Dissertations / Theses on the topic 'Ionosphere system'

Magnetized planets, such as Earth, are strongly influenced by the solar wind. The Sun is very dynamic, releasing varying amounts of energy, resulting in a fluctuating energy and momentum exchange between the solar wind and planetary magnetospheres. The efficiency of this coupling is thought to be controlled by magnetic reconnection occurring at the boundary between solar wind and planetary magnetic fields. One of the main tasks in space physics research is to increase the understanding of this coupling between the Sun and other solar system bodies. Perhaps the most important aspect regards the transfer of energy from the solar wind to the terrestrial magnetosphere as this is the main source for driving plasma processes in the magnetosphere-ionosphere system. This may also have a direct practical influence on our life here on Earth as it is responsible for Space Weather effects. In this thesis I investigate both the global scale of the varying solar-terrestrial coupling and local phenomena in more detail. I use mainly the European Space Agency Cluster mission which provide unprecedented three-dimensional observations via its formation of four identical spacecraft. The Cluster data are complimented with observations from a broad range of instruments both onboard spacecraft and from groundbased magnetometers and radars.

A period of very strong solar driving in late October 2003 is investigated. We show that some of the strongest substorms in the history of magnetic recordings were triggered by pressure pulses impacting a quasi-stable magnetosphere. We make for the first time direct estimates of the local energy flow into the magnetotail using Cluster measurements. Observational estimates suggest a good energy balance between the magnetosphere-ionosphere system while empirical proxies seem to suffer from over/under estimations during such extreme conditions.

Another period of extreme interplanetary conditions give rise to accelerated flows along the magnetopause which could account for an enhanced energy coupling between the solar wind and the magnetosphere. We discuss whether such conditions could explain the simultaneous observation of a large auroral spiral across the polar cap.

Contrary to extreme conditions the energy conversion across the dayside magnetopause has been estimated during an extended period of steady interplanetary conditions. A new method to determine the rate at which reconnection occurs is described that utilizes the magnitude of the local energy conversion from Cluster. The observations show a varying reconnection rate which support the previous interpretation that reconnection is continuous but its rate is modulated.

Finally, we compare local energy estimates from Cluster with a global magnetohydrodynamic simulation. The results show that the observations are reliably reproduced by the model and may be used to validate and scale global magnetohydrodynamic models.

The thesis investigates Ultra Low Frequency waves in the band 0.1 Hz to 5 Hz in the terrestrial magnetosphere-ionosphere system. Utilising mid-high latitude ground-based induction coil magnetometers, continuous (Pc1-2) and irregular (Pi1-c) pulsations are explored through the application of digital spectral analysis. An assessment of two spectral analysis techniques is conducted. From which it is concluded that, for routine ground-based analysis of Pc1-2 pulsations, treating the horizontal components of magnetic field variation as a single complex signal is computationally beneficial with minimal loss of useful information. Polarisation parameters and values of cross spectral phase are derived using a weighted histogram technique and are subsequently used to distinguish discrete pulsations and infer their location through simple triangulation. The results of a statistical study of ~1200 discrete Pc1-2 events over the full year of 2007, during the declining phase of solar cycle 23, are presented. This study, for the first time, reports the ground-based polarisation properties of Pc1-2 waves as a function of latitude. The derived diurnal frequency behaviour supports the suggestion that the Ionospheric Alfvén Resonator may play a part in the filtration of ground-based Pc1 observations. Pc1-2 behaviour over the course of 26 geomagnetic storms is also presented, with support being found for the association of pulsation enhancement with plasmaspheric plume formation in the recovery phase. A case study, combining coherent and incoherent radar, in situ particle measurements and ground based magnetometry, has focused on high latitude Pi-c activity during a period of enhanced dayside reconnection. This study has provided support for the association of Electromagnetic Ion cyclotron waves with the SuperDARN spectral width enhancements observed in the flanks of the ionospheric cusp.

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Kyoto University (京都大学)
0048
新制・課程博士
博士(理学)
甲第8164号
理博第2186号
新制||理||1156(附属図書館)
UT51-2000-F68
京都大学大学院理学研究科地球惑星科学専攻
(主査)教授 藤田 茂, 教授 荒木 徹, 助教授 町田 忍
学位規則第4条第1項該当

International Telemetering Conference Proceedings / October 26-29, 1992 / Town and Country Hotel and Convention Center, San Diego, California
The ionosphere is a critical link in the earth's environment for space-based navigation, communications and surveillance systems. Signals sent down by the GPS satellites can provide an excellent means of studying the complex physical and chemical processes that take place there. GPS uses two frequencies to ascertain signal delays passing through the ionosphere. These are measured as errors and used to correct position solutions. Since this process is a means of measuring columns of Total Electron Content (TEC), multiple top-soundings from the GPS constellation could provide significant detail of the ionospheric pattern and possibly lead to enhancement of predictions for selectable areas and sites. This paper addresses transforming the GPS propagation delays (errors) into TEC and providing TEC contours on a PC-style workstation in real and integrated time and discusses a worldwide ionospheric network monitoring system.

Ionospheric phase delays of radio signals from Global Positioning System (GPS) satellites have been used to compute ionospheric Total Electron Content (TEC). An extended Chapman profle model is used to estimate the electron density profles and TEC. The Chapman profle that can be used to predict TEC over the mid-latitudes only applies during day time. To model night time TEC variability, a polynomial function is fitted to the night time peak electron density profles derived from the online International Reference Ionosphere (IRI) 2001. The observed and predicted TEC and its variability have been used to study ionospheric in°uence on Radio Astronomy in South Africa region. Di®erential phase delays of the radio signals from Radio Astronomy sources have been simulated using TEC. Using the simulated phase delays, the azimuth and declination o®sets of the radio sources have been estimated. Results indicate that, pointing errors of the order of miliarcseconds (mas) are likely if the ionospheric phase delays are not corrected for. These delays are not uniform and vary over a broad spectrum of timescales. This implies that fast frequency (referencing) switching, closure phases and fringe ¯tting schemes for ionospheric correction in astrometry are not the best option as they do not capture the real state of the ionosphere especially if the switching time is greater than the ionospheric TEC variability. However, advantage can be taken of the GPS satellite data available at intervals of a second from the GPS receiver network in South Africa to derive parameters which could be used to correct for the ionospheric delays. Furthermore GPS data can also be used to monitor the occurrence of scintillations, (which might corrupt radio signals) especially for the proposed, Square Kilometer Array (SKA) stations closer to the equatorial belt during magnetic storms and sub-storms. A 10 minute snapshot of GPS data recorded with the Hermanus [34:420 S, 19:220 E ] dual frequency receiver on 2003-04-11 did not show the occurrence of scintillations. This time scale is however too short and cannot be representative. Longer time scales; hours, days, seasons are needed to monitor the occurrence of scintillations.

Global Positioning System (GPS) networks provide an opportunity to study the dynamics and continuous changes in the ionosphere by supplementing ionospheric measurements which are usually obtained by various techniques such as ionosondes, incoherent scatter radars and satellites. Total electron content (TEC) is one of the physical quantities that can be derived from GPS data, and provides an indication of ionospheric variability. This thesis presents a feasibility study for the development of a Neural Network (NN) based model for the prediction of South African GPS derived TEC. The South African GPS receiver network is operated and maintained by the Chief Directorate Surveys and Mapping (CDSM) in Cape Town, South Africa. Three South African locations were identified and used in the development of an input space and NN architecture for the model. The input space includes the day number (seasonal variation), hour (diurnal variation), sunspot number (measure of the solar activity), and magnetic index(measure of the magnetic activity). An attempt to study the effects of solar wind on TEC variability was carried out using the Advanced Composition Explorer (ACE) data and it is recommended that more study be done using low altitude satellite data. An analysis was done by comparing predicted NN TEC with TEC values from the IRI2001 version of the International Reference Ionosphere (IRI), validating GPS TEC with ionosonde TEC (ITEC) and assessing the performance of the NN model during equinoxes and solstices. Results show that NNs predict GPS TEC more accurately than the IRI at South African GPS locations, but that more good quality GPS data is required before a truly representative empirical GPS TEC model can be released.

The Global Positioning System (GPS) has been widely used for precise positioning applications throughout the world. However, there are still some limiting factors that affect the performance of satellite-based positioning techniques, including the ionosphere. The GPS Network-RTK (NRTK) concept has been developed in an attempt to remove the ionospheric bias from user observations within the network. This technique involves the establishment of a series of GNSS reference stations, spread over a wide geographical region. Real time data from each reference station is collected and transferred to a computing facility where the various spatial and temporal errors affecting the GNSS satellite observations are estimated. These corrections are then transmitted to users observations in the field. As part of a Victorian state government initiative to implement a cm-level real time positioning service state-wide, GPSnet is undergoing extensive infrastructure upgrades to meet high user demand. Due to the sparse (+100km) configuration of GPSnet's reference stations, the precise modelling of Victoria's ionosphere will play a key role in providing this service. This thesis aims is to develop a temporal model for the ionospheric bias within a Victorian NRTK scenario. This research has analysed the temporal variability of the ionosphere over Victoria. It is important to quantify the variability of the ionosphere as it is essential that NRTK corrections are delivered sufficiently often with a small enough latency so that they adequately model variations in the ionospheric bias. This will promote the efficient transmission of correctional data to the rover whilst still achieving cm-level accuracy. Temporal analysis of the ionosphere revealed that, during stable ionospheric conditions, Victoria's double differenced ionospheric (DDI) bias remains correlated to within +5cm out to approximately two minutes over baselines of approximately 100km. However, the data revealed that during more disturbed ionospheric conditions this may decrease to one minute. As a preliminary investigation, four global empirical ionospheric models were tested to assess their ability to estimate the DDI bias. Further, three temporal predictive modelling schemes were tested to assess their suitability for providing ionospheric corrections in a NRTK environment. The analysis took place over four seasonal periods during the previous solar maximum in 2001 and 2002. It was found that due to the global nature of their coefficients, the four global empirical models were unable to provide ionospheric corrections to a level sufficient for precise ambiguity resolution within a NRTK environment. Three temporal ionospheric predictive schemes were developed and tested. These included a moving average model, a linear model and an ARIMA (Auto-Regressive Integrated Moving Average) time series analysis. The moving average and ARIMA approaches gave similar performance and out-performed the linear modelling scheme. Both of these approaches were able to predict the DDI to +5cm within a 99% confidence interval, out to an average of approximately two minutes, on average 90% of the time when compared to the actual decorrelation rates of the ionosphere. These results suggest that the moving average scheme, could enhance the implementation of next generation NRTK systems by predicting the DDI bias to latencies that would enable cm-level positioning.

"Author biographies are available in the expanded on-line version of this article [http://www.insidegnss.com/auto/julyaug09-tetewsky-final.pdf]"
"July/August 2009." Web site title: Making Sense of GPS Inter-Signal Corrections : Satellite Calibration Parameters in Legacy and Modernized Ionosphere Correction Algorithms.

COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
A ionosfera de baixas latitudes tem características que poderiam causar problemas à operação do GPS/SBAS. Entre elas se encontra a anomalia equatorial, cuja densidade eletrônica pode apresentar intensos gradientes horizontais (e, portanto, no índice de refração do meio). Estes gradientes podem ser intensos o suficiente para introduzir erros nas previsões resultantes do GPS/SBAS. Para avaliar este problema, foi desenvolvido um programa de simulação em computador que integra modelos para: (i) a previsão das posições dos satélites da constelação GPS; (ii) a evolução temporal e espacial da densidade eletrônica da ionosfera equatorial; e (iii) uma rede de estações de referência de posições fornecidas para analisar os efeitos da anomalia equatorial sobre os erros causados pela ionosfera nos sinais dos satélites GPS recebidos pelas estações. Em cada passo da simulação, diversos procedimentos são realizados. Estes procedimentos são repetidos um grande número de vezes e, ao final da simulação, estatísticas dos erros são apresentadas. Este programa de simulação em computador foi utilizado para analisar a influência do número de estações de referência, assim como de suas localizações, nos erros de posicionamento de aeronaves.
The low-latitude ionosphere has some features that could cause problems even to the joint GPS/SBAS operation. Among them, one finds the equatorial anomaly, whose electronic density - and thus its refractive index - can present intense horizontal gradients. These gradients can be intense enough to induce errors in the predictions by the GPS/SBAS. To analyze this problem, a computer simulation program has been developed. This program integrates models for: (i) forecasting the satellite orbital positions of the GPS constellation; (ii) the temporal and spatial evolution of the electronic density of the low-latitude ionosphere; and (iii) a given network of reference stations to analyze the effects of the equatorial anomaly on the GPS satellite signals received by the stations and users. In each step of the simulation, several procedures are performed. These procedures are repeated several times and, at the end of the simulation, error statistics are presented. This computer simulation program has been used to analyze the influence of the equatorial anomaly and of the number and layout of reference stations upon the errors in aircraft positions provided by the GPS/SBAS.

Global Navigation Satellite Systems can provide position, velocity and time information to users using receiver hardware. The United States developed Global Positioning System (GPS) is the only current fully operational system; however further systems are in development. The GPS has shown considerable success for navigation, but it still has a number of problems that limit its accuracy. The two main problems are the ionosphere and local environment of the receiver. The ionosphere causes a delay and random rapid shifts in phase and amplitude (scintillation) to the signal. The local environment can provide the signal with multiple routes (multi-path) to the receiver. In this project a GPS signal simulator is developed, which models the effects of the ionosphere and multi-path on the modulated signals. The focus is made on the GPS system as the simulator measurements can be compared to the real measurements; however other systems will be considered in the future. A number of experiments investigating multi-path and ionospheric effects on a receiver’s ability to track the signals have been completed. The simulator has been used to replicate a real local multi-path environment and the results have been compared. Further investigations of the multi-path have shown a unique multi-path signature in the receiver power output. The later part of the thesis describes a case study investigating a short but rapid period of scintillation observed on three receivers based in Norway. An analysis of the multi-path environment was completed, but was found not to be the cause. The ionosphere was investigated using equipment based across Scandinavia. The equipment showed that geomagnetic conditions were disturbed at the time of the event. The GPS measurements were compared with all-sky camera data to show that the scintillation can be attributed to the GPS signal path crossing electron density structures associated with the aurora.

Electron density irregularities influence Global Navigation Satellite System (GNSS) signals, manifesting as ionospheric scintillation. Scintillation poses a service risk to safety-critical GNSS applications at high latitudes. It is difficult to predict, as ionospheric instability processes are not yet fully characterised. This research combines the fields of ionospheric imaging and scintillation monitoring, to investigate the causes of scintillation in the Antarctic and Arctic. Results revealed a plasma patch structure above Antarctica, in response to the impact of a solar wind shock front. Measurements from a network of Global Positioning System scintillation receivers across the continent revealed moderate levels of phase scintillation associated with Total Electron Content (TEC) gradients at the patch break-off point. Scintillation was also driven by solar particle precipitation at E and F region altitudes, verified with in situ spectrometers on polar-orbiting satellites. The current receiver coverage in the region provided the Multi-Instrument Data Analysis Software (MIDAS) tomography tool with sufficient data to track the lifetime of the plasma patch without a convection model. A second experiment was performed at the South Pole, using a collocated GPS scintillation receiver and auroral imager. This allowed simultaneous line-of-sight tracking of GPS signals through the optical auroral emissions. Results showed the first statistical evidence that auroral emissions can be used a proxy for ionospheric irregularities causing GPS scintillation. The relationship was strongest during the presence of discrete auroral arcs. Correlation levels of up to 74% were found over periods of 2-3 hours. The use of multiple emission wavelengths provided basic altitude discrimination. Current capability of ionospheric TEC mapping in the Arctic was tested, where GPS receiver distribution is extensive compared to present Antarctic coverage. Analysis of the ionosphere’s response to a storm event revealed a sequential picture of polar cap patch activity, without the aid of plasma convection modelling. The electron density enhancements of the auroral oval were imaged in completeness for the first time using GPS tomography. Reconstructions were verified using ultraviolet auroral imagery from polar-orbit satellites, and vertical profiles from an incoherent scatter radar.

An ionospheric topside sounder is a high frequency radar system that is located above the ionosphere, ideally on-board a polar orbiting satellite to provide global coverage. The previous eight satellite sounders have measured the critical frequency of the F2 ionosphere region using traditional swept frequency methods. The most expensive part of these missions however is considered to be the large network of ground support stations required for collecting and processing data. This information has been invaluable in improving our global understanding of the upper ionosphere and the accuracy of critical frequency maps used by HF radio engineers to calculate communications routes and the optimum frequencies for early warning OTH radars. A new technique for the direct detection of critical frequency has been developed, which is called the 'Dispersion Method'. Real data from previous sounders is used in the development and verification of this method. This sounder will not only provide traditional lonograms but detects critical frequency and spread echoes directly from the dispersion of a returning radar pulse. This new method does not use traditional lonograms with their inherent processing complexity and is an order faster than any previous sounder. The 'Dispersion Method' therefore resolves the problems encountered with the past topside sounder missions and produces large quantities of real time data autonomously when required. Previous sounding satellites had little memory capacity, no on-board processing capability, required large antennas and transmitters on satellites with a mass of between 150 and 250 kg. This meant power requirements of about 60 watts per orbit average. A feasibility study to place a third generation topside sounder into low Earth orbit on a 50 kg microsatellite with an orbit average power capacity of only 20 watts has been successfully completed.

This thesis examines the behaviour of large scale magnetohydrodynamic ultra low frequency (ULF) waves in the coupled magnetosphere-ionosphere system. Wave energy from solar wind driven disturbances at the magnetopause, is carried through the magnetosphere by compressional, fast mode waves, which couple to field guided Alfven mode waves on geomagnetic field lines. Field lines with eigenfrequencies corresponding to the driving frequency become resonant. Energy is dissipated in the ionosphere where partial reflection of the Alfven waves takes place, via Joule heating. The general aim of work presented in this thesis is to combine ground based measurements of the large scale structure of individual ULF waves with in-situ measurements of the small scale structure of the electric field, magnetic field and particle precipitation, made by the Fast Auroral SnapshoT (FAST) satellite. With a more specific aim to investigate the small scale structures which give rise to small regions of high current density, leading to parallel electric fields, particle acceleration and aurora. Details of the mechanisms which result in particle acceleration are not fully understood and are of considerable interest at present.;Four field line resonances (FLRs) with conjugate radar, magnetometer and FAST Satellite observations have been studied and compared. In each case a simple FLR model was created and scaled using the wave's spatial and temporal characteristics inferred from SuperDARN radar and ground magnetometer observations. The model field aligned current is compared with field aligned currents derived from the FAST energetic particle spectra and magnetic field measurements. In all four events downward currents appear to be carried, partially by upgoing electrons below the FAST energy detection threshold (5 eV), but also consist of a mixture of hotter downgoing magnetospheric electrons and upgoing ionospheric electrons of energies 30 eV -- 1 keV. In two of the events downgoing magnetospheric electrons with energies of a few keV, which are associated with upward field aligned currents of ~ 1 microA m --2, are observed. Strong intervals of upward current show that small-scale structuring of ~50 km has been imposed on the current carriers, which is thought to be associated with a mode conversion of an ideal magnetohydrodynamic (MHD) Alfven wave to an inertial Alfven wave.

University of Central Florida College of Engineering Thesis
High Frequency (HF) communication has been shown to be a useful communication technique from the very beginning of World War I and it accelerated during World War 11. This is attributed to its simplicity, ability to provide near globe connectivity at low power without repeaters, moderate cost, and ease of proliferation [I]. In fact, the HF communication system utilizes the ionosphere [2][3][4] to refract the skywave signals to a distant receiver. This ionospheric channel has some disadvantages. First, it is a non-stationary channel as the HF frequency propagation is a function of the sun spot activities, solar winds, and diurnal variations of the ionization level [5]. Second, the channel produces distortion in both signal amplitude and phase. As the different ionospheric layers move up or down, independent Doppler shifts on each propagation mode are introduced. Multipath fading [6] caused by multiple refractions of the signal fiom the ionosphere with or without ground reflection causes performance degradation in the HF system. Some techniques have been developed to improve HF performance [I]. One example is Space-Diversity [7], which uses more than one antenna at distant spaces to combine the received signal. Angle-of-Arrival Diversity that takes advantage of the fact that different modes have different arrival angles at the receiver, and so, highly directional antenna for example, can be used to improve the system performance. Another method of improving HF performance is to use different frequencies to transmit and receive messages. This method is known as Frequency diversity. Using timediversity, one can add a degree of redundancy to the transmitted message through the use of different types of coding, interleaving, etc. In the military standard, MIL-STD- 1 88- 1 1 OA [8], a convolutional encoder [9][10] followed by interleaver [Ill-[14] was used to scramble and transmit the data in different bit rates. In the presence of multipath fading [ 1 51, a training sequence is transmitted in an interleaved fashion with the data symbols with a 50% duty cycle. This has the disadvantage of losing half the bandwidth. At present, the recent advances of the Digital Signal Processing (DSP) [16][17] make it possible to reduce the bit-error-rate BEY and increase the transmission bit rate [18] through the usage of adaptive equalization [ 191-[2 11 which will be the focus of this dissertation. Equalizers such as, Transversal Equalizer [ 1 61, Blind Equalizer [22], Training waveform Equalizer [23], and Minimum Mean Square Error (MMSE) [20] Adaptive Equalizer have been applied into various communication systems. This proposal work will be to initially apply some of the previous developed equalizer to the HF channel specifically. Thereafter, new adaptive channel equalization [24],[25] will be developed to compensate for transmission channel impairments due to bandwidth limitations, multipath propagation, and rayleigh fading [2 11 conditions in mobile environments. A new technique for frequency offset prediction has been developed and finally, a new approach for MIL-STD- 1 88- 1 1 0A high frequency single-tone modem employing orthogonal Walsh-PN codes has been implemented.
Ph.D.
Doctorate;
Department of Electrical and Computer Engineering
Engineering and Computer Science
Electrical Engineering and Computer Science
198 p.
xviii, 198 leaves, bound : ill., (some col.) ; 28 cm.

Radio signals transmitted by GPS satellites orbiting the Earth are modulated as they propagate through the electrically charged plasmasphere and ionosphere in the near-Earth space environment. Through a linear combination of GPS range and phase measurements observed on two carrier frequencies by terrestrial-based GPS receivers, the ionospheric total electron content (TEC) along oblique GPS signal paths may be quantified. Simultaneous observations of signals transmitted by multiple GPS satellites and observed from a network of South African dual frequency GPS receivers, constitute a spatially dense ionospheric measurement source over the region. A new methodology, based on an adjusted spherical harmonic (ASHA) expansion, was developed to estimate diurnal vertical TEC over the region using GPS observations over the region. The performance of the ASHA methodology to estimate diurnal TEC and satellite and receiver differential clock biases (DCBs) for a single GPS receiver was first tested with simulation data and subsequently applied to observed GPS data. The resulting diurnal TEC profiles estimated from GPS observations compared favourably to measurements from three South African ionosondes and two other GPS-based methodologies for 2006 solstice and equinox dates. The ASHA methodology was applied to calculating diurnal two-dimensional TEC maps from multiple receivers in the South African GPS network. The space physics application of the newly developed methodology was demonstrated by investigating the ionosphere’s behaviour during a severe geomagnetic storm and investigating the long-term ionospheric stability in support of the proposed Square Kilometre Array (SKA) radio astronomy project. The feasibility of employing the newly developed technique in an operational near real-time system for estimating and dissimenating TEC values over Southern Africa using observations from a regional GPS receiver network, was investigated.

The Global Positioning System (GPS) has been used for more than a decade for the accurate determination of position on the earth's surface, as well as navigation. The system consists of approximately thirty satellites, managed by the US Department of Defense, orbiting at an altitude of 20 200 kilometres, as well as thousands of stationary ground-based and mobile receivers. It has become apparent from numerous studies that the delay of GPS signals in the atmosphere can also be used to study the amosphere, particularly to determine the precipitable water vapour (PWV) content of the troposphere and the total electron content (TEC) of the ionosphere. This dissertation gives an overview of the mechanisms that contribute to the delay of radio signals between satellites and receivers. The dissertation then focuses on software developed at the Hartebeesthoek Radio Astronomy Observatory's (HartRAO's) Space Geodesy Programme to estimate tropospheric delays (from which PWV is calculated) in near real-time. In addition an application of this technique, namely the improvement of tropospheric delay models used to process satellite laser ranging (SLR) data, is investigated. The dissertation concludes with a discussion of opportunities for future work.
Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2004.

In this thesis an ionospheric tomographic algorithm called Multi-Instrument Data Anal- ysis System (MIDAS) is used to reconstruct electron density profiles using the Global Positioning System (GPS) data recorded from 53 GPS receivers over the South African region. MIDAS, developed by the Invert group at the University of Bath in the UK, is an inversion algorithm that produces a time dependent 3D image of the electron density of the ionosphere. GPS receivers record the time delay and phase advance of the trans- ionospheric GPS signals that traverse through the ionosphere from which the ionospheric parameter called Total Electron Content (TEC) can be computed. TEC, the line integral of the electron density along the satellite-receiver signal path, is ingested by ionospheric tomographic algorithms such as MIDAS to produce a time dependent 3D electron density profile. In order to validate electron density profiles from MIDAS, MIDAS derived NmF2 values were compared with ionosonde derived NmF2 values extracted from their respective 1D electron density profiles at 15 minute intervals for all four South African ionosonde stations (Grahamstown, Hermanus, Louisvale, and Madimbo). MIDAS 2D images of the electron density showed good diurnal and seasonal patterns; where a comparison of the 2D images at 12h00 UT for all the validation days exhibited maximum electron concentration during the autumn and summer and a minimum during the winter. A root mean square error (rmse) value as small as 0.88x 10¹¹[el=m³] was calculated for the Louisvale ionosonde station during the winter season and a maximum rmse value of 1.92x 10¹¹[el=m³] was ob- tained during the autumn season. The r² values were the least during the autumn and relatively large during summer and winter; similarly the rmse values were found to be a maximum during the autumn and a minimum during the winter indicating that MIDAS performs better during the winter than during the autumn and spring seasons. It is also observed that MIDAS performs better at Louisvale and Madimbo than at Grahamstown and Hermanus. In conclusion, the MIDAS reconstruction has showed good agreement with the ionosonde measurements; therefore, MIDAS can be considered a useful tool to study the ionosphere over the South African region.

Global Positioning System (GPS) satellites and receivers are used to derive total electron content (TEC) from the time delay and phase advance of the radiowaves as they travels through the ionosphere. TEC is defined as the integralof the electron density along the satellite-receiver signal path. Electron densityprofiles can be determined from these TEC values using ionospheric tomographic inversion techniques such as Multi-Instrument Data Analysis System (MIDAS).This thesis reports on a study aimed at evaluating the suitability of ionospheric tomography as a tool to derive one-dimensional electron density profiles, using the MIDAS inversion algorithm over Grahamstown, South Africa (33.30◦S, 26.50◦E). The evaluation was done by using ionosonde data from the Louisvale (28.50◦S, 21.20◦E) and Madimbo (22.40◦S, 30.90◦E) stations to create empirical orthonormal functions (EOFs). These EOFs were used by MIDAS in the inversion process to describe the vertical variation of the electron density. Profiles derived from the MIDAS algorithm were compared with profiles obtained from the international Reference Ionosphere (IRI) 2001 model and with ionosonde profiles from the Grahamstown ionosonde station. The optimised MIDAS profiles show a good agreement with the Grahamstown ionosonde profiles. The South African Bottomside Ionospheric Model (SABIM) was used to set the limits within which MIDAS was producing accurate peak electron density (NmF2) values and to define accuracy in this project, with the understanding that the national model (SABIM) is currently the best model for the Grahamstown region. Analysis show that MIDAS produces accurate results during the winter season, which had the lowest root mean square (rms) error of 0.37×1011[e/m3] and an approximately 86% chance of producing NmF2 closer to the actual NmF2 value than the national model SABIM. MIDAS was found to also produce accurate NmF2 values at 12h00 UT, where an approximately 88% chance of producing an accurate NmF2 value, which may deviate from the measured value by 0.72×1011[e/m3], was determined. In conclusion, ionospheric tomographic inversion techniques show promise in the reconstruction of electron density profiles over South Africa, and are worth pursuing further in the future.

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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)

Orientador: Paulo de Oliveira Camargo
Banca: João Francisco Galera Monico
Banca: Edvaldo Simões da Fonseca Junior
Banca: Cláudia Pereira Krueger
Banca: Moisés Ferreira Costa
Resumo: 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) ... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: 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)
Doutor

The High Frequency (HF) band, 3-30 MHz, is used when no infrastructure for long-range communications is available. New technology, such as digital signal processing enables higher data rate in the HF band, which in 2000s has resulted in increased commercial use. Reflection of radio waves in the ionosphere allows for beyond horizon communication, and are a unique property of the HF band. However, properties of the ionosphere are highly dependent of radiation from the sun, which varies with geographical location, season and time. The use of unmanned areal vehicle (UAV) has increased during the past years. In this project it is investigated if a HF transmitter can be placed on a small UAV platform. The objective is to get an estimation of the probabilities for successful HF transfer of real-time data from a small UAV. For example, the data could be sensor- or position data. When studying a complex problem having several parameters, such as a HF communication system, it is necessary to use the systems approach. This report illustrates the impact of size of the transmitting antenna, transmitter output power and bandwidth as well as different sources of noise and its levels. The results and analysis, made in this project, shows that there are feasible solutions for every tested case except at very high latitudes. Frequency planning, that is finding the less occupied channel, is almost as important as maximizing the signal to noise ratio. This project has been carried out on behalf of ÅF Technology in Solna, Sweden.

The genetic algorithm (GA) is a popular random search and optimization method inspired by the concepts of crossover, random mutation, and natural selection from evolutionary biology. The real-valued genetic algorithm (RGA) is an improved version of the genetic algorithm designed for direct operation on real-valued variables. In this work, a modified version of a genetic algorithm is introduced, which is called a modified genetic algorithm with micro-movement (MGAM). It implements a particle swarm optimization(PSO)-inspired micro-movement phase that helps to improve the convergence rate, while employing the e'cient GA mechanism for maintaining population diversity. In order to test the capability of the MGAM, we firrst implement it on five generally used test functions. Then we test the MGAM on two typical nonlinear dynamical systems. The performance of the MGAM is compared to a basic RGA on all these applications. Finally, we implement the MGAM on the most important application, which is the plasma physics-based model of the solar wind-driven magnetosphere-ionosphere system (WINDMI). In order to use this model for real-time prediction of geomagnetic activity, the model parameters require up-dating every 6-8 hours. We use the MGAM to train the parameters of the model in order to achieve the lowest mean square error (MSE) against the measured auroral electrojet (AL) and Dst indices. The performance of the MGAM is compared to the RGA on historical geomagnetic storm datasets. While the MGAM performs substantially better than the RGA when evaluating standard test functions, the improvement is about 6-12 percent when used on the 20D nonlinear dynamical WINDMI model.

Thesis (M.S. in Systems Engineeering (Electronic Warfare))--Naval Postgraduate School, December 1990.
Thesis Advisor(s): Adler, Richard W. Second Reader: Jauregui, Stephen. "December 1990." Description based on title screen as viewed on March 30, 2010. DTIC Descriptor(s): Ionosphere, Parameters, Electron Density, Ionospheric Disturbances, Theses, Estimates, Sampling, Value, Measurement, Paths. DTIC Identifier(s): Ionospheric Disturbances, Radio Direction Finders, Atmospheric Refraction, Theses. Author(s) subject terms: Single-Site-Location, Direction-Finding, High-Frequency, Estimation, Sampling. Includes bibliographical references (p. 57-58). Also available in print.

This thesis is executed in cooperation with RUAG Space AB, which specializes in highly reliable on-board satellite equipment. The thesis focuses on the effect, which disturbs the amplitude and phase of a Global Positioning System (GPS) signal, called scintillation effect. It has a substantial impact on a GPS signal, during Radio Occultation (RO). RO is a method of analysis of a refracted signal which passes through the atmosphere. RO can be used for measuring climate change and for weather forecasting. By retrieving the bending angle of a GPS signal, three basic parameters of the Earth’s atmosphere can be obtained at different heights: temperature, pressure and humidity. As the scintillation effect causes prominent errors in the bending angle calculations, it is crucial to provide possibly the most precise mathematical model, which allows to conceive proper ionospheric corrections. In this thesis, the model using Rytov approach is implemented and optimised with different optimisation functions. It is shown that the scintillation model can be optimized, which may contribute to a more accurate retrieval of the atmospheric profiles.

Includes bibliographical references
Global Navigation Satellite Systems (GNSS) are used to provide information on position, time and velocity all over the world at any time of the day. Currently there are four operational GNSS and one of them is GPS (Global Positioning System) that is developed and maintained by U.S Department of Defence (DoD), which is widely used and accessible all over the world. The accuracy of the output or even the availability of the navigation system depends on current space weather conditions, which can cause random fluctuations of the phase and amplitude of the received signal, called scintillation. Interference of GNSS signals that are reflected and refracted from stationary objects on the ground, with signals that travel along a direct path via the ionosphere to the antenna, cause errors in the measured amplitude and phase. These errors are known as multipath errors and can lead to cycle slip and loss of lock on the satellite or degradation in the accuracy of position determination. High elevation cut off angles used for filtering GNSS signals, usually 15-30°, can reduce non-ionospheric interference due to multipath signals coming from the horizon. Since a fixed-elevation threshold does not take into consideration the surrounding physical environment of each GPS station, it can result in a significant loss of valuable data. Alternatively, if the fixed-elevation threshold is not high enough we run the risk of including multipath data in the analysis. In this project we characterized the multipath environment of the GPS Ionospheric Scintillation and TEC (Total Electron Content) Monitor (GISTM) receivers installed by SANSA (South African National Space Agency) at Gough Island (40:34oS and 9:88° W), Marion Island (46:87° S and 37:86° E), Hermanus (34:42° S and19:22° E) and SANAE IV (71:73° S and 2:2° W) by plotting azimuth-elevation maps of scintillation indices averaged over one year. The azimuth-elevation maps were used to identify objects that regularly scatter signals and cause high scintillation resulting from multipath effects. After identifying the multipath area from the azimuth-elevation map, an azimuth-dependent elevation threshold was developed using the MATLAB curve fitting tool. Using this method we are able to reduce the multi-path errors without losing important data. Using the azimuth-dependent elevation threshold typically gives 5 to 28% more useful data than using a 20° fixed-elevation threshold.

La transformada de Abel es una técnica de inversión usada frecuentemente en radio ocultaciones (RO) que, en el contexto ionosférico, permite deducir densidades electrónicas a partir de datos de STEC (Slant Total Electron Content) derivados a partir de observaciones de la fase portadora. Esta técnica está basada en medidas precisas en doble frecuencia de fase portadora ( banda L) de un receptor GPS a bordo de un satélite de órbita baja (Low Earth Orbit -LEO-) rastreando un satélite GPS detrás del limbo de la tierra. Al combinar tales medidas con la información de posiciones y velocidades de los satélites GPS y LEO, es posible deducir el cambio en el camino de la señal debido a la presencia de la atmósfera y, consecuentemene, convertirlo en ángulos de curvatura (bending angles). A partir de ellos, información sobre el índice de refracción vertical puede ser obtenida a través de técnicas de inversión, y transformarlo en perfiles verticales de densidad electrónica y/o perfiles de atmósfera neutra. Una de las hipótesis básicas de la inversión clásica es suponer que el campo de densidades electrónicas tiene simetría esférica en la vecindad de una ocultación. Sin embargo, a la práctica, la huella de una ocultación generalmente cubre regiones de miles de km que puede presentar variabilidad ionosférica importante; por lo cuál, la hipótesis de simetría esférica no puede ser garantizada. De hecho, las inhomogeneidades de la densidad electrónica en la dirección veritcal para una ocultación dada son una de las principales causas de error cuando se usa la inversión de Abel inversion. Para corregir el error debido a la hipótesis de simetría esférica, se introduce el concepto de separabilidad. Ello implica que la densidad electrónica puede ser expresada como una combinación de datos de Vertical Total Electron Content (VTEC) derivados externamente, los cuales asumen la dependencia horizontal de la densidad, y una función de forma, que a su vez asume la dependencia en altura que es común a todas las observaciones para una ocultación dada. Nótese que el espesor de capa permanece constante cerca de la región de la ocultación debido a la hipótesis de separabilidad en vez de la densidad, como ocurriría en el caso de usar simetría esférica. Esta técnica fue aplicada exitosamente a la combinación lineal de fases de GPS L1 y L2, , LI= L1-2, la cuál proporcionar un observable libre de geometría que depende sólo del retraso ionosférico, la ambigüedad de fase, biases instrumentales y wind-up. Los resultados presentaban una mejora del 40% en RMS al comparar frecuencias del pico de la capa F2 con datos de ionosonda respecto la inversión clásica de Abel. Sin embargo, la potencial influencia de la diferencia de caminos ópticos entre L1 y L2 fue despreciada. Esta tesis doctoral muestra que ello no es un problema para la inversión a alturas ionosféricas. Una alternativa para la inversión de perfiles que evita esta desventaja es usar la curvatura de la señal como dato principal. La implementación de la separabilidad para ángulos de curvatura no es inmediata y ha sido uno de los objetivos de esta tesis. En este sentido, el principio de la separabilidad ha sido aplicado a los ángulos de curvatura de L1 en vez de la la combinación LI como en trabajos anteriores. Además, trabajando con ángulos de curvatura, la separabilidad puede ser también trasladada a la obtención de perfiles troposféricos. Varias aproximaciones para obtener la contribución de las partes altas de la ionosfera han sido también estudiadas, aparte del hecho de simplemente prescindir de esta contribución. Se ha usado un modelo climatológico, una extrapolación exponencial y el hecho de considerar las implicaciones de usar separabilidad. También se ha propuesto una manera para obtener funciones de mapeo (mapping functions) deducidas a partir de perfiles RO. Sin embargo, trabajando sólo con datos derivados únicamente de RO, se está sistematicamente despreciando la contribución de la protonosfera al TEC. Con la propuesta inicial de función de mapeo sólo la contribución ionosférica es tenida en cuenta. La solución ideal para aplicaciones de datos de tierra GNSS sería usar un modelo de dos capas, una para modelar la ionosfera y otra para la protonosfera, o alternativamente, si se quisiera alta resolución tomográfica, combinar observaciones RO y con elevación positiva de LEOs con datos de tierra. Se ha probado que modelando con dos capas, los resultados que se habían obtenido con el análisis de datos RO han podido ser validados. La conclusión más importante es que la proporción entre la contribución ionosférica y protonosférica es el parámetro que explica la localización de las alturas efectivas.
La transformada d’Abel és una tècnica emprada freqüentment en radio ocultacions (RO) que, en el context ionosfèric, permet deduir densitats electròniques a partir de dades de STEC (Slant Total Electron Content) derivats a partir d’observacions de la fase portadora. Aquesta tècnica està basada en mesures precises en doble freqüència de fase portadora (banda L) d’un receptor GPS a bord d’un satèl·lit d’òrbita baixa (Low Earth Orbit-LEO-) rastrejant un satèl·lit GPS darrere del limb de la terra. En combinar les dites mesures amb la informació de posicions i velocitats dels satèl·lits GPS i LEO, és possible deduir el canvi en el camí del senyal degut a la presència de l’atmosfera i, conseqüentment, convertir-lo en angles de curvatura (bending angles). A partir d’ells, informació sobre l’índex de refracció vertical pot ser obtinguda mitjançant tècniques d’inversió i transformar-lo en perfils verticals de densitat electrònica i/o perfils d’atmosfera neutra. Una de les hipòtesis bàsiques de la inversió clàssica és suposar que el camp de densitats electròniques té simetria esfèrica en el veïnatge d’una ocultació. Tanmateix, a la pràctica, la petjada d’una ocultació generalment cobreix regions de milers de quilòmetres que pot presentar variabilitat ionosfèrica important; per la qual cosa, la hipòtesi de simetria esfèrica no pot ser garantida. De fet, les inhomogeneitats de la densitat electrònica en la direcció vertical per a una ocultació donada són una de les principals causes d’error quan es fa servir la inversió d’Abel. Per a corregir l’error a causa de la hipòtesi de simetria esfèrica, s’introdueix el concepte de separabilitat. Això implica que la densitat electrònica pot ser expressada com una combinació de dades de Vertical Total Electron Content (VTEC) derivats externament, els quals assumeixen la dependència horitzontal de la densitat, i una funció de forma, la qual alhora assumeix la dependència en altura que és comuna a totes les observacions per a una ocultació donada. Cal notar que l’espessor de capa roman constant a prop de la regió de l’ocultació a causa de la hipòtesi de separabilitat en comptes de la densitat, tal i com succeiria en el cas de fer servir simetria esfèrica. Aquesta tècnica fou aplicada amb èxit a la combinació lineal de fases de GPS L1 i L2, LI=L1-2, la qual proporciona un observable lliure de geometria que depèn només del retard ionosfèric, l’ambigüitat de fase, biases instrumentals i wind-up. Els resultats presenten una millora del 40% en RMS en comparar freqüències del pic de la capa F2 amb dades de ionosonda respecte la inversió clàssica d’Abel. No obstant, la potencial influència de la diferència de camins òptics entre L1 i L2 fou menyspreada. Aquesta tesi doctoral mostra que això no és pas un problema per a la inversió a altures ionosfèriques. Una alternativa per a la inversió de perfils que evita aquesta desavantatge és emprar la curvatura del senyal com a dada principal. La implementació de la separabilitat per a angles de curvatura no és immediata i ha estat un dels objectius d’aquesta tesi. En aquest sentit, el principi de la separabilitat ha esta aplicat als angles de curvatura de L1 en comptes de la combinació LI com en treballs anterior. A més, treballant amb angles de curvatura, la separabilitat pot ser també traslladada a l’obtenció de perfils troposfèrics. Varies aproximacions per a obtenir la contribució de les parts altes de la ionosfera han estat també estudiades, apart del fet de prescindir simplement d’aquesta contribució. S’ha fet servir un model climatològic, una extrapolació exponencial i el fet de considera les implicacions d’usar separabilitat. També s’ha proposat una manera pera obtenir funcions de mapeo (mapping functions) deduïdes a partir de perfils RO. Tanmateix, treballant només amb dades derivades únicament de RO, s’està menyspreant sistemàticament la contribució de la protonosfera al TEC. Amb la proposta inicial de funció de mapeo només tenim en compte la contribució ionosfèrica. La solució ideal per a aplicacions de dades de terra GNSS seria fer servir un model de dues capes, una per a modelar la ionosfera i una altra per la protonosfera, o alternativament, si es volgués alta resolució tomogràfica, combinar observacions RO i amb elevació positiva de LEOs amb dades de terra. S’ha provat que modelant amb dues capes, els resultats obtinguts amb l’anàlisi de dades RO han pogut estar validats. La conclusió més important és que la proporció entre la contribució ionosfèrica i protonosfèrica és el paràmetre que explica la localització de les altures efectives.
The Abel transform is a frequently used radio occultation (RO) inversion technique which, in the ionospheric context, allows retrieving electron densities as a function of height from STEC (Slant Total Electron Content) measurements derived from carrier phase observations. The GPS RO technique is based on precise carrier dual-frequency phase measurements (L-band) of a GPS receiver onboard a Low Earth Orbit satellite (LEO) tracking a rising or setting GPS satellite behind the limb of the earth. When combining such measurements with the information from the positions and velocities of GPS and LEO satellites, it is possible to derive the phase path change due to the atmosphere during an occultation event which subsequently can be converted into bending angles. From these, information about the vertical refraction index can be obtained by means of inversion techniques, which can then be converted into ionospheric vertical electron density profiles and/or neutral atmospheric profiles. One of the basic assumptions in the classical approach is to assume the spherical symmetry of the electron density field in the vicinity of an occultation. However, in practice, the footprint of an occultation generally covers wide regions of thousands of kilometres in length that may show significant ionospheric variability; therefore this hypothesis cannot be guaranteed. Indeed, inhomogeneous electron density in the horizontal direction for a given occultation is believed to be one of the main sources of error when using the Abel inversion. In order to correct the error due to the spherical symmetry assumption, the separability concept is introduced and applied. This implies that the electron density can be expressed by a combination of externally derived Vertical Total Electron Content (VTEC) data, which assumes the horizontal dependency, and a shape function, which in turn assumes the height dependency that is common to all the observations for a given occultation. Note that the slab thickness remains constant near the occultation due to the separability hypothesis instead of the density as is the case of the spherical symmetry. This technique was successfully applied to the linear combination of the GPS carrier phases L1 and L2, , LI= L1-2 which is a geometric free observable that depends only on the ionospheric delay, phase ambiguity, instrumental bias and wind-up. The result was an improvement of about 40% in RMS when comparing frequencies of the F2 layer peak with ionosonde data and the classical Abel inversion. The main advantage of such developed technique is its simple computation. Nevertheless, the potential influence of the different signal paths between L1 and L2 was neglected. Regarding this aspect, this Ph.D. dissertation shows that is not a problem for inversion at ionospheric heights. An alternative to inverting the profile, which overcomes this disadvantage, is to use the bending angle of the signal as the main input data. The implementation of separability when using the bending angle is not immediate and was, actually, one of the goals of this thesis. In this sense, the separability approach has been applied to measured L1 bending angle, instead of LI combination as reported in previous work. Additionally, this approach could also be translated to tropospheric profile retrievals. Several approaches to account for the upper ionospheric contribution have been also tackled, apart from the fact of neglecting such contribution: a climatological model, an exponential extrapolation and condisering the nature of the separability concept. it has been proposed a way to obtain mapping functions derived from RO profiles. Such mapping functions can be easily derived from usual ionospheric parameters. For the contribution of this part of the ionosphere, it has been shown that it is capable to account for the total electron content (TEC). However, by working solely with RO derived data, we are systematically neglecting the contribution of the protonosphere to the total electron content. With the initial proposed mapping function based on the analysis of effective heights derived from RO, only the ionospheric contribution is accounted for. The ideal solution for ground-based GNSS data applications would be to use a two-layer model, one to model the ionosphere and another one for the protonosphere, or alternatively, if we are looking for high tomographic resolution, to combine RO and topside LEO observations with ground data. It has been shown that by modelling in such way, the results that were obtained with RO data analysis can be validated. The most important conclusion is that the ratio between ionospheric and protonospheric contribution is the driver for the location of the effective heights.

We have been actively involved in the research and management activities of European Co-Operation in the Field of Scientific and Technical Research (EU COST) actions such as COST 238 Prediction and Retrospective Ionospheric Modeling over Europe (PRIME), COST 251 Improved Quality of Service in Ionospheric Telecommunication System Planning and Operation, COST 271 Effects of the Upper Atmosphere on Terrestrial and Earth-Space Communications, COST 296 Mitigation of Ionospheric Effects on Radio Systems (MIERS) and COST 724 Developing the Scientific Basis for Monitoring, Modeling and Predicting Space Weather. In this thesis High Frequency (3-30 MHz) (HF) radar system under ionospheric disturbances has been identified globally and some operational suggestions have been presented. The use of HF radar system is considered from the identification of ionospheric propagation medium point of view. Doppler velocity is considered as the characteristic parameter of the propagation medium. ap index is chosen as the parameter for disturbance characterization due to geomagnetic storms in the ionosphere. The main difficulty is the scarcity of data, which is rare and confidential. Therefore semi-synthetic data are generated. Dependence of Doppler velocity and group range of the echo signal on ap index is examined and some details of dependence are studied and demonstrated. Thus, effects of space weather on the ionosphere and as a result on HF radar wave propagation are displayed. These results are examples of system identification. This can be used in communication system planning and operation.

At the department of Space and Plasma Physics a small wire boom system has been developed. The system’s cable is stored around a stationary cylinder. The system uses a gear to feed out the cable in an axial direction. The purpose is to measure the electric and magnetic fields in the ionosphere. The wire boom system has had problems with the functionality and friction. In the REXUS 10 project, the system concept was to be proven through a space flight. This Master’s Thesis describes the process of bringing the Boom system from concept to a fully functional and flight proven physical model. The results came in the form of a successful space flight, where two of the four systems deployed and retracted successfully. Another important result was the identification of and solution for, areas of the concept that are critical for its design, manufacturing and function.

Travelling Ionospheric Disturbances (TIDs) are said to be produced by atmospheric gravitational waves propagating through the neutral ionosphere. These are smaller in amplitude and period when compared to most ionospheric disturbances and hence more difficult to measure. Very little is known about the properties of the travelling ionospheric disturbances (TIDs) over the Southern Hemisphere regions since studies have been conducted mostly over the Northern Hemisphere regions. This study presents a framework, using a High Frequency (HF) Doppler radar to investigate the physical properties and the possible driving mechanisms of TIDs. This research focuses on studying the characteristics of the TIDs, such as period, velocity and temporal variations, using HF Doppler measurements taken in South Africa. By making use of a Wavelet Analysis technique, the TIDs’ characteristics were determined. A statistical summary on speed and direction of propagation of the observed TIDs was performed. The winter medium scale travelling ionospheric disturbances (MSTIDs) observed are generally faster than the summer MSTIDs. For all seasons, the MSTIDs had a preferred south-southwest direction of propagation. Most of the large scale travelling ionospheric disturbances (LSTIDs) were observed during the night and of these, the spring LSTIDs were fastest when compared to autumn and summer LSTIDs. The general direction of travel of the observed LSTIDs is south-southeast. Total Electron Content (TEC), derived from Global Positioning System (GPS) measurements, were used to validate some of the TID results obtained from the HF Doppler data. The Horizontal Wind Model (HWM07), magnetic K index, and solar terminators were used to determine the possible sources of the observed TIDs. Only 41% of the observed TIDs were successfully linked to their possible sources of excitation. The information gathered from this study will be valuable in future radio communications and will serve as means to improve the existing ionospheric models over the South African region.

Global Navigation Satellite Systems (GNSS) have been in the center stage of the recent technological upheaval that has been initiated by the rise of smartphones in the last decade. This is clearly reflected in the development of many applications based on GNSS technology as well as the emergence of multi-constellation GNSS with the launch of the first Galileo satellites at the end of the year 2011. GNSS does not only guarantee global positioning, navigation and timing services but also extends to applications in banking, agriculture, mapping, surveying, archaeology, seismology, commerce, ionosphere scintillation monitoring, remote sensing (soil moisture, ocean salinity, type of surface), wind speed monitoring, ocean surface monitoring, altimetry and many others. In the last decade, Location Based Services (LBS) have increased significant market demand where GNSS has been coupled with technologies based on terrestrial communication links in order to meet strict positioning accuracy requirements. In these conditions, relying on GNSS technology alone, raises a few challenges for signal synchronization even before positioning attempts and are mainly due to a considerable signal attenuation as it propagates through construction material and into indoor environments. Ionosphere scintillation induces a similar challenge where in addition to amplitude fading, the carrier phase and frequency suffer from indeterministic fluctuations. This research activity is devoted to explore and design the elements constituting pilot channel scalar tracking loop systems, specifically tailored to Galileo signals. It is expected that running such systems with extended integration intervals offers robust synchronization of the incoming signal which is heavily affected by external indeterministic fluctuations. In some conditions, it is desired to follow these fluctuations as in ionosphere scintillation monitoring while in other instances it is mainly desired to filter them out as noise to guarantee positioning capabilities. This is the objective of this research study which applies for both indoor environments and ionosphere scintillation affected signals. Towards this endeavor, a comprehensive theoretical study of the carrier and code tracking loops elements is undertaken, and particular attention is directed to the following aspects: • carrier frequency and phase discriminators and the relative optimum integration time • Galileo specific code discriminators and code tracking architecture especially tailored to Composite Binary Offset Carrier (CBOC) modulated signals. • optimum loop filters designed in the digital domain for different types of phase input signals • local signal generation using a numerically controlled oscillator and loop filter estimates • front-end filter bandlimiting effects on the tracking performance. This design is further tested with simulated Galileo signals with and without ionosphere scintillation as well as raw Galileo signals in an equatorial region during March 2013. Tracking performance comparison is carried out between the customized Galileo receiver developed in this research activity and an ionosphere scintillation dedicated professional GNSS receiver, the Septentrio PolaRxS PRO R receiver.

We investigate the possibility of measuring and using the phase delay of radio frequency transmissions in the amateur satellite band as a method to determine the distribution of currents systems in the ionosphere. The amateur satellite transmissions at 7MHz, 14M Hz, and 144M Hz are low enough for Faraday rotation to cause a significant phase delay on the propagating signals in addition to the phase delay produced by the total electron content (TEC) in the ionosphere. The ionosphere in the E and F regions is modeled as an equivalent thin planar shell of collision free cold plasma 100 km in thickness located in an altitude range of 100 􀀀 200 km. The earth's magnetic field is superposed with a weaker magnetic field due to a narrow Gaussian strip of current representing an ionospheric electrojet. The prole of the current system is obtained by numerically optimizing the Appleton-Hartree dispersion relation for rays of simulated radio frequency (RF) signals that propagate through the ionosphere shell. The optimization procedure is performed with a differential evolution algorithm. From the optimization procedure, we obtain the ionosphere total electron content (TEC) and the strength, prole, and orientation of the ionospheric current system.

2012/2013
L’attività di Ricerca svolta durante il ‘Corso di Dottorato in Ingegneria Civile e Ambientale ha riguardato l’attività in collaborazione con il Prof. Radicella presso il Telecommunications/ICT for Development Laboratory (T/ICT4D), the Abdus Salam International Centre of Theoretical Physics, Miramare, e il periodo formativo di tre mesi all’estero presso l’EGNOS Project Office (EPO), sede dell’European Space Agency (ESA) a Toulouse (Francia), sotto la supervisione di Dott. Stefan Schlueter dell’ EPO. Il lavoro svolto, che consiste nello studio dell’effetto della ionosfera sul segnale satellitare di EGNOS (European Geostationary Navigation Overlay System e del conseguente degrado del calcolo della posizione plano altimetrica che ne deriva, si intende come un contributo per l’ottimizzazione di uno dei servizi forniti da EGNOS denominato EGNOS Open Service, ovvero del primo servizio reso disponibile agli utenti a partire dal 2009. Le caratteristiche del servizio EGNOS Open Service sono presentate in un documento ufficiale chiamato EGNOS SDD-OS dell’Agenzia Spaziale Europea. Il suo obiettivo principale è di aumentare l’accuratezza nel posizionamento correggendo i differenti errori che influenzano il segnale GPS a singola frequenza: gli errori di orologio e di orbita dei satelliti e gli errori legati al ritardo di propagazione del segnale in ionosfera. Altre tipologie di errori, come quelli legati alla propagazione del segnale in troposfera e gli errori da multipath, essendo dovuti a effetti locali non possono essere corrette dai sistemi SBASs (Satellite Based Augmentation System). Lo scopo del documento è di suggerire le tecniche e linee guida per i produttori di ricevitori satellitari e di mettere in evidenza l’importanza del posizionamento e delle prestazioni temporali attualmente disponibili per utenti equipaggiati con strumentazione in grado di ricevere sia il segnale di trasmissione GPS in modalità Standard Positioning Service, con utilizzo della sola frequenza L1, sia il segnale fornito da EGNOS. L’importanza di questo lavoro risiede nel fatto che l’EGNOS OS, essendo un Open Service, può facilmente essere usato da differenti utenze, in una vasta gamma di settori quali la navigazione stradale, l’agricoltura di precisione e le applicazioni personali su palmari. Il lavoro svolto quest’anno s’inserisce pertanto in uno degli obiettivi proposti dall’EGNOS SDD OS. L’aspetto fondamentale riguarda i dati ionosferici forniti da EGNOS tramite i messaggi navigazionali MT 18 e MT26. E’ ormai noto come la precisione nel calcolo del posizionamento, che si ottiene mediante un sistema GNSS a singola frequenza, sia dipendente da vari fattori tra i quali quello dominante è legato alla propagazione del segnale in ionosfera. Sebbene i sistemi SBAS, (del quale EGNOS è la componente Europea) abbiano consentito di ridurre notevolmente gli effetti legati a quest'ultima tipologia di errore, anch'essi soffrono di una forte riduzione di prestazioni in condizioni di grande variabilità ionosferica, come quelle legate a alle basse latitudini o alla presenza di tempeste geomagnetiche. Per questo motivo è importante valutare a livello quantitativo le prestazioni di EGNOS in termini di range delay e di posizionamento tramite un adeguato confronto dei dati trattati con software dedicati, come quelli utilizzati dalla scrivente durante i mesi trascorsi all’EPO. Il dato principale fornito da EGNOS consiste nella correzione ionosferica in punti ionosferici di griglia (IGPs). A causa della presenza di punti IGP “non monitorati” in alcune zone dell’area di copertura del sistema (ECAC, European Civil Aviation Conference), non e' possibile calcolare in modo appropriato il ritardo ionosferico nei corrispondenti punti ionosferici (IPPs) in accordo con la tecnica di interpolazione prevista nel documento ufficiale dell’EGNOS, RTCA Do 229C-D Minimal Operational Performance Standard for GPS/WAAS. Questo incide ovviamente sulle prestazioni globali del sistema EGNOS che, in condizioni nominali, presenta una disponibilità massima di punti ionosferici di griglia (IGPs) monitorati nelle zone centrali dell’area ECAC. In questo contesto, il lavoro svoltosi all’EPO si è' diviso in 3 fasi principali: 1. Sono state considerate le possibili aree di miglioramento delle prestazioni del sistema; per queste aree sono state proposte diverse soluzioni di implementazione. Ognuna di queste soluzioni è stata scelta con l’obiettivo di migliorare la disponibilità dell’EGNOS OS e allo stesso tempo di soddisfare le richieste di accuratezza specificate nel documento. 2. Una volta identificate le possibili aree di miglioramento e di studio, sono state valutate le prestazioni associate a ciascuna soluzione proposta, usando un prototipo di ricevitore GNSS. L’utilizzo di un ricevitore/software è motivato dal fatto che questo permette grande flessibilità quando si testano le differenti opzioni. 3. Infine, sulla base della valutazione delle prestazioni ottenute, sono state considerate e discusse le soluzioni in ciascuna area analizzata. Nella prima fase della ricerca è stato studiato una strategia di calcolo per ovviare alla mancanza dei dati di EGNOS ed estendere la disponibilità per i dati di griglia, in particolare nella parte sud della zona ECAC: e’ stato effettuato uno studio delle mappe globali, in particolare quelle fornite dall’International GNSS Service (IGS) e dal CODE, dell’Università di Berna, eseguendo un confronto fra i ritardi ionosferici verticali e quelli obliqui (slant). Lo scopo di questa analisi e' stato quello di determinare il ritardo ionosferico e correggerne l'influenza in un settore critico dell'area ECAC, come quello delle regioni meridionali, a causa dell'assenza dei dati di correzione ionosferica EGNOS. L’obiettivo principale di questa fase è stato quello di confrontare il TEC verticale estratto dai dati EGNOS con quello ottenuto utilizzando le mappe globali di CODE in tutta la zona d’interesse, e infine l'utilizzo dei valori di correzione dei punti di griglia del CODE in sostituzione dei valori mancanti nei punti di griglia non monitorati da EGNOS. L’analisi è stata eseguita per gli anni 2012 e 2013 in giorni caratterizzati sia da condizioni di quiete che da condizioni di tempeste geomagnetiche, in un’area di copertura [40°W, 40°E ] in longitudine e [20°N, 60°N] in latitudine, in modo da valutare quantitativamente in che misura le mappe globali riproducano le condizioni regionali descritte dalla griglia di EGNOS in termini di gradiente spaziale di TEC (sia in longitudine che latitudine). Nella seconda fase della ricerca sono stati calcolati i valori di TEC verticale e obliquo e le coordinate dei punti IPP. In seguito sono stati calcolati i diversi contributi agli IPP ( in termini di TEC) con l’algoritmo di interpolazione bilineare a quattro punti. Sono stati considerati i valori del TEC negli IGPs: - di EGNOS - di CODE - di una griglia sintetica ottenuta sostituendo i dati di CODE nei punti di griglia non monitorati da EGNOS. Utilizzando le diverse correzioni ionosferiche sono stati ricalcolati i valori dei relativi pseudoranges da utilizzare per il calcolo della posizione con correzione WADGPS. I nuovi valori pseudorange così calcolati sono stati inseriti all'interno dei files rinex di diverse stazioni di riferimento, operanti nella parte sud dell’area ECAC. Con l'utilizzo di diversi software utilizzati all’EGNOS Project Office, sono stati valutati gli errori di posizionamento per i diversi modelli di TEC assunti al fine di valutare l’attendibilità delle precisioni planimetriche e altimetriche ottenute. Si è' deciso di usare software di posizionamento flessibili per le varie esigenze che simulassero i ricevitori, in modo da poter applicare le varie correzioni in modo sistematico. Lo scopo dell'attivita' di ricerca svolta, è quello di permettere l’estensione della disponibilità dei dati EGNOS nei valori di griglia in caso di condizioni “non monitorate” del sistema, attraverso lo studio degli effetti dell’impatto ionosferico su EGNOS.
XXV Ciclo
1984

Las tecnologías de posicionamiento por satélite (GNSS, del inglés global navigation satellite systems) se han convertido en una herramienta indispensable en diferentes ámbitos de nuestra sociedad moderna. Algunos ejemplos de aplicaciones son el posicionamiento y la navegación en entornos terrestre, marítimo y aéreo, así como usos destinados a la agricultura, topografía o aplicaciones de sincronización precisa en sistemas de telecomunicaciones o finanzas. El módulo de tracking es una de las etapas centrales para mantener los receptores alineados con los satélites, y hasta ahora se han empleado técnicas de tracking convencionales de fácil implementación que son suficientes para operar en escenarios con unas condiciones de trabajo favorables. Sin embargo, en los últimos años, el éxito de GNSS en entornos a cielo abierto ha propiciado su expansión hacia aplicaciones en escenarios más exigentes, tales como cañones urbanos o interiores. La tendencia es dotar a los terminales móviles (smartphones) de capacidades de posicionamiento en entornos en donde se enfrentan a nuevos retos tecnológicos dados por los problemas de propagación que abundan. En este sentido, el centelleo ionosférico (ionospheric scintillation en inglés) es uno de los problemas que degradan las prestaciones de los receptores, particularmente en zonas ecuatoriales y a altas latitudes. Es un efecto que introduce rápidas variaciones aleatorias en la fase y la potencia de la señal útil, y tiene un efecto perjudicial precisamente en la etapa de tracking del receptor. El objetivo de esta tesis es diseñar y desarrollar nuevas técnicas para el tracking robusto de señales GNSS afectadas por el efecto de centelleo ionosférico. La propuesta que se presenta está basada en el uso de técnicas de filtrado de Kalman, y las contribuciones principales de esta tesis son tres. En primer lugar se estudia el efecto de centelleo ionosférico y el tracking de la dinámica del receptor a pesar de su presencia. Diseñamos un filtro de Kalman con una formulación híbrida que permite monitorizar ambas contribuciones por separado de manera robusta. Esto surge de realizar un análisis detallado del centelleo ionosférico en el que se concluye que las variaciones de fase se pueden caracterizar a través de procesos autoregresivos, los cuales se pueden tratar mediante el filtro de Kalman de manera natural. En segundo lugar se diseñan técnicas de filtrado de Kalman adaptativas que permiten ajustar su ancho de banda en función de las condiciones de centelleo, las cuales suelen ser variantes en el tiempo en la práctica. Esta parte incluye un detector de presencia de centelleo, un estimador en tiempo real de los parámetros del modelo autoregresivo, y una implementación para lidiar con las atenuaciones no lineales introducidas por el mismo centelleo. El funcionamiento de las técnicas propuestas se valida posteriormente mediante una campaña extensiva de simulaciones utilizando tanto datos sintéticos como datos reales de centelleo ionosférico, y se cuantifica la región de ganancia respecto a las técnicas convencionales. Por último se propone un innovador método para derivar expresiones para la denominada cota Bayesiana de Cramér-Rao (BCRB, del inglés Bayesian Cramér-Rao bound) que permiten caracterizar el comportamiento de los filtros de Kalman de manera cerrada. Esto supone una contribución a la literatura de gran interés práctico para diseñar filtros de Kalman para cualquier tipo de aplicación.
Global Navigation Satellite Systems (GNSS) have become an indispensable tool in different areas in our modern society for positioning purposes using radio-frequency ranging signals. Some application examples are the positioning and navigation in ground, maritime and aviation environments, as well as their use in agriculture, surveying and precise timing and synchronization in communication systems and finances. The tracking stage is one of the core tasks within a GNSS receiver to keep aligned with the satellites and, to date, most receivers equip conventional tracking techniques with ease of implementation that suffice to operate in environments with favorable working conditions. However, in the recent years, the success of GNSS in open-sky environments has led to the emergence of applications that expand toward scenarios with harsher conditions, such as urban canyons and soft-indoor environments. The trend is to provide user mobile terminals such as smartphones with positioning capabilities in scenarios where receivers face new technological challenges owing to the abounding propagation impairments. In this sense, the so-called ionospheric scintillation is one of the issues degrading the performance of GNSS receivers, particularly in equatorial regions and at high latitudes. It introduces rapid carrier phase and signal power variations, and has a detrimental effect particularly onto the tracking stage. The objective of this thesis is to design and develop new techniques for the robust tracking of GNSS signals affected by ionospheric scintillation disturbances. The presented approach is based on the use of Kalman filtering techniques, and the main contributions of the thesis are three. First, the analysis of ionospheric scintillation and the tracking of carrier dynamics despite the presence of the former. We design a Kalman filter with a hybrid formulation that allows the robust monitoring of both contributions separately. This arises from carrying out a detailed analysis of ionospheric scintillation which concludes that scintillation phase variations can be characterized through autoregressive processes, and thus be dealt with within the Kalman filter in a natural manner. Second, the design of adaptive Kalman filter-based techniques that allow self-adjusting their loop bandwidth to the actual scintillation conditions, which are rather time-varying in practice. This part includes a scintillation detector, a real-time estimator of the autoregressive model parameters, and an implementation to address the problem of non-linear signal amplitude attenuation introduced by scintillation itself. The goodness of the proposed techniques is later validated by carrying out an extensive simulation campaign using both synthetic data and real scintillation time series, and the outperformance region with respect to conventional tracking techniques is quantified. Third, a novel method for the derivation of expressions for the termed Bayesian Cramér-Rao bound (BCRB), which allow characterizing the behavior of Kalman filters in a closed-form manner, thus becoming a contribution to the literature of practical usefulness to design Kalman filters for any kind of application.

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This thesis reports the design, performance evaluation and construction of a transmitting antenna for an HF communications probe system. A short range ionospheric communication link between Monterey, CA, (transmit site) and San Diego, CA, (receive site) was established to test the software and hardware of this probe system. The Multiband Dipole Antenna was selected as the more practical antenna for this link, using less real estate and support structure than other alternatives. The antenna was constructed and installed at the NPS beach site where the ground constants were measured accurately. Numerical Electromagnetics Code (NEC) analysis and measurements show that the antenna operates with low input VSWR ( < 1.5), is insensitive to electrical ground characteristics and has excellent radiation patterns for short range ionospheric communication links. Based on the observed signal strengths at San Diego, the antenna appears to be performing very well.

>Magister Scientiae - MSc
The development of regional ionospheric Total Electron Content (TEC) models has contributed to understanding the behavior of ionospheric parameters and the coupling of the ionosphere to space weather activities on both local and global scales. In the past several decades, the International Global Navigation Satellite Systems Service (GNSS) networks of dual frequency receiver data have been applied to develop global and regional models of ionospheric TEC. These models were mainly developed in the Northern Hemisphere where there are dense network of ground based GPS receivers for regional data coverage. Such efforts have been historically rare over the African region, and have only recently begun. This thesis reports the investigation of the effect of mid-latitude magnetic storms on TEC over South Africa for portions of Solar Cycles 23 and 24. The MAGIC package was used to estimate TEC over South Africa during Post Solar Maximum, Solar Minimum, and Post Solar Minimum periods. It is found that TEC is largely determined by the diurnal cycle of solar forcing and subsequent relaxation, but effects due to storms can be determined

Two research studies have been addressed in this thesis. Both of them are of actual scientific interest and are based on processing GNSS data. The first part of this thesis is devoted to GNSS detection and monitoring of solar flares. The second one is devoted to GNSS prediction of ionospheric Total Electron Content. Regarding the first study, a new solar flare detector called SISTED has been designed and implemented. Its goal is to provide a simple and efficient way of detecting the most number of powerful X-class solar flares in real time operation. In addition, it can send early warning messages to prevent the harmful consequences of the increase of ejected particles from the Sun that may reach the Earth after a solar flare, especially in case of a Coronal Mass Ejection. The main benefit of SISTED regarding other detection techniques is that it does not require data from external providers out of the GNSS community. In addition, it can run in real-time operation and could provide value added data to GNSS users. The results show that SISTED was able to detect up to the 95% of the X-class flares reported by GOES for more than a half solar cycle. Regarding the second study, a new approach to predict Global Ionospheric vertical TEC Maps has been designed and implemented in the context of the IGS Ionosphere Working Group. The motivation to develop a UPC Predicted product was the interest of ESA's SMOS mission. A recent application using UPC Predicted products is the generation of real-time global VTEC maps as background model. In addition, the predicted VTEC maps are used to generate the combined IGS Predicted products. The results obtained in this thesis show that the model performs well when the results are compared with those obtained by the other IGS analysis centers. In addition, applying the prediction model leads to better results than the use of time-invariant ionosphere for two days ahead. In relation with this research, 4 publications in international journals indexed in JCR/ISI have been generated (and another one is under review process), and 7 presentations have been authored in international meetings, among the new UPC predicted product contributing to IGS, and the contribution to two competitive projects funded by the European Space Agency (AGIM and MONITOR).