1,720,992 research outputs found
Ephemeris Protection Level Equations and Monitor Algorithms for GBAS
No full textOne troublesome failure mode for Ground Based Augmentation Systems (GBAS) is the possibility of large discrepancies between satellite locations in space and the locations derived by the ephemeris data that they broadcast. For the Global Positioning System (GPS), nominal ephemeris errors are typically 10 meters or less, and it would take large errors (typically greater than 1 km) to be hazardous to GBAS users making precision approaches to Category I minima. Although most large errors will be detected by the GBAS ground segment Message Field Range Test, ephemeris errors orthogonal to the line-of-sight between a failed satellite and a GBAS ground station are not detectable by this simple test. To counter this possibility, RTCA has adopted new protection levels to quantify the potential impact of undetected ephemeris failures on user position errors for both precision approach and terminal area applications. These equations define position error bounds as functions of the approximate aircraft location with respect to each satellite and the GBAS ground station as well as the magnitude of the satellite orbit error detectable by the ground station. This Minimum Detectable Error (MDE) determines the "P-value" that is broadcast by the GBAS ground station for each satellite it has approved for use. Several GBAS monitor algorithms have been developed and tested for use in GBAS installations that lack SBAS coverage. One of these is a comparison between satellite positions given by the current satellite ephemeris and the ephemeris broadcast by the same satellite on its previous pass. Variants of this "YE-TE" test have been shown to support GBAS MDE's as low as 1100 meters, which will minimize the resulting impact on Category I user availability due to the ephemeris protection level equations. In addition, means of using raw measurements to initialize this monitor and to separately verify ephemerides in real-time are proposed.The authors would like to thank Todd Walter and Gang
Xie for their help during this research. This work was
also supported by the efforts of Barbara Clark of FAA
AIR-130, Tom Hsiao, Curt Shively, Chris Varner, and
Ron Braff of MITRE, Victor Wullschleger and John
Warburton of FAA ACT-360, and others participating in
RTCA SC-159 WG-4 requirements development. The
advice and interest of many other people in the Stanford
GPS research group is appreciated, as is funding support
from the FAA LAAS Program Office (AND-710). The
opinions discussed here are those of the authors and do
not necessarily represent those of the FAA or other
affiliated agencies
Integrity Design and Updated Test Results for the Stanford LAAS Integrity Monitor Testbed
No full textThe authors would like to give thanks to many other people in the Stanford GPS research group for their advice and interest. Funding support from the FAA Satellite Navigation LAAS Program Office (AND-710) is appreciated. The opinions discussed here are those of the authors and do not necessarily represent those of the FAA
and other affiliated agencies
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
The Next Generation Integrity Monitor Testbed (IMT) for Ground System Development and Validation Testing
No full textThe Stanford University Integrity Monitor Testbed (IMT)is a prototype of the Local Area Augmentation System (LAAS) Ground Facility (LGF). It is used to evaluate whether the LGF can meet the integrity and continuity requirements that apply to Category I precision approach. With support from the U.S. Federal Aviation Administration (FAA), Stanford University has developed IMT algorithms and has implemented them in real-time with special emphasis on automated fault diagnosis and recovery. The first generation IMT hardware was designed in the mid-1990s, and since then computer power and receiver technology has evolved significantly. Therefore, a transition has been made to a new and improved system to further development and testing for Category I precision approach and to use as a starting point for Category II/III LGF development. This paper describes the hardware and motivation behind the second-generation IMT system. One key element of the upgrade has been the development of new software to communicate with the receivers. This function, known as Signal-in-Space Receive and Decode (SISRAD), is now a modular means of integrating different receiver types, providing synchronization of receiver measurement packets, and extracting receiver measurement packets into a specified IMT data format. With these modifications, the new IMT is able to support more extensive and efficient nominal and failure testing. The upgrade has been completed, and in this paper present nominal data fault free data is presented along with how the IMT responds to a satellite clock ramp failure.The authors would like to give thanks to many other people in the Stanford GPS research group for their advice and interest. Funding support from the FAA Satellite Navigation LAAS Program Office (AND-710) is appreciated. The opinions discussed here are those of the authors and do not necessarily represent those of the
FAA and other affiliated agencies
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
URA/SISA Analysis for GPS and Galileo to Support ARAIM
No full textThis paper presents a User Range Accuracy (URA)/Signal-in-Space Accuracy (SISA) analysis to support ARAIM based on a time-dependent statistical characterization of orbit and clock error observations. By comparing precise orbits to broadcast ephemeris for each individual GPS and Galileo satellite, this work computes the Signal-in-Space Range Error (SISRE) that needs to be overbounded by the URA/SISA value included in the Integrity Support Message (ISM). Service data from January 2008 to February 2015 for GPS and from March to June 2015 for Galileo are processed, showing that range error is mainly driven by satellite's clock performance. In order for the ISM generation to account for the variation in error biases and standard deviation, GPS service history is broken down into monthly, quarterly, and yearly datasets. Results reveal that orbit and clock error distributions are non-zero mean on a monthly basis, although biases tend to reduce as sample set size increases
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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