a specified path. The path parameterization is performed by using cubic B- splines and by having the curvilinear abscissa as parameter. The proposed optimization algorithm uses dynamic programming to find quasi-optimal con- trols. The number of optimization parameters is drastically reduced by working only on the acceleration component along the vehicle trajectory. The shape of the path can be chosen according to the specific maneuver requirements. In particular, the optimization algorithm is split into a trajectory planner which generates the best tangential acceleration sequence through backward explo- ration of a tree of possible policies, and a control generator which inverts the parameterized dynamics in order to get the thrusters commands sequence. The optimization algorithm has been coded in Simulink as a library of embedded functions and has been experimentally proved to run in real time
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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