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Analytical design of end-of-life disposal manoeuvres for highly elliptical orbits under the influence of the third body's attraction and planet's oblateness
Full text linkLAUREA MAGISTRALEL’incremento del numero di satelliti in orbita terrestre fa nascere la necessità di studiare metodi di rimozione dei veicoli spaziali, per mantenere gli standard di sicurezza necessari in orbita per le missioni future. Nel corso degli anni, diversi studi hanno analizzato strategie di rimozione passiva. Lo scopo di questa tesi è lo studio e la determinazione di metodi analitici per il rientro e la rimozione di satelliti in orbite fortemente ellittiche. Seguendo la teoria classica, è stata sviluppata una formulazione analitica del potenziale dovuto al disturbo del terzo corpo e di J2, nel sistema di riferimento equatoriale, eliminando i contributi ad alta frequenza. Lo scopo di questa tesi è l’introduzione di un approccio innovativo, basato su algoritmi analitici, con lo scopo di ridurre il costo computazionale. In passato, diversi studi si sono basati su un approccio semi analitico: l’agoritmo risulta computazionalmente inefficiente per l’ottimizzazione delle manovre. Si è sviluppata quindi un’analisi semplificata,
considerando l’effetto della perturbazione del terzo corpo e di J2, sviluppando una descrizione dell’Hamiltoniana bidimensionale. La manovra è stata modellata tramite un processo di
ottimizzazione su diversi parametri, e il risultato è stato ottenuto tramite le equazioni di Gauss per una variazione dei parametri orbitali. Come casi studio, le possibili traiettorie a fine vita sono state valutate sia per missioni terrestri che intorno a Venere, considerando
anche il limite dovuto alla quantità di combustibile disponibile a bordo. Dai risultati, è stato dimostrato che la perturbazione orbitale dovuta
all’attrazione del terzo corpo può essere
sfruttato per il design di manovre nello spazio delle fasi, e questo può essere considerato come il punto di partenza per ulteriori analisi, che comprendano modelli più accurati, dimostrando la potenziale efficienza dell’utilizzo di codici analitici che sfruttino le perturbazioni orbitali per il controllo dei satelliti. Questa tesi è parte del progetto COMPASS: “Control for orbit manoeuvring by surfing through orbit perturbations”(Grant agreement No 679086). Questo progetto è un progetto finanziato dall’European Research Council (ERC) nell’ambito della ricerca European Unions Horizon 2020.The increasing number of satellites orbiting the Earth gives rise to the need for investigating disposal strategies for space vehicles, to keep operative orbits safe for future space missions. In the last years, several studies have been conducted focused on designing
end-of-life trajectories. The aim of this thesis is the feasibility analysis for a fully-analytical method for end-of-life de-orbit strategy of spacecraft in Highly Elliptical Orbits. The main perturbations to the Keplerian motion are planet’s oblateness and third bodies’ gravitational attraction. Following the classical theory, the analytical expression of the double-averaged potential due to a third body’s perturbations and zonal J2 effect is derived in the planet
Centred Equatorial frame. This allows for a simplified formulation of the system’s long-term dynamics. This thesis aims to introduce an innovative approach for the manoeuvre design, relying on a fully-analytical method to reduce the computational cost. Some studies were already developed but using a semi-analytical approach: the resulting algorithm is time consuming for manoeuvres’ optimisation. The analysis is done considering the third body and J2 contributions and the re-entry is modelled using the two-dimensional Hamiltonian
phase space. The model is used to estimate the eccentricity variations of the large set of orbits required during the optimisation process. The disposal manoeuvre is selected through a multi-criteria optimisation. As real cases scenarios, the disposal manoeuvre for Earth’s
and Venus’ satellite missions are designed, considering the limitation upon the available propellant onboard. It is demonstrated that the third-body gravitational perturbation provides a suitable environment for manoeuvres design. These results could serve as initial
conditions for more accurate analysis with a high-fidelity model and confirm the potential efficiency of exploiting the use of orbital perturbations for satellite navigation. This thesis was part of the COMPASS project: “Control for orbit manoeuvring by surfing through orbit perturbations”(Grant agreement No 679086). This project is European Research Council (ERC) funded project under the European Unions Horizon 2020 research
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
Recovery of the gravity field of a small body through autonomous and relative navigation between two spacecraft
No full textLAUREA MAGISTRALEL’interesse crescente nello studio degli asteroidi e dei Near Earth Objects in generale ha dato origine alla necessità di trovare nuove e diverse strategie per indagare l’ambiente intorno ad essi. Negli ultimi anni diversi studi hanno considerato la possibilità di aumentare l’autonomia e la sicurezza delle missioni nello spazio profondo introducendo l’utilizzo di più satelliti.
L’obiettivo di questa tesi è l’utilizzo di un modello analitico per stimare il campo gravitazione attorno a questi corpi minori attraverso la navigazione autonoma e relativa tra due satelliti. A differenza di altri studi, per supportare il processo di stima del campo gravitazione, viene utilizzata una conversione analitica tra gli elementi osculatori e gli elementi medi, che sono utilizzati per descrivere la dinamica relativa; si vogliono infatti fornire al modello le informazioni più accurate possibili per facilitare la valutazione dei coefficienti del campo gravitazionale. L’utilizzo degli elementi medi permette di ottenere una propagazione della dinamica del satellite senza doverli integrare, raggiungendo allo stesso tempo una propagazione accurata per periodi temporali più lunghi. In questo lavoro sono stati
utilizzati due filtri per studiare i coefficienti delle armoniche sferiche del campo gravitazionale dei target: un filtro batch e un filtro kalman. Partendo dall’ipotesi di non poter sapere l’accuratezza con cui lo stato iniziale è noto, l’obiettivo principale del filtro batch è quello di raggiungere una soluzione finale indipendente dalle condizioni iniziali e dal livello di confidenza con cui le stesse sono note; per verificare che questa ipotesi sia soddisfatta, è stata effettuata un’analisi statistica su un campione di
casi analizzando il valore medio ottenuto e la deviazione standard. Questa tipologia di filtro risulta infatti più robusta quando si è in presenza di condizioni iniziali, nel caso in esame i coefficienti zonali delle armoniche sferiche, con grandi errori rispetto al loro vero valore. Con il filtro di Kalman esteso, invece, si vuole rifinire la soluzione trovata riducendo sia l’errore assoluto che quello relativo. Il modello utilizzato per costruire gli algoritmi di questi filtri è semplificato, nel senso che comprende solo i termini zonali fino al quinto grado. Per testare il modello sono stati selezioni tre diversi target: Eros, Bennu e Ryugu. I tre asteroidi hanno caratteristiche differenti per dimensioni, forma e campo gravitazionale; tuttavia Bennu e Ryugu sono accomunati da dimensioni contenute e una forma più
regolare rispetto ad Eros.
Per la determinazione delle orbite da utilizzare, è stata condotta un’analisi sulle orbite frozen rispetto ai termini zonali fino al quinto grado. Questa scelta è consistente con il modello utilizzato per i filtri.
Le orbite selezionate sono polari per garantire una migliore copertura rispetto al corpo che si sta analizzando e sono stabili per un tempo sufficiente a garantire un sicuro volo in formazione autonomo.The increasing interest in the study of asteroids and Near Earth Objects in general, gives rise to the need for investigating different and new strategies to investigate the environment around them. In recent years several studies have considered the possibility of increasing the autonomy and the safety of the missions in deep space by introducing the use of multiple spacecraft.
The aim of this thesis is the exploitation of an analytical model to recover the gravity field of these small bodies through autonomous and relative navigation between two spacecraft. Unlike other studies, to support the process of estimating the gravitation field, an analytical conversion is used between the osculating elements and the relative orbital elements are employed to describe the relative dynamics. The aim is to provide the model with the most accurate information possible to facilitate the assessment of gravitational field coefficients. The use of the mean elements allows to obtain a propagation of the dynamics of the satellite without integration, reaching at the same time an accurate propagation for
longer periods of time. In this work two filters are used to estimate the coefficients of the spherical harmonics of the gravity field of the target: a batch least square method and a Kalman filter. Assuming that is not known the accuracy with which the initial state is not known, the main objective of the batch filter is to reach a final solution independent on the initial conditions and the level of confidence with which they are known. In order to verify that this hypothesis is respected, a statistical analysis is carried out on a sample of cases analysing the mean value and the standard deviation obtained. This type of filter is in fact more robust when one is in the presence of initial conditions, in the case under investigation the zonal coefficients of the spherical harmonics, with large errors with respect to the
true values. With the Extended Kalman filter, however, the solution found is refined to reduce both the relative and the absolute errors. The model used to build the algorithms of those filters is simplified in sense that it includes the zonal terms up to fifth grade only.
To test the proposed model three different asteroids have been selected: Eros, Bennu and Ryugu. The three asteroids differ in size, shape, and gravity field; however Bennu and Ryugu share a smaller size and a more regular shape with respect to Eros.
For the determination of the orbits to be used, an analysis on the frozen orbits with respect to the zonal coefficients up to fifth order is performed. This choice is consistent with the model used for the filter.
As a result, the orbits selected are polar to guarantee a better coverage with respect to the body under examination and stable for a time enough to enable safe autonomous formation flying
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
End-of-life disposal of geosynchronous satellites
No full textEnd-of-life disposal of spacecraft in the GEO region is required for the further exploitation of this particularly important orbital regime. The orbital dynamics around the geostationary ring can be exploited for designing graveyard orbits or looking for re-entry solutions. Here we present an end-of-life trajectory design method based on a detailed cartography of the orbital space. Given a post-mission orbit of a decommissioned satellite and the available fuel on board, efficient two-burn transfers are calculated for each reachable orbit on the grid. Furthermore, an analysis of cost (delta-v) versus stability of target orbit or re-entry time is performed by means of finding the Pareto optimal solutions for each case
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