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Correlation of space debris observations by the virtual debris algorithm
No full textThe main problem in the orbit determination of the space debris population is the correlation of independently observed tracklets, that are sets of observations over a short time. The information contained in such data are not sufficient for a complete determination of an orbit, thus we need to find two or more tracklets belonging to the same physical object and an orbit fitting all the observations. In this paper we will show how to use the admissible region tool to generate a set of virtual objects (Virtual Debris) which can be used as alternate preliminary orbits and as starting points for a recursive procedure of correlation. We shall focus on optical observations of GEO showing some preliminary results testing the 2007 data by the ESA Optical Ground Station (OGS) at Teide Observatory (Canary Islands), after an astrometric reduction performed by University of Bern
Correlation and orbit determination of space objects based on sparse optical data
Get PDFWhile building up a catalogue of Earth-orbiting objects, the available optical observations are typically sparse. In this case, no orbit determination is possible without previous correlation of observations obtained at different times. This correlation step is the most computationally intensive, and becomes more and more difficult as the number of objects to be discovered increases. In this paper, we tested two different algorithms, and the related prototype software, recently developed to solve the correlation problem for objects in geostationary orbit (GEO). The algorithms allow the accurate orbit determination by full least-squares solutions with all six orbital elements. The presence of a significant subpopulation of high area-to-mass ratio objects in the GEO region, strongly affected by non-gravitational perturbations, required to solve also for dynamical parameters describing these effects, that is to fit between six and eight free parameters for each orbit.
The validation was based upon a set of real data, acquired from the European Space Agency (ESA) Space Debris Telescope (ESASDT) at the Teide Observatory (Canary Islands). We proved that it is possible to assemble a set of sparse observations into a set of objects with orbits. This would allow a survey strategy covering the region of interest in the sky just once per night. As a result, it would be possible to significantly reduce the requirements for a future telescope network, with respect to what would have been required with the previously known algorithms for correlation and orbit determination
Innovative methods of correlation and orbit determination for space debris
Get PDFSUMMARY We propose two algorithms to provide a full preliminary orbit of an Earth-orbiting object with a number of observations lower than the classical methods, such as those by Laplace and Gauss. The first one is the Virtual debris algorithm, based upon the admissible region, that is the set of the unknown quantities corresponding to possible orbits for a given observation for objects in Earth orbit (as opposed to both interplanetary orbits and ballistic ones). A similar method has already been successfully used in recent years for the asteroidal case. The second algorithm uses the integrals of the geocentric 2-body motion, which must have the same values at the times of the different observations for a common orbit to exist. We also discuss how to account for the perturbations of the 2-body motion, e.g., the effect
Orbit determination with the two-body integrals. II
No full textInternational audienceThe first integrals of the Kepler problem are used to compute preliminary orbits starting from two short observed arcs of a celestial body, which may be obtained either by optical or by radar observations. We write polynomial equations for this problem, which can be solved using the powerful tools of computational Algebra. An algorithm to decide if the of two short arcs is successful, i.e. if they belong to the same observed body, is proposed and tested numerically. This paper continues the research started in Gronchi et al. (Celest. Mech. Dyn. Astron. 107(3):299-318, 2010), where the angular momentum and the energy integrals were used. The use of a suitable component of the Laplace-Lenz vector in place of the energy turns out to be convenient, in fact the degree of the resulting system is reduced to less than half
Optimization of Space Surveillance Resources by Innovative Preliminary Orbit Methods
No full textThe number and performance of the sensors to be used
for a survey is a function of the minimum number of observations
required to determine an orbit. This is critical
for the definition of the sensor network and the observation
planning.
Our goal is to obtain an orbit with a smaller number of
observations with respect to the classical methods, such
as Gauss/Laplace. In the context of space debris surveys,
the goal is a full 6-elements orbit from just 2 tracklets,
which could be obtained with only 2 exposures. The information
contained in a tracklet can be summarized in a
4-dimensional vector called attributable, thus two tracklets
are enough for the orbit determination problem to be
over-determined.
We have proposed an algorithm based upon the integrals
of the 2-body problem. We outline the equations and
the solution methods which are used in our implementation.
We report on the results of a validation test, based
upon the processing of one year of data from ESA Optical
Ground Station. We conclude that the method is
very effective and can be used to find correlations between
tracklets, to be confirmed with additional correlations,
thus providing a catalog of full 6-elements orbits
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
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