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Introduction to geodetic time series analysis
No full textThe previous chapter gave various examples of geophysical time series and the various trajectory models that can be fitted to them. In this chapter we will focus on how the parameters of the trajectory model can be estimated. It is meant to give researchers new to this topic an easy introduction to the theory with references to key books and articles where more details can be found. In addition, we hope that it refreshes some of the details for the more experienced readers. We pay special attention to the modelling of the noise which has received much attention in the literature in the last years and highlight some of the numerical aspects. The subsequent chapters will go deeper into the theory, explore different aspects and describe the state of art of this area of research
Estimation of the vertical land motion from GNSS time series and application in quantifying sea-level rise
No full textSea-level rise observed at tide gauges must be corrected for vertical land motion, observed with GNSS, to obtain the absolute sea-level rise with respect to the centre of the Earth. Both the sea-level and vertical position time series contain temporal correlated noise that need to be taken into account to obtain the most accurate rate estimates and to ensure realistic uncertainties. Satellite altimetry directly observes absolute sea-level rise but these time series also exhibit colored noise. In this chapter we present noise models for these geodetic time series such as the commonly used first order Auto Regressive (AR), the General Gauss Markov (GGM) and the ARFIMA model. The theory is applied to GNSS and tide gauge data from the Pacific Northwest coast
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
Desenvolvimento de um sistema de gravimetria escalar baseado em GNSS/IMU acoplado a veículos aéreos não tripulados
Get PDFAirborne gravimetry is the determination of the Earth’s gravity field, using aircraft
as a mobile measurement platform. Nowadays, airborne measurements cover an
important intermediate area between terrestrial and satellite gravity methods in
terms of both resolution and coverage. However, the aircraft rental cost is high,
and strict rules and regulations exist for acquiring permission to conduct a flight
mission in most countries. A possible alternative investigated in this thesis is based
on the use of an Unmanned Aerial Vehicle (UAV). Its advantages include high manoeuvrability,
operational flexibility and no airport support.
The development of a compact size low-cost strapdown scalar gravimetry system
for hexacopter UAV is attempted and its performance is evaluated. This gravimetry
system is based on a combination of one Global Navigation Satellite System
(GNSS) receiver and a (strapdown) Inertial Measurement Unit (IMU) that are the
two components of tactical-grade INS Quanta.
IMU observations are integrated with respect to time and the gravity disturbance is
obtained from the comparison of IMU-predicted position and velocity with GNSSobserved
position and velocity, i.e. inertial navigation approach was adopted.
Raw GNSS observations are processed into velocity and position estimates using
Qinertia® software. All observations, IMU and GNSS data, are processed within
a single loosely coupled Extended Kalman Filter (EKF) that includes the system
state for gravity. For the implementation of the filter, the random constant process
was adopted to model the errors in the gyros and accelerometers. Whereas, the
gravity disturbance was modelled as a simple random walk process. In this way, the
total state vector was composed of the position (3 states), the velocity (3 states),
the attitude (3 states), the accelerometer and gyro biases (6 states), and one state
for the gravity disturbance. In the scope of this thesis the UAV-borne gravimetry
processing tool called gravito was developed.
Evaluation of the system was primarily based on collecting the statistics of innovation
sequences and comparing EKF’s estimates of position, velocity and attitude
with those obtained by a commercial software Qinertia®. Based on the real campaign
data, it was concluded that the internal gyroscope noise of IMU Quanta
does not allow the estimation of the heading, as well as the roll and pitch angles,
with sufficient accuracy for scalar gravimetry. Also increased dynamics of UAV (in
comparison with manned aircraft) add additional errors to the gravity disturbance
estimation by making invalid assumptions of linearity for the dynamic model of the
system and through the scale factor errors of IMU.
In this way, the current low-cost system with the processing method implemented
is not suitable for scalar gravimetry mainly due to the noise in the IMU Quanta
gyroscopes measurements.A gravimetria aérea é a determinação do campo de gravidade da Terra, usando
aeronave como a plataforma móvel de medição. Atualmente, as medições aéreas
cobrem uma importante área intermediária entre os métodos de gravimetria terrestre
e por satélite em termos de resolução e cobertura. No entanto, o custo
do aluguer da aeronave é alto e as regras e regulamentos para adquirir permissão
de efetuar missão na maioria dos países são exigentes. Uma possível alternativa
investigada nesta tese baseia-se na utilização de um Veículo Aéreo Não Tripulado
(UAV). As suas vantagens incluem alta manobrabilidade, flexibilidade operacional
e o facto do UAV não precisar de suporte aeroportuário.
Foi desenvolvido um sistema de gravimetria escalar de tamanho compacto e baixo
custo para UAV hexacóptero e o seu desempenho foi avaliado. O sistema de gravimetria
é baseado na combinação de um receptor do Sistema de Navegação Global
por Satélite (GNSS) e uma Unidade de Medição Inercial (IMU) do tipo strapdown,
estes são os dois componentes do Sistema de Navegação Inercial (INS) Quanta de
grau tático.
As observações do IMU são integradas em relação ao tempo e a perturbação da
gravidade é obtida a partir da comparação da posição e velocidade previstas pelo
IMU com a posição e velocidade observadas pelo GNSS, ou seja, a abordagem
de navegação inercial foi adotada. As observações GNSS brutas são processadas
em estimativas de velocidade e posição usando o software Qinertia®. Todas as
observações, IMU e GNSS, são processados dentro de um único Filtro de Kalman
Estendido (EKF) fracamente acoplado que inclui o estado do sistema para
a gravidade. Na implementação do filtro para modelar os erros nos giroscópios e
acelerómetros foi adotado o processo de constante aleatória. Enquanto que a perturbação
da gravidade foi modelada como o processo de passeio aleatório. Desta
forma, o vetor de estado total é composto por posição (3 estados), velocidade (3
estados), atitude (3 estados), viéses do acelerómetro e do giroscópio (6 estados) e
um estado para a perturbação da gravidade. No âmbito desta tese foi desenvolvida
a ferramenta chamada gravito que processa os dados recolhidos por UAV.
A avaliação do sistema baseou-se principalmente na análise estatística das sequências
de inovação e na comparação das estimativas de posição, velocidade e atitude
do EKF com as obtidas por um software comercial Qinertia®. Com base nos dados
reais da campanha gravimétrica, concluiu-se que o ruído interno dos giroscópios do
IMU Quanta não permite estimar o ângulo de guinada, bem como os ângulos de
rolamento e picada, com precisão suficiente para a gravimetria escalar. Além disso,
a dinâmica altamente instável do UAV (em comparação com aeronave tripulada)
invalida a hipótese do comportamento linear do modelo dinâmico do sistema e
torna consideráveis os erros do fator de escala do IMU o que acrescenta os erros à
estimativa da perturbação da gravidade.
Desta forma, o atual sistema de baixo custo com o método de processamento
implementado não é adequado para gravimetria escalar principalmente devido ao
ruído nas medições dos giroscópios IMU Quanta.Mestrado em Engenharia Computaciona
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
Use of GPU for time series analysis
Get PDFThe Global Navigation Satellite System (GNSS) provides the position of stations with millimetre
accuracy. These positions are not constant over time due to motion of the tectonic plates on
which they are installed. The motion of tectonic plates is not the same all over the world,
leading stress to build up in some areas, thus increasing the occurrence of earthquakes. The
knowledge of such movements is of extreme importance, as is the determination of their uncertainty.
The tectonic motion is a very slow process that is constant over thousands of years. As a result,
the motion can be represented by a linear trend. Programs such as Hector can estimate linear
trends in time series with temporal correlated noise. Correlated noise means that GNSS observations
made today are similar to those of the days before, and it implies that one actually has
less information than when all observations were independent. This is the reason why the real
uncertainty of the estimated tectonic motion is between 5 to 11 times larger than when this
temporal correlation was not considered.
Unfortunately, taking this temporal correlation into account in the analysis slows down the computations
considerably. With the ever growing number of GNSS stations and the increasing length
of the existing time series, it is necessary to speed up these computations. This behaviour can
also be found in other geodetic time series such as sea level observed at tide gauges, and surface
temperature time series. For climate change studies it is important that besides estimating sea
level rise or an increase in temperature, also the associated uncertainties are realistic.
The Hector software package was developed with also the aim to reduce the computation time as
much as possible. In this thesis we will investigate if the use of a powerful Graphics Processing
Unit (GPU) can further reduce this computation time. So far, no Hector-like software uses
GPUs to perform data processing. This dissertation is therefore the first attempt to go in that
direction.Os Sistemas de Navegação por Satélite, (do inglês Global Navigation Satellite System - GNSS) fornecem a posição de estações com uma precisão milimétrica. Estas posições não são constantes ao longo do tempo devido ao movimento das placas tectónicas nas quais as estações se encontram instaladas. O movimento das placas tectónicas não é igual em todas as partes
do globo, o que leva à acumulação de stress em algumas áreas, aumentado assim a ocorrência
de movimentos sísmicos. O conhecimento de tais movimentos é de extrema importância, bem
como a determinação da sua incerteza.
O movimento tectónico é um processo muito lento e constante ao longo de milhares de anos.
Como resultado, o movimento pode ser representado por uma tendência linear. Programas
como o Hector podem estimar tendências lineares em séries temporais com ruído temporal
correlacionado. Ruído correlacionado significa que as observações dos Sistemas de Navegação
por Satélite feitas hoje são similares às dos dias anteriores, o que implica que haja menos
informação do que se as observações fossem independentes. Esta é a razão pela qual a incerteza
real do movimento tectónico estimado é entre 5 a 11 vezes maior do que quando esta correlação
temporal não é considerada.
Infelizmente, ter em consideração essa correlação temporal na análise aumenta consideravelmente
o tempo de computação. Com o crescente número de estações GNSS e o aumento da
duração das séries temporais existentes, é necessário acelerar essa computação. Este comportamento
também pode ser encontrado em outras séries temporais geodésicas, como as do nível
do mar observado em medidores de maré ou séries temporais da temperatura da superfície. Para
estudos das alterações climáticas, é importante que, para além de estimar a elevação do nível
do mar ou um aumento da temperatura, também as incertezas associadas sejam realísticas.
O conjunto de software Hector foi desenvolvido com o objetivo de reduzir o tempo de computação
tanto quanto possível. Nesta tese, investigaremos se o uso de uma potente Unidade
de Processamento Gráfico (do inglês Graphics Processing Unit), vulgarmente conhecida como
placa gráfica, pode reduzir ainda mais este tempo de computação. Até ao momento, nenhum
software semelhante ao Hector usa placas gráficas para realizar o processamento de dados. Esta
dissertação é, portanto, a primeira tentativa a ir nesse sentido
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