1,720,954 research outputs found
Design of Natural Collision-Free Trajectories for the Mission Extension Phase of a Remote Sensing Formation Flying Mission
Full text linkThe safety concept is of paramount importance in the trajectory design of formation flying missions. The selection of natural collision-free trajectories is based on the analysis of natural relative motion among the satellites. This is beneficial for the mission both in terms of propellant consumption and control effort, allowing a naturally safe environment. This paper focuses on a formation flying for remote sensing missions in low Earth orbit, in the direction of future Earth observation missions. We consider as a baseline the Formation Flying L-band Aperture Synthesis mission concept, proposed by the European Space Agency. This work proposes natural collision-free trajectories to extend the scientific campaign at the end of the nominal operative life of the FFLAS mission. The possibility to extend the scientific operations, before the final atmospheric re-entry phase, could provide a significant amount of data to improve meteorological and climate prediction. The scenario selected is based on a close formation flying, with a nominal inter-satellite distance in the order of tens of meters. This allows the satellites to behave as a node of a distributed payload for Earth observation, increasing the accuracy in the measurements, thanks to the increase of the radiometer's aperture size. We present two possible strategies to design the extension phase of this mission. First, a single satellite science mission is envisioned, moving the satellites on the same reference orbit, with a certain separation angle. In this new configuration, each satellite operates singularly on different orbits, as a single satellite science mission. Second, we consider the possibility to increase the relative distance among the satellites, maintaining the possibility to do Earth observation with distributed payloads. We perform some analyses to select the augmented geometry, with a bigger formation baseline in the order of tens to hundreds of meters. The analyses are driven by the need to use natural collision-free relative trajectories since at the end-of-life the low thrust control is limited. The final decay of the satellites is provided via a deorbiting low-thrust manoeuvre, to comply with the 25-year mitigation rule. The main idea is to propose disposal in less than three months. The results of the analyses are obtained including the effects of the orbital perturbations, thanks to a high-fidelity relative motion propagator. The relative orbital elements environment is introduced to assess the formation safety more straightforwardly
Formation Flying L-Band Aperture Synthesis: Design Challenges and Innovative Formation Architecture Concept
Full text linkIt is well known that global maps of soil moisture and sea surface salinity are required to improve meteorological and climate prediction, as demonstrated by the Soil Moisture and Ocean Salinity (SMOS) satellite flying since 2009. In the context of future L-band missions for land and oceans applications, the spatial resolution should increase from 40 km, as for SMOS, to 1-10 km. Such improvement in the resolution of a radiometer is typically obtained increasing the aperture size: formation flying is envisioned as a potential way to achieve that spatial resolution. This paper presents the results of the formation flying design and trade-off analysis. It shows the selection of the baseline geometry for the Formation Flying L-band Aperture Synthesis (FFLAS) mission concept, proposed by the European Space Agency. FFLAS foresees the use of three SMOS-like satellites, flying in a tight - rigid - formation in the low Earth orbit region. Starting from the scientific mission requirement of the L-band interferometer, an analysis, for possible three satellite planar formation flying architectures, has been carried out in the local orbital frame. For the sake of generality, a yaw angle is introduced to parametrise the possible orientation of the virtual instrument into the local orbital frame. The selection of the proper yaw angle, with respect to the transversal direction, is performed for fuel consumption balancing and optimisation. It also affects the sun-angle in the satellites' body-fixed frame, which is essential for solar panels dimensioning. Furthermore, the selection of the yaw angle is essential to evaluate the along-track and cross-track displacements, influencing the control effort and the inter-satellite collision risk. An established methodology to ensure collision avoidance for neighbouring satellites exploits the eccentricity and inclination separation between the spacecraft. The design investigates the possibility to implement such safety policies with the baseline formation flying geometry. The paper presents the outcome of these trade-off analyses. Results are obtained leveraging the Hill-Clohessy-Wiltshire formulation with the relative orbital elements environment, to include the effects of orbital perturbations and to assess the formation safety more straightforwardly
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
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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