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Orbit determination with the two-body integrals
No full textWe investigate a method to compute a finite set of preliminary orbits
for solar system bodies using the first integrals of the Kepler problem. This method is thought for the applications to the modern
sets of astrometric observations, where often the information
contained in the observations allows only to compute, by interpolation,
two angular positions of the observed body and their time derivatives
at a given epoch; we call this set of data attributable.
Given two attributables of the same body at two different epochs we
can use the energy and angular momentum integrals of the two-body
problem to write a system of polynomial equations for the topocentric
distance and the radial velocity at the two epochs. We define two
different algorithms for the computation of the solutions, based on
different ways to perform elimination of variables and obtain a
univariate polynomial.
Moreover we use the redundancy of the data to test the hypothesis
that two attributables belong to the same body (linkage problem).
It is also possible to compute a covariance matrix, describing the
uncertainty of the preliminary orbits which results from the
observation error statistics.
The performance of this method has been investigated by using a large
set of simulated observations of the Pan-STARRS project.
We expect that this method can be used when the two epochs are well
separated, even if the time span is such that the algorithms based
upon propagation of a swarm of virtual objects become inadequate
Orbit determination for the radio science experiment of the NASA mission Juno
Get PDFJuno is a NASA New Frontiers mission to the planet Jupiter, launched from Cape Canaveral on August 5,
2011. The spacecraft will arrive to Jupiter in 2016 and will be placed for one year in a polar high-eccentric
orbit to study the composition of the planet, the gravity and the magnetic field, and the magnetosphere. The
Italian Space Agency (ASI) contributed to the mission providing the radio science instrument KaT (Ka-Band
Translator, developed by the University of Rome “La Sapienza” and Thales Alenia Space) used for the gravity
experiment, which has the goal of studying the Jupiter’s deep structure by mapping the planet’s gravity. Such
instrument takes advantage of synergies with a similar tool in development for BepiColombo, the ESA cornerstone
mission to Mercury. The Celestial Mechanics Group of the University of Pisa and SpaceDyS s.r.l. are responsible,
under an ASI contract, for the development of an orbit determination and parameters estimation software for
processing the real data independently from NASA software ODP. We shall present the state of the art of such
software highlighting the theoretical models used, the problems addressed and first results about the scientific
goals obtained with simulated data
On the computation of preliminary orbits for Earth satellites with radar observations
Get PDFWe introduce a new method to perform preliminary orbit determination
for satellites on low Earth orbits (LEO). This method works with
tracks of radar observations: each track is composed by
topocentric position vectors per pass of the satellite, taken at
very short time intervals. We assume very accurate values for the
range , while the angular positions (i.e. the line of sight,
given by the pointing of the antenna) are less accurate. We wish to
correct the errors in the angular positions already in the
computation of a preliminary orbit. With the information contained
in a pair of radar tracks, using the laws of the two-body dynamics,
we can write 8 equations in 8 unknowns. The unknowns are the
components of the topocentric velocity orthogonal to the line of
sight at the two mean epochs of the tracks, and the corrections
to be applied to the angular positions. We take advantage
of the fact that the components of are typically small.
We show the results of some tests, performed with simulated
observations, and compare this method with Gibbs'
and the Keplerian integral
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
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
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