In global navigation satellite systems (GNSS) a fundamental operational component is the calculation of the orbits of the system spacecraft. This requires understanding and modelling the forces that act on the spacecraft. Solar radiation pressure (SRP) is the force caused by the impact of solar photons on the spacecraft surface. For GNSS spacecraft this is a significant force. If SRP is not included in the force model, then the calculated position of the spacecraft can be in error by between one and two hundred metres after one 12-hour orbit. SRP can be modelled using either analytical or empirical methods, or by some combination of the two. Historically, analytical SRP modelling has been somewhat neglected and high precision orbit estimation has relied upon empirical methods to account for SRP. Even so, most of these empirical methods start the estimation process with an a priori analytical model. The success of this empirical approach relies upon having many observations of the range between the system spacecraft and ground-based tracking stations, and works well within the context of the International Global Positioning System Service (IGS) network, which provides the necessary data volume. However, empirical methods do not work as well in operational GNSS, as these typically have a relatively small number of tracking stations. Moreover, empirical methods cannot be applied at the GNSS design stage, where knowledge of the system dynamics plays a key role. Existing methods for calculating analytical SRP models can only be used with relatively simple spacecraft structures, and lack flexibility as tools for analysis. In this study a new method is developed for calculating analytical SRP models that can cope with a high level of complexity in the spacecraft structure. The method is based upon simulating the solar photon flux with a pixel array. Using the method, models are calculated and tested for the Russian GLONASS IIv spacecraft. This particular spacecraft was used as the testbed because, at the time the study was being conducted, an international scientific campaign - called IGEX-98, the International GLONASS Experiment - was being carried out to analyse the Russian system. Developing force models for the spacecraft was one of the campaign goals, and the IGEX-98 steering committee accepted a proposal to use SRP models for GLONASS from this study. A detailed description is given of all the mathematics and physics that was used to develop the modelling technique. The method by which the models can be calculated and applied in practical orbit determination is also provided. In order to test the performance of the SRP models computed for the GLONASS spacecraft using the new method, comparisons were made between two kinds of trajectory. The first kind was calculated by numerical integration of the spacecraft's second order differential equation of motion, where this force model included the custom SRP models developed in the thesis. The second kind of trajectory, which is used as a 'truth' model in the study, was a precise orbit computed by the University of Berne using IGS range data and an empirical SRP model. Such precise orbits are the best estimates available of the true trajectories, as they are derived from the simultaneous estimation of multiple receiver tracking station network positions and spacecraft force model parameters. The repeatability of the Berne orbit is circa 0.75m. The RMS differences between the two trajectories over one twelve-hour orbit (an arc length of circa 160,000km) were 0.7m in height, 1.3m across track and 3.5m along track. This shows that the trajectory derived from the force model alone is very close to the precise orbit. The time-varying pattern of the differences between the two trajectories strongly indicates that the residual mismodelling of the forces acting on the spacecraft is due to thermal re-radiation effects. Further tests of the method were also conducted using satellite laser ranging (SLR) data to calculate arc lengths of 400 days, again using SRP models from the study. This enabled the calculation of model scale factors and additional empirical terms. The average SRP model scale factor was circa 1.01, which implies that the average error in the a priori SRP models calculated for the GLONASS IIv spacecraft is at the 1% level. This is consistent with an error budget based on an assessment of the accuracy of the source data supplied by the Russian authorities. The magnitude and parameterisation of the SLR empirical terms again strongly suggest that most of the remaining mis-modelling is caused by thermal effects. An analysis is given of the effect on the a priori SRP model of unmodelled, SRP-related forces acting along the spacecraft Y-axis. This is the so-called Y-bias. It is shown that whilst Y-bias effects are important in orbit determination, they are less critical in the process of calculating the a priori SRP model. A discussion is provided on how the new method can be adapted to improve the modelling and understanding of thermal re-radiation and Y-bias effects, and also on what benefits might accrue from such studies. The new method is an improvement over existing techniques as it enables the calculation of high precision SRP models that can be applied in the design, operation and scientific analysis of GNSS. A UK patent application has been made in respect of the new method.

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Dissertação (mestrado)—Universidade de Brasília, Instituto de Geociências, Programa de Pós-Graduação em Geociências Aplicadas, 2017. / Submitted by Raquel Almeida (raquel.df13@gmail.com) on 2017-11-20T16:13:23Z No. of bitstreams: 1 2017_DenisViniciusRicardodaSilva.pdf: 3726942 bytes, checksum: 9a7076dc55d9f42c5868d0539bdb17ec (MD5) / Approved for entry into archive by Raquel Viana (raquelviana@bce.unb.br) on 2018-01-04T21:46:28Z (GMT) No. of bitstreams: 1 2017_DenisViniciusRicardodaSilva.pdf: 3726942 bytes, checksum: 9a7076dc55d9f42c5868d0539bdb17ec (MD5) / Made available in DSpace on 2018-01-04T21:46:28Z (GMT). No. of bitstreams: 1 2017_DenisViniciusRicardodaSilva.pdf: 3726942 bytes, checksum: 9a7076dc55d9f42c5868d0539bdb17ec (MD5) Previous issue date: 2018-01-04 / O modelo geoidal é parte fundamental na transformação entre as altitudes ortométricas e geométricas. Existem aspectos positivos na sua utilização quando comparados a métodos clássicos de levantamento. O surgimento das técnicas de posicionamento por GNSS (Global Navigation Satellite System) impulsionou de maneira significativa diversas linhas de pesquisa, na busca de um modelo geoidal cada vez mais preciso. A disponibilidade de dados altimétricos, gravimetria terrestre e orbital também contribuíram neste sentido. Deste então, várias abordagens para a obtenção de um modelo geoidal tem sido apresentadas. Atualmente a integração de diferentes métodos se mostra uma alternativa promissora para o cálculo do geoide. Neste contexto, o emprego da técnica Remove-Calcula-Restaura (RCR) tem demonstrado resultados importantes no Brasil e em outras partes do mundo. A base de todas as formulações da técnica RCR envolve métodos gravimétrico e orbital, por isto, utiliza Modelos Digitais de Terreno (MDT), dados gravimétricos terrestres, Modelos do Geopotencial Global (MGG) e valor de densidade para o cálculo de modelos geoidais. Neste trabalho é apresentado um levantamento das diferentes formulações utilizadas no processo de redução gravimétrica. Também uma análise das principais variáveis que possam influenciar no cálculo das anomalias gravimétricas e na elaboração de modelos geoidais, a partir da técnica RCR. Para o cálculo, utilizou-se um pacote denominado GRAVTool, baseado no software MATLAB®. No final da pesquisa, tem-se também, como marco, a determinação de um modelo geoidal local para o Distrito Federal. / The geoidal model is a fundamental part of the transformation between orthometric and geometric heights. There are positive aspects in its use when compared to classical survey methods. The emergence of GNSS (Global Navigation Satellite System) positioning techniques has significantly boosted several lines of research in the search for an increasingly accurate geoidal model. The availability of altimetric data, terrestrial and orbital gravimetry also contributed in this sense. From this, several approaches to obtaining a geoid model have been presented. Currently the integration of different methods shows a promising alternative for the calculation of the geoid. In this context, the use of the Remove-Compute-Restore technique (RCR) has shown important results in Brazil and in other parts of the world. The basis of all RCR technique formulations is derived from gravimetric and orbital methods, using Digital Terrain Models (DTM), terrestrial gravimetric data, Global Geopotential Models (GGM) and density value for the calculation of geoid models. This work presents a revision of the different formulations used in the gravimetric reduction process. Also an analysis of the main variables that can influence the calculation of the gravimetric anomalies and the elaboration of geoid models from the RCR technique. For the calculation, a package called GRAVTool, based on the MATLAB® software, is used. At the end of the research, we also have as a landmark, the determination of a local geoidal model for the Brazilian Federal District.

The goal of this thesis is to improve the understanding of the performance of Global Navigation Satellite System (GNSS) receivers that use assistance data provided by cellular networks. A typical example of such a receiver is a mobile phone including a Global Positioning System (GPS) receiver. Using assistance data such as an accurate estimate of the GPS system time is known to improve the availability and the time-tofirst- fix performance of a GNSS receiver. However, the performance depends on the architecture of the cellular network and may vary significantly across networks. This thesis presents three new contributions to the performance analysis of assisted-GNSS receivers in cellular networks. I first introduce a mathematical framework that can be used to calculate a theoretical lower bound of the time-to-first-fix (TTFF) in an assisted-GNSS receiver. Existing methods, for example the flow-graph method, generally focus on calculating the theoretical mean acquisition time of a pseudo-noise signal for one satellite only. I extend these methods to calculate the full probability distribution of the joint acquisition of several satellites, as well as the sequential acquisition of satellites, which is commonly performed in assisted receivers. The method is applied to real measurements made in a multipath fading channel. I next consider time assistance in unsynchronised cellular networks. It is often argued that unsynchronised networks can not provide fine-time aiding since they do not have a common clock, although few experimental results have been reported in the existing literature. I carried out experiments on a GSM network, a second-generation cellular network, in Cambridge, UK, in order to measure the time stability of the synchronisation signals. The results showed a large variability in the time stabilities across different base stations and I evaluated the performance of an ensemble filter that combines the measurements into a single, more accurate, estimate of the universal time. The main contribution is to show that the performance of such a filter is adequate to provide fine-time assistance to a satellite navigation receiver. Finally, I address the positioning performance of an assisted receiver in synchronised cellular networks. Cellular positioning has been often investigated in the literature, but few results on real networks have been presented. Many positioning methods are proprietary and little information about their performance in real networks haven been published publicly. A CDMA2000 cellular network in Calgary, Canada, was used to collect experimental data. The time stability and the synchronisation of the CDMA2000 pilot signals were excellent and were used to evaluate the performance of CDMA2000-based cellular positioning system. I then developed a method to combine the pseudo-range measurements from the GPS signals and the CDMA2000 base stations. I evaluated the performance of positioning in both outdoor and indoor environments, and I analysed the effects and the possible mitigation of non-line-of-sight signals. The main contribution is to show that additional satellite navigation signals can improve the accuracy of cellular positioning beyond what is theoretically expected from the improvement in the geometry.

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This study used airborne gravity data, the 2008 Earth Gravity Model (EGM08) and Shuttle Radar Topographic Mission (SRTM) digital elevation data in a ‘Remove-Compute-Restore’ process to determine absolute vertical reference system for Ethiopia. This gives a geopotential height at any isolated field point where there is a Global Navigation Satellite System (GNSS) measurement without reference to a vertical network or a regional datum point. Previously, height was determined conventionally by connecting the desired field point physically to a nearby bench mark of a vertical network using co-located measurements of gravity and spirit levelling. With the use of precise GNSS positioning and a gravity model this method becomes obsolesce. The new approach uses the ‘Remove-Restore’ process to eliminate longer to shorter wavelengths from the measured gravity data using EGM08 and geometrical and condensed gravity models of the SRTM data. This provides small, smooth and localised residuals so that the interpolation and integration involved is reliable and the Stokes-like integral can be legitimately restricted to a spherical cap. A very fast, stable and accurate computational algorithm has been formulated by combining ‘hedgehog’ and ‘multipoint’ models in order to make tractable an unavoidably huge computational task required to remove the effects of about 1.5 billion! SRTM topographic mass elements representing Ethiopia and its immediate surroundings at 92433 point airborne gravity observations. The compute stage first uses an iterative Fast Fourier Transform (FFT) to predict residual gravity at aircraft height as a regular grid on to the surface of the ellipsoidal Earth and then it used a Fourier operation equivalent to Stokes’ integral to transform the localised gravity disturbance to residual potential. The restore process determines the geopotential number on or above the Earth’s surface where practitioners need it by restoring the potential effects of the removed masses. The accuracy of the geopotential number computed from gravity and topography was evaluated by comparing it with the one derived directly from EGM08 and precise geodetic levelling. The new model is in a good agreement across 100 km baseline with a standard deviation of 56 10−2 2 −2 × m s and 39 10−2 2 −2 × m s relative to EGM08 and levelling, respectively ( 10−2 2 −2 m s is approximately equivalent to 1mm of height). The new method provides an absolute geopotential height of a point on or above the Earth’s surface in a global sense by interpolating from geopotential models prepared as the digital grids carried in a chip for use with the GNSS receiver in the field.

This thesis argues for the use of what I term `poetic methods' in approaching the study of Global Navigation Satellite Systems (GNSS) infrastructures. Poetic methods frame research experiments with techniques drawn from art practice and build understandings of an infrastructure's actions, or the \texture" [1] of those actions by leveraging symbol and metaphor. This work is situated in an interdisciplinary space across Human Computer Interaction (HCI), art, design and the study of Science, Technology and Society (STS). The theoretical grounding of the work draws on Actor Network Theory (ANT) and Karen Barad's intra-activity [2] to emphasise sociotechnical practices and objects as emergent across combinations of material agency. From Bowker and Star's concept of infrastructural inversion [1], I argue that infrastructures become visible through points of breakdown. To probe GNSS infrastructure, I stage an experiment where its smooth operation is disrupted. Re-framing the infrastructure in this way draws participant's attention to its influence in forming practices. I then use a second method of short form speculative design [3] workshops to have participants think about scenarios where the user is pushed further into the background and user and infrastructure are understood as nodes in `more-than-human-networks'. Alongside this participant-facing research work, I give an account of artworks that developed from my own practice in response to the research questions. These are understood as deep responses to the design workshops' prompt to re-think how we understand the actions, influence and ontology of GNSS infrastructures. At the end of this art and research process I have a finished artwork and several sets of rich qualitative data. I use these to understand how effective my techniques are in achieving infrastructural inversion, diagnostically understanding the actions of GNSS infrastructure and the texture of how those actions are felt by participants, and interrogating ontological questions around concepts of `user' and `infrastructure'. I argue that the poetic methods offer an innovative set of techniques that can be added to a wider research project to help interrogate blackboxing, practically achieve infrastructural inversion and begin to move towards ontological critique. These offer a new methodological tool to STS researchers and contribute to HCI debates around non- user-centered design practices. I offer some suggestions for further refinements to these techniques and point towards some possible future work.

Made available in DSpace on 2014-06-11T19:30:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-11-03Bitstream added on 2014-06-13T19:00:44Z : No. of bitstreams: 1 dalpoz_wr_dr_prud.pdf: 7310354 bytes, checksum: 0dad0c578066121061e36552e4e9f136 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O erro devido à ionosfera nas observáveis GNSS (Global Navigation Satellite System) é diretamente proporcional à densidade de elétrons presente na ionosfera e inversamente proporcional a frequência do sinal. Da mesma forma que no posicionamento por ponto, os resultados obtidos no posicionamento relativo são afetados pelo efeito sistemático da ionosfera, que é uma das maiores fontes de erro no posicionamento com GNSS. Mesmo considerando que parte dos erros devido à ionosfera é cancelada na dupla diferenciação, a ionosfera pode causar fortes impactos no posicionamento relativo. O problema principal neste método de posicionamento é a variação espacial na densidade de elétrons, que pode ocorrer em função de vários fatores, tais como hora local, variação sazonal, localização do usuário, ciclo solar e atividade geomagnética. Dependendo das condições do clima espacial, que é controlado pelo Sol, a atividade geomagnética pode ser alterada de forma significativa, dando origem a uma tempestade geomagnética. Nesta pesquisa foram avaliados os efeitos da ionosfera no posicionamento relativo, com observações GNSS da fase da onda portadora (L1), nas regiões ionosféricas de latitude média e alta e na região equatorial. Nas duas primeiras regiões foram analisados os efeitos da ionosfera em períodos de irregularidades, decorrentes de tempestades geomagnéticas. Na região equatorial, que engloba o Brasil, foram analisados os efeitos da ionosfera em função da variação diária e sazonal. No processamento dos dados GNSS foi utilizado o GPSeq, que processa os dados na forma recursiva e fornece os Resíduos Preditos da Dupla Diferença da Fase (RPDDF)... / The error caused by ionosphere on GNSS (Global Navigation Satellite System) is directly proportional to the density of electrons from ionosphere and inversely proportional to the frequency squared of the signal GNSS. As in the case of point positioning, results in relative positioning are affected by systematic effect from ionosphere, which is one of major error sources in the GNSS positioning. Although some errors caused by ionosphere are canceled in double difference, strong impacts may be caused by ionosphere on the relative positioning. In this positioning the main problem is the spatial variation in electron density that can occur due local time, seasonal variation, user location, solar cycle, geomagnetic activity, etc. Depending on the conditions of space weather, in which is controlled by the Sun, the geomagnetic activity can be changed inducing geomagnetic storms. In this research the effects from ionosphere has been evaluated in GNSS relative positioning using L1 carrier phase observations, at the three regions of the ionosphere: middle and high latitudes and equatorial region. In regions of middle and high latitudes have been analyzed the effects from ionosphere in irregularities periods, caused by geomagnetic storms. In the equatorial region, including Brazil, have been analyzed the effects from ionosphere according daily and seasonal variation. In the processing GNSS data has been used GPSeq software. This software processes the data in a recursive form and provides the Predicted Residual of Carrier Phase Double Difference (PRCPDD) ... (Complete abstract click electronic access below)

The aim of the study was to investigate how users experience two different global navigation systems of an e-commerce site in the home furnishing industry. Furthermore, the purpose of the study was to identify which aspects, in a global navigation system, affect the usability of an e-commerce website in the home furnishing industry and investigate what, in connection with a global navigation system, affects the usability. A user centered method called Contextual Inquiry was adopted to define users’ experiences, needs and expectations on global navigation systems in the home furnishing industry. Five participants participated and studied two e-commerce sites: H&M Home and Ellos Home. Furthermore, a content analysis was used to analyze the empirical material. Based on the participants' experiences, needs and expectations concerning/regarding the global navigation systems four themes emerged: to have a distinct search function and an easily accessible shopping cart, a global navigation should be easy to navigate, scrolling is experienced less efficiently, descriptive labels increase efficiency and visibility. The results indicate that the horizontal global navigation menu was considered more useful than the vertical navigation menu, according to the participants, mainly because the drop-down menu made it easier to overview the website's offerings. The vertical menu was considered to have poorer positioning and the biggest problem was that the participants had to scroll to take part of the information. Further on the results also indicate that clear labels, search box and drop-down menu affect the usability.