1,720,958 research outputs found
Target-Based Guidance Method for Trajectories with Multiple Flybys
Get PDFIIN A real-life scenario, a spacecraft will not follow the prescribed
nominal path. As a matter of fact, uncertainty in the dynamic
model (e.g., gravitational parameters or radiation pressure noisy
profiles) can lead to relevant drifts during ballistic arcs, while errors
in command actuation (i.e., thrust magnitude and pointing angles
error) can inject the spacecraft on unwanted trajectories [1]. Moreover, the spacecraft state cannot be known exactly, since it is inferred
only from indirect measurements subjected to noise
Radio Science Investigations with the Asteroid Impact Mission
Get PDFThe Asteroid Impact & Deflection Assessment (AIDA) mission is a joint ESA/NASA collaboration to study the binary Near-Earth Asteroid (65803) Didymos and assess the feasibility of the kinetic impactor technique to deflect an asteroid. The European contribution to AIDA is the Asteroid Impact Mission (AIM), which will characterize in detail the Didymos system, investigating the surface, subsurface, and internal properties of the asteroid.
This paper presents a possible Radio Science Experiment (RSE) to be performed with AIM focused at its precise orbit determination within the Didymos system, providing an assessment of the accuracies achievable in the estimation of the scientific parameters of interest, like the masses and the extended gravity fields of Didymos primary and secondary, their relative orbit, and their rotational states. The experiment expected performances were assessed through numerical simulations, based upon a complete and realistic dynamical model of the Didymos system and the AIM spacecraft.
Given the small mass of the Didymos system, optical navigation images proved to be crucial to obtain good accuracies for the scientific parameters of interest, even keeping AIM at relatively large distances from Didymos. At 10 km, after 8 flybys dedicated to gravity science, the masses of the primary and secondary can be estimated to about 0.2% and 1.6% (1-sigma), respectively, with the mass of the secondary being mainly given by observing the wobble of the primary around the common center of mass due to the mutual orbital motion; the orbital motion of the secondary around the primary can be estimated to about 1 m, and the pole orientation of the primary and the secondary can be estimated to about 0.1 deg and 0.4 deg, respectively (1-sigma)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Nonlinear Guidance for a Transfer Orbit to the Moon
No full textThe purpose of this work is to present
a nonlinear guidance algorithm for spacecraft
navigation that can be applied in any mission
scenario. This approach allows to reliably estimate
the navigation Delta-V (DV) budget and obtain a
more accurate and reliable orbit insertion
performance.
The proposed nonlinear guidance algorithm is
applied for a lunar mission scenario, focusing on the
transfer orbit up to lunar orbit injection. The key
performance parameters under analysis include the
navigation DV budget for the mission and the orbital
parameters scattering at arrival with respect to the
targets. To perform a realistic and reliable analysis,
the study takes into account the main factors that
affect spacecraft navigation, namely the accuracy of
the launcher insertion, the manoeuvres execution
errors, and the knowledge of the spacecraft state. As
a last step, we investigate how the navigation
performances are affected by the ground station
operations schedule, namely by the orbit
determination arc and the flight dynamics
operations cycle. The duration of the orbit
determination arc is related to the observation time
window used to estimate the spacecraft's position.
The flight dynamics operations cycle consists of the
timing for the ground infrastructure procedures,
such as orbit determination, manoeuvres planning,
and TT&C
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