1,721,379 research outputs found
Optical design of the single-detector planetary stereo camera for the BepiColombo European Space Agency mission to Mercury
No full textWe present the catadioptric optical design solution for the stereo channel of the imaging system
SIMBIOSYS for the BepiColombo European Space Agency mission to Mercury. The main scientific objectives
of the instrument are the three-dimensional global mapping of the entire surface of Mercury in
the panchromatic band and imaging of selected areas in four broad colored bands; both tasks have to be
accomplished with a scale factor of 50mper pixel at periherm. The system consists of an original compact
layout in which the two stereo subchannels share a common detector; also, the optical components are
common to the two subchannels, with the exception of the first element, which is a rhomboid prism. The
field of view of each subchannel is about 5° × 5° with a scale factor of 23 arcsec/pixel. The ray-tracing
simulation of the system shows that the design guarantees optimal aberration balancing over the entire
field of view and the entire wavelength range covered by the instrument, with ensquared energy of the
order of 80% in one pixel
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
A New Stereo Algorithm based on Snakes
No full textAs part of the ongoing activities for the European space
mission BepiColombo to Mercury, a new stereo-matching
algorithm is here proposed: this algorithm uses deformable
surfaces, or snakes, to find a dense disparity map. Subject
to both external and internal forces, respectively represented
by the similarity function and by smoothness constraints on
the disparity map, a “deformable” disparity map evolves
from an initial approximate state to an optimal one in
which the algorithm has reached convergence.
This algorithm is expected to provide one of the image
matching tools for the Digital Terrain Model generation
procedure that will be used by the BepiColombo stereo
camera. To check the algorithm, tests have been performed
on synthetic images derived from 3D models of geological
features relevant to planetary science. The results show that
it is possible to obtain an image measurement accuracy
comparable to the one attainable with the Least Squares
Matching algorithm. In addition, less object smoothing can
be obtained since the object points are not derived by a
large scale averaging over a terrain patch, as for example, in
area-based methods; this means that more details of the
terrain can be captured. Finally, because of the continuity
constraint, this method is also expected to be robust in case
of blunders in the reconstruction of the parallax field
Hybrid GIS-Transformer Approach for Forecasting Sentinel-1 Displacement Time Series
No full textThis study presents a deep learning-based approach for forecasting Sentinel-1 displacement time series, with particular attention to irregular temporal patterns-an aspect often overlooked in previous works. Displacement data were generated using the Parallel Small BAseline Subset (P-SBAS) technique via the Geohazard Thematic Exploitation Platform (G-TEP). Initial experiments on a regular dataset from Lombardy employed Long Short-Term Memory (LSTM) models to forecast multiple future time steps. Empirical analysis determined that optimal forecasting is achieved with a 50-time-step input sequence, and that predicting 10% of the input sequence length strikes a balance between temporal coverage and accuracy. The investigation then extended to irregular datasets from Lisbon and Washington, comparing two preprocessing strategies: imputation and the inclusion of time intervals as a second feature. While imputation improved one-step predictions, it was inadequate for multi-step forecasting. To address this, a Time-Gated LSTM (TG-LSTM) was implemented. TG-LSTM outperformed standard LSTM for irregular data in one-step prediction but faced limitations in handling heteroscedasticity and computational cost during multi-step forecasting. These issues were effectively resolved using Temporal Fusion Transformers (TFT), which achieved the best performance, with RMSE values of 1.71 mm/year (Lisbon) and 1.26 mm/year (Washington). A key contribution of this work is the development of a GIS-integrated forecasting toolbox that incorporates LSTM models for regular sequences and TG-LSTM/TFT models for irregular ones. The toolbox enables both single- and multi-step displacement predictions, offering a scalable solution for geohazard monitoring and early warning applications
Link budget and background noise for satellite quantum key distribution
No full textOptical quantum communication exploiting satellites is the most promising field to enable global quantum communication.
Bonato et al. (2009) discussed the feasibility of quantum key distribution (QKD), for vertical space to Earth and Earth to space links
at different time conditions and transmitter and receiver specifications, outlining a set of needed technical requirements.
In a real scenario a satellite is seen at any time at a different position from an Earth station: we analyze the link attenuation and the
signal to noise ratio (SNR) for its visibility time. In particular, we study the atmosphere effects on the beam propagation for different
altitudes and zenith angles, modelling the dynamic quantum link for an orbiting satellite.
Our results show the feasibility of QKD from low-Earth orbit satellites to Earth during night time; QKD uplinks are much more
difficult to achieve, and their implementation is feasible reducing the field of view of the receiving telescope. Feasibility of QKD with
satellite at higher altitude is a technological challenge due to the narrow portion of sky that an Earth station can effectively use for
communication
Feasibility of satellite quantum key distribution
Full text linkIn this paper, we present a novel analysis of the feasibility of
quantum key distribution between a LEO satellite and a ground station.
First of all, we study signal propagation through a turbulent atmosphere for
uplinks and downlinks, discussing the contribution of beam spreading and
beam wandering. Then we introduce a model for the background noise of the
channel during night-time and day-time, calculating the signal-to-noise ratio for
different configurations.We also discuss the expected error-rate due to imperfect
polarization compensation in the channel. Finally, we calculate the expected key
generation rate of a secure key for different configurations (uplink, downlink)
and for different protocols (BB84 with and without decoy states, entanglementbased
Ekert91 protocol)
Characterization of the polarizing filters for the EnVisS camera
No full textThe Comet Interceptor mission has been adopted by the European Space Agency (ESA) Science Programme Committee in June 2022 as the first ”F” mission in the Science Programme. The aim of the mission is to increase the knowledge on comets and on the Solar System formation by encountering and exploring a Dynamically New Comet (DNC) or an Interstellar Object (ISO) originating at another star. EnVisS (Entire Visible Sky) is an all-sky camera designed to fly on Comet Interceptor and whose scientific task is to study the radiance and the polarization properties of the comet coma in the visible spectrum. The camera is composed of an optical head, a filter strip assembly and a detector. The Institute for Photonics and Nanotechnologies (CNR-IFN) of Padova and Leonardo SpA (Campi BisenzioFlorence) are in charge of the design of the filter package, which currently consists of three filter strips glued side by side. The central strip is a high transmission broadband (BB) filter in the range 550–800 nm with no polarization properties, while the side ones are linear polarization filters with the same transmission bandpass as the BB and with polarization axis at 45° from one another. In the CNR-IFN laboratories different types of polarizers have been tested to establish which one has the most fitting properties for EnVisS’s purposes. The analyzed filters are Moxtek Visible Light Polarizer RCV8N2EC and Ultra BroadBand Polarizer UBB01A, and Polarcor Wide Band Polarizer. For each type of polarizing filter, both transmissivity and reflectivity have been measured and compared both with those of the other filters as well as data provided by the manufacturer. Overall, measurements of the filters’ transmissivity and reflectivity agree with those provided by the supplier and mostly fit EnVisS’ purposes. Thanks to its optimal performance and the fused silica substrate, Moxtek UBB01A is considered the best candidate filter for the instrument between the polarizers that have been characterized
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