1,720,958 research outputs found
MISSION ANALYSIS AND ORBIT CONTROL STRATEGY FOR A SPACE MISSION ON A POLAR TUNDRA ORBIT
No full textIn this paper a Polar Tundra Orbit is investigated for a space mission aiming at improving weather monitoring and forecasting on the North Pole and Canada. Specific requirements are given in terms of satellite operation lifetime, satellite-bus class, desired coverage of particular latitudes and observation sensor aperture. Orbit design is performed so to identify the set of orbital parameters which satisfy the requirements and, at the same time, limit the propellant consumption required for station keeping manoeuvres. Orbit design effectiveness is investigated by means of numerical simulations and comparison with a traditional Tundra orbit. It is also showed that with a particular selection of the initial values of the perigee argument and of the right ascension of the ascending node it is possible to limit the orbit correction to in-plane manoeuvres with a reduced propellant consumption
Applications of Genetic Algorithms in Mission Design
No full textGenetic algorithms (GAs) have been showed to match the requirements of optimization problems in mission design. The paper presents the approach pursued at Universita' di Roma "La Sapienza" in order to solve, by means of GAs, two different kinds of problems typical to Astrodynamics. First, the design of a constellation devoted to zonal coverage on a non-continuous time basis has been faced. GAs-specific unneeded request of the derivatives of the cost function is especially advantageous in this problem, faced as a discrete optimization one. Second, the application of GAs to identify optimal solutions to transfer between different altitudes, circular to eccentric terrestrial orbits and to the orbit acquisition since the release from the launcher in parking orbit has been carried on. The possibility to provide an efficient tool for the low-thrust techniques, requiring a detailed and therefore computation expensive analysis is the reason to prefer GAs in the latter case. While all these applications, primarily intended to test the validity of the approach, are aimed to keep as simple as possible the technique, several issues and findings deemed to be of general validity when using GAs are highlighted. More important, GAs confirm once more the interest they recently received in literature as far as it concerns Astrodynamics applications
Interferometric Missions: Applications within ASI Space Programs
No full textThe Italian Space Agency (ASI), together with Ministry of Defence (I-AD), is involved in the realization of the COSMO-Skymed program that foresees the deployment of a constellation of 4 SAR satellites in Low Earth Orbit, dedicated to the management, control and exploitation of Earth resources for civilian and defence applications. ASI is responsible for the definition, implementation and management of the Program, while Alcatel Alenia Space Italia is the Prime Contractor responsible for the design, development and verification of the entire system. The nominal phasing of the satellites on the orbit plane has been selected in order to achieve the optimisation of the performance in terms of accessibility and revisit time w.r.t. the number of satellites. Moreover COSMO-SkyMed system is able to perform interferometric acquisitions, which yield three dimensional images and/or provide various information on land conformation change, subsidence, land-slides and so on. Then, periodically, one of the four satellites will be moved to a fixed distance with respect to another for reaching the interferometric configuration. In order to increase the operating capacity of the COSMO-Skymed system, ASI intends to start the definition of a complementary civilian program with the purpose of covering bi-static and across-track/along-across interferometry applications. Such a mission will be referred as “SABRINA”, acronym of “System for Advanced Bistatic and Radar INterferometry Applications”, based on the use of a “passive” satellite (“BISSAT”, acronym for “Bistatic and Interferometric SAR SATellite”). This satellite will fly in formation with COSMO-Skymed and then a suitable strategy shall be determined for reaching and maintaining the interferometric configuration. The paper will describe the manoeuvre strategies necessary for reaching the interferometric configuration, both for COSMO-Skymed and for BISSAT. Such strategies have been defined taking into account all the constraints coming from the satellite (Δv size, minimum interval time between two consecutive manoeuvres, propulsion S/S performance, AOC requirements). After the description of the manoeuvres requested for achieving the interferometric configurations foreseen for COSMO-Skymed (tandem and “tandem-like”), the paper will focus on the configuration requested for the BISSAT satellite for flying in formation with a single COSMO-SkyMed satellite. Two formation configurations will be selected: pendulum or cartwheel. The mission will carry out interferometric acquisitions in the following configurations: 1. Across track interferometry 2. Along track interferometry The mission will carry out bistatic acquisition multi-angle and multi-polarization with a baseline variable in the range 100-500 km
Collision Avoidance Activities for COSMO-SkyMed Constellation
No full textCOSMO-SkyMed (COnstellation of Satellites for Mediterranean basin Observation) is an Earth Observation space program funded by the Italian Ministry of Research and Ministry of Defence and
managed by the Italian Space Agency (ASI) in conjunction with Italian MoD.
COSMO-SkyMed (CSK) consists of a constellation of four Low Earth Orbit mid-sized satellites, each carrying a multi-mode, high resolution, electronically steerable Synthetic Aperture Radar (SAR)
instrument operating at X-band and a full featured Ground Segment to properly exploit space capabilities. CSK System is fully qualified and now in operative phase.
This paper describes the characteristics and functionalities of COSMO-SkyMed collision avoidance system, highlighting its versatility and effectiveness for constellation in flight operations. Moreover, the application of the COSMO-SkyMed risk mitigation daily procedure during two year of operations is described
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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