1,720,966 research outputs found
Active Gravity-Gradient Stabilization of a Satellite in Elliptic Orbits
No full textAbstract - A tether can be utilized as an efficient actuator for the purpose of satellite attitude control. The tether tension generates a strong passive gravity stabilization effect. The combination of tether tension with a movable attachment point mechanism results in an effective and low cost active controller. This paper reviews the concept and presents the dynamical characteristics of the configuration. It then demonstrates that a feasible three-axis control can be achieved by means of only two actuators. The primary contribution is the development of a control law that works for eccentric orbits. In this case, the dynamical system that represents the configuration becomes periodic in time, and thus impossible to control by using constant gains. The solution is a periodic control law. A combination of Floquet transformation and sampled state periodic hold feedback control is proposed as an effective pointing method for small-eccentricity orbits
Dynamical Characteristics of a Tethered Stabilized Satellite.
No full textAbstract - Results of a preliminary study of the stability characteristics of a tethered satellite attitude with respect to the local vertical are presented. The configuration investigated consists of a satellite, a tether, and a ballast mass at the opposite end of the tether. The position of the movable tether attachment is controlled by the stabilization control logic. The parameters that affect the attitude control are the tether length, the offset between the satellite center of mass and the attachment point, and the ballast mass. The effect of orbital eccentricity and tether oscillations on the satellite dynamics and identification of possible resonances are also discussed
Dynamics of a Dual-Probe Tethered System
No full textAbstract - The dynamics of a dual probe is examined for the case where two probes, close to each other, are located at the far end of the tether. The system is designed to operate at the desired low altitudes and keep the tether oscillations bounded and small. Small perturbations and modal analysis techniques are used to analyze the linear (i.e., small amplitude) motions. The nonlinear motion is analyzed by means of a systematic parametric study using numerical simulations and stroboscopic mapping techniques. It is shown that the dual-probe configuration examined here is suitable for reaching altitudes as low as 130 km in the Earth's atmosphere. A single-probe tether system is preferable for reaching even lower altitudes
Gravitational Torque Frequency Analysis for the Einstein Elevator Experiment
No full textAbstract. Testing the principle of equivalence with a differential acceleration detector that
spins while free falling requires a reliable extraction of a very small violation
signal from the noise in the output signal frequency spectrum. The experiment
is designed such that the violation signal is modulated by the spin of the
test bodies. The possible violation signal is mixed with the intrinsic white
noise of the detector and the colored noise associated with the modulation of
gravitational perturbations, through the spin, and inertial-motion-related noise.
In order to avoid false alarms the frequencies of the gravitational disturbances
and the violation signal must be separate. This paper presents a model for
the perturbative gravitational torque that affects the measurement. The torque
is expanded in an asymptotic series to the fourth order and then expressed as
a frequency spectrum. A spectral analysis shows the design conditions for
frequency separation between the perturbing torque and the violation signal
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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