1,720,991 research outputs found
Rain and ice flagging of Envisat altimeter and MWR data
Full text linkAltimetry range, wave height, and wind speed measurements are often corrupted by two effects over the ocean: rain and sea-ice. Radiometer measurements, which provide the altimetric wet troposphere correction, are similarly corrupted by the presence of rain or sea-ice in the instrument's footprint. To avoid contamination of
sea surface height measurements, it is imperative that data influenced by either of these effects be edited out. The waveform “peakiness” parameter, available on the
GDR data sets is effective at identifying sea-ice returns when stringent thresholds are applied. The mean relationship between backscatter (sigma0) at the two
altimeter frequencies allows one to flag data impacted by both rain and sea-ice. We present here a new method for flagging rain or sea-ice contaminated data, based on
two-dimensional histograms of sigma0
Improved modelling of surface forces in the orbit determination of ERS and ENVISAT
No full textPrecise orbit determination of the European remote sensing satellites ERS-2 and ENVISAT, a prerequisite for the processing of their altimeter data, requires highly accurate force models. Inaccuracies in the modelling of non-gravitational forces, caused by interaction of photons, molecules, and atoms with the spacecraft surfaces, form a major error source. A highly detailed new non-gravitational force modelling package named ANGARA was implemented and tested for ERS-2 and ENVISAT. The resulting orbits for ERS-2 were compared with those from an existing box-wing panel model and a simple constant-area model. In addition, the effects on orbit accuracy of three thermospheric density models (DTM94, MSIS-86, and NRLMSISE-00) and a horizontal wind model (HWM-93) were evaluated. Force model parameters, estimated from tracking data during the precise orbit determination process, absorb most of the differences between the considered models. At the same time, an analysis of these parameters, and orbit determination runs with a limited parameterization, can give important clues about modelling accuracy. Atmospheric density is the dominant error source at high solar activity, degrading the radial orbit accuracy. Radiation pressure forces, for which uncertainties in material properties are the largest error source, play an equally important role at low solar activity
Improved modelling of surface forces in the orbit determination of ERS and ENVISAT
No full textPrecise orbit determination of the European remote sensing satellites ERS-2 and ENVISAT, a prerequisite for the processing of their altimeter data, requires highly accurate force models. Inaccuracies in the modelling of non-gravitational forces, caused by interaction of photons, molecules, and atoms with the spacecraft surfaces, form a major error source. A highly detailed new non-gravitational force modelling package named ANGARA was implemented and tested for ERS-2 and ENVISAT. The resulting orbits for ERS-2 were compared with those from an existing box-wing panel model and a simple constant-area model. In addition, the effects on orbit accuracy of three thermospheric density models (DTM94, MSIS-86, and NRLMSISE-00) and a horizontal wind model (HWM-93) were evaluated. Force model parameters, estimated from tracking data during the precise orbit determination process, absorb most of the differences between the considered models. At the same time, an analysis of these parameters, and orbit determination runs with a limited parameterization, can give important clues about modelling accuracy. Atmospheric density is the dominant error source at high solar activity, degrading the radial orbit accuracy. Radiation pressure forces, for which uncertainties in material properties are the largest error source, play an equally important role at low solar activity
A decade of ERS satellite orbits and altimetry
No full textThe First European Remote Sensing Satellite, ERS-1, was launched in July 1991, fol- lowed by ERS-2 in April 1995. Both satellites carry a radar altimeter to serve oper- ational applications and scientific research in the fields of geodesy, oceanography, glaciology and meteorology. Together, the satellites have now been operating for over twelve years. This thesis embarks on the a voyage along several milestones in the lifetime of these satellites, and particularly of its altimeters. Both altimeter missions were hampered by a number of misfortunes, ranging from hardware failures, via design mishaps, to a sub-optimally designed ground segment. Yet, the altimeter data, when properly processed and supplied with a precise orbit, can compete favourably with data supplied by specialised altimeter satellites like TOPEX/Poseidon and Jason-1. The upgrading of all altimeter cor- rections and the tweaking of the basic altimeter measurements of range, significant wave height and backscatter coefficient, are essential steps in this process, outlined in this thesis. Having accomplished that, the ERS altimeters allow the monitor- ing of annual, semi-annual and secular changes in sea level, which are important factors in study of climate change. A key role in the processing of altimeter data plays the position of the satellite, and its progress in time, the satellite orbit. The quality of the recovered sea surface height is limited by the precision at which the orbit can be determined. A long journey along several stepwise improvements of the orbit determination process, both in the technique as well as in the modelling, have resulted in the reduction of the orbit error by a factor of 30! A major player in this process was the estab- lishment of a gravity field model tailored to the ERS satellites that, when applied to ERS orbit determination, outperformed any of the previous and contemporary general-purpose models. Yet, it is shown that this model, DGM-E04, is not just a fabrication for the benefit of ERS only, but is equally suitable for the orbit determ- ination of TOPEX/Poseidon and as a long-wavelength geoid model. The calibration of the ERS-1 altimeter in 1991 required, at a time when the orbit errors were still counted in decimetres, if not metres, a short-arc orbit determin- ation technique centred over the calibration site off the coast of Venice. Again, through elaborate surveying, gathering of altimeter corrections and tide gauge measurements, problem solving and elimination of errors, a final absolute range bias of 41.5 cm was established, a value that is still within the error margins of more recent estimates. A timing error in the ERS altimeter data was discovered in 1995 and its cause remained a mystery ever since, especially when the timing error appeared to ex- hibit daily, annual and inter-annual variations as well. Although the exact cause may never be found, the issue is examined in fine detail and a model is constructed to eliminate the timing error for the benefit of the quality of the altimeter data.Aerospace Engineerin
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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