RESSALVA Atendendo solicitação do(a) autor(a), o texto completo desta dissertação será disponibilizado somente a partir de 16/06/2025. UNESP – UNIVERSIDADE ESTADUAL PAULISTA CAMPUS DE PRESIDENTE PRUDENTE FACULDADE DE CIÊNCIAS E TECNOLOGIA PROGRAMA DE PÓS-GRADUAÇÃO EM CIÊNCIAS CARTOGRÁFICAS PRESIDENTE PRUDENTE 2023 ANA LUCIA CHRISTOVAM DE SOUZA GNSS SINGLE FREQUENCY IONOSPHERIC MODEL FOR EQUATORIAL PLASMA BUBBLES DETECTION IN THE BRAZILIAN REGION ANA LUCIA CHRISTOVAM DE SOUZA GNSS SINGLE FREQUENCY IONOSPHERIC MODEL FOR EQUATORIAL PLASMA BUBBLES DETECTION IN THE BRAZILIAN REGION Thesis presented to “Programa de Pós- Graduação em Ciências Cartográficas” at São Paulo State University – UNESP School of Technology and Sciences. Supervisor: Dr. Paulo de Oliveira Camargo Co-Supervisor: Dr. Fabricio dos Santos Prol Co-Supervisor: Dr. Manuel Hernández- Pajares PRESIDENTE PRUDENTE 2023 S729g Souza, Ana Lucia Christovam de GNSS Single Frequency model for equatorial plasma bubbles detection in the Brazilian region / Ana Lucia Christovam de Souza. -- Presidente Prudente, 2023 94 f. Tese (doutorado) - Universidade Estadual Paulista (Unesp), Faculdade de Ciências e Tecnologia, Presidente Prudente Orientador: Paulo de Oliveira Camargo Coorientador: Fabricio dos Santos Prol 1. GNSS. 2. Ionospheric Modeling. 3. Ionospheric plasma bubbles. 4. TEC single frequency. I. Título. Sistema de geração automática de fichas catalográficas da Unesp. Biblioteca da Faculdade de Ciências e Tecnologia, Presidente Prudente. Dados fornecidos pelo autor(a). Essa ficha não pode ser modificada. ACKNOWLEDGMENTS I would like to thank my supervisor, Prof. Paulo O. Camargo, who was the main motivator of this research. I would also like to thank Fabricio S. Prol from Finnish Geospatial Research Institute (FGI) for their support and contributions as co-supervisors. I am also grateful to Prof. Manuel Hernández-Pajares from Universitat Politècnica de Catalunya (UPC) for kindly sharing his knowledge about ionospheric modeling and geodetic positioning during the internship supervision. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. Ph.D. scholarship from 03/2019 to 02/2023 (Grant: 88882.433958/2019-01) and from 03/2023 to 05/2023 (Grant: 88887.817768/2023-00). Internship abroad at Universitat Politècnica de Catalunya Capes-PrInt (PrInt - Programa Institucional De Internacionalização) from 11/2022 to 01/2023 (Grant: 88887.569761/2020-00). This works was also supported by: GNSS in Support of Air Navigation (INCT GNSS-NavAer), funded by CNPq (Grant: 465648/2014-2), FAPESP (Grant: 2017/50115-0) and CAPES (Grant: 88887.137186/2017-00). Postgraduate program in Cartographic Sciences, São Paulo State University (UNESP) – School of Technology and Sciences. Department of Mathematics at the Universitat Politècnica de Catalunya, Barcelona, Spain. ABSTRACT The ionospheric layer can be considered difficult to model due to the high variation in electron density presents in the upper atmosphere. In regions of low latitude, such as Brazilian region, the ionosphere presents a peculiar dynamic, considerably affecting communication systems and global navigation satellite systems (GNSS). Total Electron Content (TEC) is the main parameter capable of describing the ionospheric layer. Therefore, ionospheric models based on TEC values become critical for monitoring space weather and the ionospheric layer. However, in low latitude regions, the performance of these models degrades due to the high sensitivity of the global positioning system (GPS) L2 frequency to ionospheric scintillation. In an effort to overcome this problem, this thesis investigates the potential of utilizing only GPS L1 frequency for imaging the ionospheric bubbles during ionospheric scintillation. Two methods for estimating TEC using single frequency are presented. The first one is based on the calibration method, i.e., information retrieved by external ionospheric models are used to correct the TEC using single frequency measurements. The second one is based on independent regional ionospheric model to calibrate GPS using single frequency measurements. Both methods successfully detected plasma bubbles by mapping TEC using single frequency. In addition, single frequency model presented similar accuracy to the dual frequency models and, at the same time, provided less observations affected by ionospheric scintillations. These results demonstrate the feasibility of using single frequency GNSS data to develop ionospheric models and to improve GNSS positioning in low latitudes. Keywords: Ionosphere; spherical harmonics; TEC; single frequency; plasma bubbles; airglow images. RESUMO A camada ionosférica pode ser considerada de difícil modelagem devido à alta variação na densidade de elétrons presente na atmosfera superior. Em regiões de baixa latitude, como a região brasileira, a ionosfera apresenta uma dinâmica peculiar, afetando consideravelmente os sistemas de comunicação e os sistemas GNSS (Global Navigation Satellite System). O Conteúdo Total de Elétrons (TEC) é o principal parâmetro capaz de descrever a camada ionosférica. Portanto, os modelos ionosféricos baseados em valores de TEC tornam-se essenciais para o monitoramento do clima espacial e da camada ionosférica. No entanto, em regiões de baixa latitude, o desempenho desses modelos é prejudicado devido à alta sensibilidade da frequência L2 do GPS (Global Positioning System) à cintilação ionosférica. Em um esforço para superar esse problema, esta tese investiga o potencial de utilizar apenas a frequência L1 do GPS para obter imagens das bolhas ionosféricas durante a cintilação ionosférica. São apresentados dois métodos para estimar o TEC utilizando simples frequência. O primeiro método é baseado na calibração do TEC, ou seja, as informações obtidas por modelos ionosféricos externos são usadas para corrigir o TEC usando medições de simples frequência. O segundo método, baseia- se em um modelo ionosférico regional independente para calibrar o GPS usando medições de simples frequência. Ambos os métodos detectaram com sucesso bolhas de plasma mapeando o TEC usando frequência única. Além disso, o modelo de simples frequência apresentou precisão semelhante à dos modelos de dupla frequência e, ao mesmo tempo, forneceu menos observações afetadas pelas cintilações ionosféricas. Esses resultados demonstram a viabilidade do uso de dados GNSS de simples frequência para desenvolver modelos ionosféricos e melhorar o posicionamento GNSS em baixas latitudes. CONTENTS CHAPTER 1 INTRODUCTION 1 OVERVIEW AND MOTIVATION .................................................................. 9 1.1 Objectives .......................................................................................................... 12 1.2 Content and Contributions .............................................................................. 12 CHAPTER 2 ASSESSMENT OF GPS POSITIONING PERFORMANCE USING DIFFERENT SIGNALS IN THE CONTEXT OF IONOSPHERIC SCINTILLATION 2 SCOPE ............................................................................................................... 14 2.1 Overview ............................................................................................................ 14 2.2 Methodology ...................................................................................................... 16 2.3 Results and Discussion ..................................................................................... 17 2.3.1 Quantitative analysis of the ionospheric scintillation impact on GPS frequencies ............................................................................................................................ 17 2.3.2 Assessment of GPS point positioning at different frequencies .......................... 23 2.4 Conclusion ......................................................................................................... 26 CHAPTER 3 SINGLE FREQUENCY TEC CALIBRATION TO DETECT EQUATORIAL PLASMA BUBBLES 3 SCOPE ............................................................................................................... 28 3.1 Overview ............................................................................................................ 28 3.2 TEC Calibration ............................................................................................... 30 3.3 Results ................................................................................................................ 31 3.3.1 Comparison between single and dual frequency STEC values .......................... 31 3.3.2 Ionospheric Maps ................................................................................................ 35 3.4 Conclusion ......................................................................................................... 38 CHAPTER 4 PPP AT LOW LATITUDES WITH IONOSPHERIC MODEL EXCLUSIVELY BASED ON SINGLE FREQUENCY GNSS MEASUREMENTS 4 SCOPE ................................................................................................................ 40 4.1 Overview ............................................................................................................. 40 4.2 TEC estimation using single frequency data by regional modeling .............. 42 4.2.1 Conversion of single frequency GNSS observations to non-calibrated TEC data ............................................................................................................................. 43 4.2.2 Bias term estimation by Spherical Harmonic Modeling ...................................... 45 4.2.3 Regional Interpolation ........................................................................................ 47 4.3 Results ................................................................................................................. 48 4.3.1 Case study: DOY 003, 2014 ................................................................................ 49 4.3.2 Efficiency of the Plasma Bubble Detections by Single-Frequency TEC Keograms ............................................................................................................. 52 4.3.3 Drift Velocities by Single-Frequency TEC Keograms ........................................ 57 4.4 Conclusion .......................................................................................................... 60 CHAPTER 5 PPP AT LOW LATITUDES WITH IONOSPHERIC MODEL EXCLUSIVELY BASED ON SINGLE FREQUENCY GNSS MEASUREMENTS 5 SCOPE ................................................................................................................ 62 5.1 Overview ............................................................................................................ 62 5.2 Method ................................................................................................................ 64 5.2.1 Modeling .............................................................................................................. 64 5.2.2 PPP Settings ......................................................................................................... 67 5.3 Results ................................................................................................................. 69 5.3.1 VTEC plasma bubbles maps with analyzed models ............................................ 69 5.3.2 Ionospheric variability on GPS frequencies ........................................................ 70 5.3.3 Assessment by single frequency PPP .................................................................. 73 CHAPTER 6 FINAL REMARKS FINAL REMARKS ......................................................................................................... 82 REFERENCES ................................................................................................................ 85 9 CHAPTER 1 INTRODUCTION 1 OVERVIEW AND MOTIVATION The ionosphere is a layer of the Earth's atmosphere, located at approximately 50 to 1000 km altitude, characterized by a high concentration of ionized particles. The propagation of radio signals through the ionosphere with irregularities in the electron density distribution may produce phase and amplitude scintillations in radio frequency (RF) signals (Aarons, 1982; Yeh and Liu, 1982). These scintillations degrade the accuracy of navigation and positioning applications based on Global Navigation Satellite Systems (GNSS). Several authors point out (Skone et al., 2001; Sreeja et al. 2012; Vani et al. 2019; Guo et al 2019; 2020) that rapid phase variations lead to a higher probability of cycle slip. Depending on its severity, scintillation is capable of leading to the loss of satellite signal tracking. These extreme phenomena are more usual near the equatorial and low-latitude regions. In such regions, several electrodynamics processes may lead to large-scale plasma depletions in the F region of the ionosphere, called as Equatorial Plasma Bubbles (EPBs). The EPBs are large-scale ionospheric structures characterized by depleted electron density in comparison to the surrounding ionosphere, developed after the local sunset at the magnetic equator (Li et al., 2021; Bhattacharyya, 2022). The pre- reversal enhancement in the evening zonal electric field causes an increase in the eastward electric field that causes initial plasma depletions to rise to higher altitudes. This uplift contributes to an increase in the vertical growth of the magnetic aligned plasma-depleted structures formed by the Rayleigh–Taylor instability mechanism. (de Paula et al. 2007; Kelley, 2009; Hasse et al. 2010; Abdu 2016; Muella et al. 2017; Moraes et al., 2018b; Barros et al. 2018; Bhattacharyya, 2022). During the development of the EPBs, they tend to extend to latitudes of larger background plasma density in the Equatorial Ionization Anomaly (EIA), located at around 15°- 20° north and south of the magnetic equator (Muella et al., 2017; Li et al., 2021). Several researches describing the dynamics of the ionosphere and the ionospheric scintillation-producing irregularities are supported by GNSS data(for example see Mannucci et al. 1993; de Paula et al., 2007; Haase et al. 2010; Spogli et al., 2013; Guo et al., 2019; Macho et al., 2022). The GNSS simultaneous information in multiple https://www.sciencedirect.com/science/article/pii/S1364682622000463#bib37 10 frequencies can be used as a tool for ionospheric sounding, due to the dispersive medium of the ionosphere. From GNSS data, it is possible to perform ionospheric modeling using the total electron content (TEC) parameter capable to describe the properties of the ionosphere. TEC is the integral of electron density along the signal path counted passing through the ionosphere. The TEC is calculated in a column along a signal path between a satellite based GNSS transmitter and a receiver, typically ground-based (Seeber, 2003; Leick, 2004). Therefore, if the signal path from transmitter to receiver is known, the differential delay by two signals can be used to derive the TEC along the signal path. As the ionospheric delay is in very good approximation (better than 99.9%) proportional to TEC (see for instance Hernández-Pajares et al. 2014), combinations of signal frequencies and GNSS observables can be performed to obtain a relationship with ionospheric delay. Considering dual frequency measurements, TEC can be obtained using a linear combination, since the non-frequency dependent terms are cancelled out. The eliminated terms in the geometric-free combination are the geometric distance between the receiver and the satellite, the satellite and receiver clocks, and the tropospheric delay. If only single frequency is available, TEC can be eliminated or estimated using combinations between GNSS observables, for instance, the GRoup and Phase Ionosphere Correction (GRAPHIC), proposed by Yunck (1992), and the Code-Minus-Carrier (CMC) proposed by Xia (1992). Both combination exploits the opposite sign of the ionospheric error in the pseudorange and carrier-phase. The GRAPHIC combination is computed by the arithmetic mean of the code and carrier phases, removing the TEC term; while the CMC is given by differencing the pseudorange and carrier-phase measurements, all geometry- dependent components are cancelled. However, the main problem about these combinations are the multipath and noise of the pseudorange. It is worthwhile to highlight that the dual frequency ionosphere-free combinations for pseudorange and carrier-phase are only constructed from observations of the same type. In contrast to the GRAPHIC and CMC combinations, the dual frequency combinations are constructed from different observables, they maintain their basic characteristic properties and can be processed similarly to the original uncombined observations except for the increased noise (Teunissen; Montenbruck, 2017; Hernández-Pajares et al, 2018). Techniques and algorithms are being continuously developed and evaluated to improve the accuracy and resolution of TEC estimates. Accurately determining TEC values continues to be a challenging, particularly in low latitude regions and especially during ionospheric scintillation events. Prol et al. (2018a), for instance, evaluated the 11 performance of TEC calibration procedures by analyzing the improvement in single frequency PPP (Precise Point Positioning) considering several latitudes. The authors have found a worse PPP performance in low latitudes, mainly due to the ionospheric variability associated with the EIA and ionospheric scintillation. According to Moraes et al. (2018b) the distribution of plasma bubble irregularities in relation to the geomagnetic field configuration can amplify the effects of scintillation. In low latitude regions, such as the Brazilian region, the geomagnetic field configuration, characterized by a large magnetic declination angle, provides a particular and favorable condition for the occurrence of such effects. The authors showed a correlation between the intensified scintillation occurrence and Global Positioning System (GPS) signals that propagated through plasma bubbles aligned to the magnetic field's direction. In addition, the authors showed large PPP errors due to these events in such regions. Several authors (Delay et al. 2015, Spogli et al. 2016; Xiong et al.2016; Moraes et al. 2017; Souza et al. 2022) showed a close relationship between the occurrences of EPBs and severe ionospheric scintillations at different radio bands at low latitudes. In such regions, the lack of TEC data especially obtained using dual frequency measurements, can be associated with disruptions in the received signal caused by ionospheric plasma bubbles under ionospheric scintillation, which are zones with high electron density variability (Li et al. 2021). During ionospheric scintillation, the GPS L2 signal presents greater ionospheric interference when compared to the GPS L1 signal. Therefore, during the presence of ionospheric scintillation, it is expected that a single frequency can provide better alternative for ionospheric plasma bubbles imaging. Since the single GPS L1 frequency is supposed to minimize the loss of synchronism, ionospheric models based on GPS L1 frequency should improve the imaging of ionospheric bubbles. To enhance the imaging of ionospheric bubbles during ionospheric scintillation in low latitude regions, this work aims to develop and analyze a regional single frequency ionospheric model for TEC estimation. 12 1.1 Objectives The main goal of this research is to develop a methodology capable to detect equatorial plasma bubbles using only GPS single frequency measurements. In this regard, the Ph.D. research has the following specific objectives: (1) investigate which GPS signal frequency is the best one to detect equatorial plasma bubble; (2) develop an ionospheric regional model exclusively based on single-frequency GPS measurements; (3) understand if plasma bubbles can be detected using single frequency GPS measurements; (4) verify if the single frequency precise point positioning (PPP) based on single frequency ionospheric regional model can provide a comparable level to those ionospheric models obtained by dual frequency GPS measurements. 1.2 Content and Contributions The thesis can be summarized in two purposes: First, we evaluate and understand which GPS signal has the best capability of ionospheric bubble imaging and whether it is possible to detect the ionospheric plasma bubbles using only a single frequency GNSS measurements. Second, we present an ionospheric model for the Brazilian region using a single frequency and evaluate its quality. Chapter 2 shows the study developed by Souza et al. (2022). The authors presented an assessment of GPS positioning performance using different signals in the context of ionospheric scintillation. Chapters 3 and 4 are devoted to investigating the feasibility of detecting ionospheric plasma bubbles using GNSS single frequency measurements, i.e., it answers if it is possible to detect ionospheric plasma bubbles using single frequency TEC maps. In Chapter 3, we present a technique for TEC single frequency calibration, and we use a previously estimated TEC to generate ionospheric maps. As a result, ionospheric plasma bubbles are successfully detected using single frequency data. However, the calibration method uses external ionospheric models based on dual frequency data. Therefore, in order to use only single frequency data in all steps, 13 Section 4, also summarized in Christovam et al. (2023a), shows a methodology based on a regional model to calibrate GNSS single frequency measurements and derive ionospheric maps with no external information. In Chapter 5, Christovam et al. (2023b) present an evaluation of a new regional model by means of single frequency precise point positioning (PPP), comparing the positioning results against the correction using dual- frequency GPS signals, as well as compared to the corrections provided by global ionospheric models produced by the international GNSS service (IGS). 82 CHAPTER 6 FINAL REMARKS In this work, a methodology capable to detect equatorial plasma bubbles using only GPS single frequency measurements was proposed. For this purpose, it was investigated which GPS signal frequency is best suited to detect equatorial plasma bubbles. Two TEC single frequency estimation procedures were presented. Finally, the quality of TEC obtained by the proposed method was evaluated based on PPP. Chapter 2 presented the results of a quantitative analysis of the scintillation amplitude of GPS signals at L1, L2 and L5 frequencies, aiming to evaluate the impact of the ionospheric scintillations on the GPS frequencies. During ionospheric scintillations, a smaller magnitude was observed for the GPS L1 frequency. Therefore, it can be concluded that L1 frequency is the most suitable signal for ionospheric plasma bubble imaging, since it is the least affected by the ionosphere. Chapters 3 and 4 answer if it is possible to detect plasma bubbles using single frequency measurements of GNSS. In Chapter 3, TEC single frequency calibration procedure was proposed. TEC single frequency was calibrated using TEC values retrieved by global ionospheric models. As expected, TEC values obtained with single frequency are noisier than the dual frequency values, due to the combination between the observables CMC. However, it is important to highlight that code noise and multipath issues from the combination were not able to impair the plasma bubble imaging. Therefore, it can be confirmed that it is possible to detect ionospheric plasma bubbles by single frequency TEC data. The calibration procedure is an alternative to estimate TEC with less computational effort. This approach can provide new opportunities, particularly for ionospheric sounding. In addition, an increasing interest in the precise positioning with low-cost receivers is recognized. The number of low-cost GNSS receivers has been continuously increasing, especially with the growing number of cell phones that make single frequency GPS receivers with performance available for positioning (Gikas and Perakis 2016, Hernandez-Pajares et al. 2018). In future, it would be relevant to evaluate the capabilities of the proposed calibration method with low-cost cell phone data. Although the calibration method requires less computational effort, it also requires external information, which was estimated using dual frequencies. Therefore, in order to estimate the TEC using exclusively single frequency data, a regional model to calibrate the single frequency GNSS measurements and derive TEC values was proposed in 83 Chapter 4. The regional model also confirmed the possibility to detect ionospheric plasma bubbles by single frequency TEC data in regions with high number of GNSS stations. The developed method was capable of diagnosing the spatial and temporal progress of ionospheric plasma bubbles in VTEC maps and keograms. The results were confirmed by comparing the estimated TEC depletions with accurate airglow images. In addition, it can be seen a good agreement between the STEC data obtained with dual and single frequency. Finally, Chapter 5 presents an evaluation of the performance of the regional model based on single frequency measurements. The assessment is made by means of single frequency PPP, comparing the positioning results against the correction using dual- frequency GPS signals, as well as compared to the corrections provided by GIMs produced by IGS. As a result, the positioning performance using single frequency model presented similar accuracy to the dual frequency models and, at the same time, provided less observations affected by ionospheric scintillations. These results demonstrate the feasibility of using single frequency GNSS data to develop ionospheric models and to improve the positioning over low latitudes. In the future, the following investigations are recommended: (1) to perform an assessment of single frequency ionospheric models using other evaluation techniques, like using ionosonde data (Jerez et al. 2020) and dSTEC values (Hernadez-Pajares et al, 2017; Zhao et al. 2021) and (2) to perform the estimation of TEC single frequency uncertainties since they can provide an additional improvement in the single frequency PPP (Jerez et al. 2022), in particular for low latitude regions. With the GNSS modernization program, the new frequencies present more complex signal structure with specific functionalities. Therefore, it would be interesting investigate the performance of the ionospheric model proposed for different channels/codes at L1 modernized signal (e.g., L1C(D), L1C(P), L1C(D+P), L1(Y), L1(M)). 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