1,720,958 research outputs found
Gaborized MCS for Precise Code Phase Offset Estimation in Radionavigation
Full text linkIn radar and radionavigation systems, waveform design optimization has mostly focused on Gabor bandwidth (GB), as a metric to infer code phase offset estimation accuracy. While optimization algorithms have maximized GB w.r.t. to specific design parameters and receiver bandwidth, they lack closedform solutions for pulse shapes. Inspired by Gabor pulse theory, this paper presents a family of waveforms shaping multi-level coded spreading (MCS) coefficients with a Gaussian envelope, namely the Gaborized-MCS (G-MCS). These waveforms achieve nearly optimal GB performance, reflecting the trend of current optimization techniques to agnostically converge to such pulse shapes. Numerical analyses characterize the GB of the proposed signals across varying receiver bandwidths and compare them to optimized waveforms and legacy binary offset carrier modulations largely adopted in Global Navigation Satellite Systems
GNSS Precise Point Positioning in Cislunar Space: A Study on Regularized Least Squares and Availability
Full text linkThe ongoing Lunar GNSS Receiver Experiment (LuGRE) mission is demonstrating that Global Navigation Satellite System (GNSS) can be a major enabler for radionavigation in cislunar space and on the Moon, offering a complementary solution to ground-based tracking infrastructures. However, cislunar Orbit Determination (OD) and timing with GNSS signals remains challenging due to severe pathloss effects, frequent side lobe receptions, and degraded satellite geometry. This study evaluates a single-frequency precise point positioning (SF-PPP) approach for kinematic OD, leveraging the group and phase ionospheric calibration (GRAPHIC) model to process undifferenced code and phase observations. The method incorporates Tikhonov regularization within a batch nonlinear least square (LS) estimator to tackle the ill-conditioning caused by the inherent rank deficiency of the positioning model. The algorithm is assessed through post-processing of raw GNSS observables collected during a hardware-in-the-loop (HIL) test, simulating representative LuGRE payload operations. Results show that the proposed regularized estimator ensures more than 89 % solution availability in most of the scenarios and achieves sub-kilometer positioning accuracy, even in scenarios with insufficient measurement redundancy
Surveying GNSS Carrier Offset Modulations: Investigating Gabor Uncertainty Principle for Precise Time Delay and Frequency Offsets Estimation
Full text linkIn the last decades, the adoption of offset carrier modulations represented one of the main aspects in the modernization of Global Navigation Satellite System (GNSS) signals. Offset carrier modulations provide indeed specific signal characteristics and guarantee the desired performance trade-off in terms of bandwidth utilization and tracking jitter at the receiver. In light of this, ongoing signal design proposals for modernized GNSS, Low-Earth Orbit and navigation services cannot neglect fundamental findings in this direction. At the same time, the theoretical bounds governing time delay and frequency offset estimation have a direct impact on receivers state estimation when this task rely on the inference of signal-derived observables. In this context, the aim of this work is to investigate the inherent relationship between offset carrier modulation, i.e., spreading code chip shaping, and the bounds set by uncertainty principle about time delay and frequency offsets estimation of GNSS signals. The research addresses a surveying analysis of currently-adopted offset carrier modulations and the evaluation of their theoretical bounds associated to the respective analytical ambiguity functions. The study offers a methodology to synoptically compare different chip shaping and to characterize how this influences signals' time-frequency localization precision and estimation errors at the receiver, which has a direct impact on delay and frequency lock loops performance at the receiver tracking stage
Pseudorange and Doppler-Based Positioning: Enabling Convergence of Least-Squares Estimation from MEO to LEO
Full text linkThe growing interest of the space industry in satellite systems within the low Earth orbit (LEO) region has prompted attention to their potential for positioning, navigation, and timing applications. This study addresses the convergence issue highlighted in the literature when the Gauss Newton (GN) method is applied to least squares (LS) position estimation algorithms in LEO scenarios, with analyses conducted independently for pseudorange and Doppler shift measurements. To address these limitations, this paper examines two line search techniques in combination with the GN method. A comprehensive analysis of the LS method is conducted through tests on satellite constellations at various orbital altitudes, from medium Earth orbits to LEOs. The results, evaluated in terms of the number of iterations required to achieve convergence, show that adjusting the GN step using a damping factor, namely the damped GN factor, effectively resolves convergence issues, even in LEO scenarios. In particular, the proposed algorithm consistently converges in the LEO region within an average of seven iterations
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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