ABSTRACT
Low latitude areas are highly exposed to the effects of ionospheric scintillations caused by the activities in the sun’s corona. Ionospheric scintillation occurs when there are rapid deviations in the signal amplitude or phase frequency as radio signals travel through areas of non-uniformity in the ionosphere. These changes cause adverse effects on Global Navigation and Satellite Systems (GNSS) by producing propagation impairment hence disturbing the satellite systems accuracy in positioning. When scintillations are stronger, satellite receivers can lose lock posing threats to GNSS applications and positioning. Scintillation effects worsen during solar maximum. The receiver-satellite geometry is poorly messed up when the effects of scintillations are higher. Due to the adverse effects posed by irregularities in the ionosphere, there is need to carry out research to study the extent to which scintillation effects can affect navigation tools and propose solutions to improving precision on positioning as well as receiver satellite geometry. This research work presents a detailed study of occurrence of scintillations in the region of Darwin, Australia, at latitude 12.4637° S, a low latitude area. Ionospheric data for Darwin from sws.bom.gov.au/aims are used to analyze the extent of scintillations for the period ranging between January 2020 and April 2021. Amplitude scintillation index and phase scintillation index are used to plot graphs using the gnuplot. The results from the graph show that scintillation is prevalent throughout the period of study, an indication that radio frequency signals passing through the ionosphere is affected adversely. Being a low latitude area, GPS stations located in this place, will need to be monitored for scintillation. Ground based GPS data receivers can be positions at angles that lower the receiver distance from the space satellites. Radio signals with scintillations will have to be corrected before they can give a clear signal for communication.
TABLE OF CONTENTS
DECLARATION ii
DEDICATION iii
ACKNOWLEDGMENTS iv
ABSTRACT v
TABLE OF CONTENTS vi
LIST OF FIGURES viii
LIST OF ABBREVIATIONS/ACRONYMS AND SYMBOLS ix
CHAPTER 1
INTRODUCTION
1.1 Research background 1
1.2 Statement of the Problem 2
1.3 Research objectives 2
1.3.1 Main objective 2
1.3.2 Specific objectives 3
1.4 Research justification and significance 3
CHAPTER 2
LITERATURE REVIEW
CHAPTER 3
THEORETICAL FRAMEWORK
3.1 Ionosperic irregularities cause scintillation 8
3.2 Categorizing scintillation intensity 8
3.3 Signal amplitude and phase modeling 10
3.4 Amplitude and phase scintillations equations 10
CHAPTER 4
METHODOLOGY
4.1 Source of Data 12
4.2 Data analysis and tools 12
CHAPTER 5
RESULTS AND DISCUSSION
5.1 Scintillation index, S4 variation with time for January 2021 to April 2021 13
5.2 Amplitude and phase scintillation index for January to December 2020? 15
5.3 How satellite’s angle of elevation affects ionospheric scintillations intensity 17
5.4 Plotting angle of elevation for Satellites for May-December 2020 20
CHAPTER 6
CONCLUSION AND RECOMMENDATIONS
6.1 Conclusions 24
6.2 Recommendations 24
REFERENCES 25
LIST OF FIGURES
Figure 1: Wave propagation coordination system through the ionospheric irregularities 9
Figure 2: Variation of S4 as a function of time for January 2020 to august 2020. 13
Figure 3: S4 against time for September 2020 to April 2021. 14
Figure 4: Plots showing S4 and Sigma60 as a function of universal time for the months of January to April 2020. The red points show amplitude scintillations index, S4. Yellow plots represent phase scintillations index sigma60. All plots are for 20th day of each month 15
Figure 5: S4 and Sigma60 plotted against time for May to August 2020. 16
Figure 6: Phase and amplitude scintillations graphs for September to December 2020. 16
Figure 7: Plotting S4 as a function of satellite Elevation Angle (Elv) for tracked signal. The minimum Elevation Angle is 5o below which no signal can be tracked. The Maximum Elevation Angle is 90o. 17
Figure 8: S4 as a function of satellite Elevation Angle (Elv) for March 2021. 18
Figure 9: S4 as a function of satellite Elevation Angle (Elv) for April 2020. 19
Figure 10: S4 as a function of satellite Elevation Angle (Elv) for April 2021. 19
Figure 11: Graphs of S4 index against the angle of elevation for May and June 2020. 20
Figure 12: August and September 2020: the graph of S4 against the angle of elevation 21
Figure 13: Scintillations s4 index and angle of elevation for October and November 2020. 22
Figure 14: Scintillation S4 Index for December 2020: all days recorded scintillations greater than 0.3, adversely affecting GPS receivers. 23
LIST OF ABBREVIATIONS/ACRONYMS AND SYMBOLS
C/NOFS - Communications/Navigation Outage Forecasting System
GLONASS - GLObal NAvigation Satellite System
GNSS - Global Navigation Satellite System
GPS - Global Positioning System
ISMR - Ionospheric Scintillation Monitor Receivers
L1 - Level 1
L2 - Level 2
PLL - Phase Locked Loop
RF - Radio Frequency
SCINDA - Scintillation Network Decision Aid
CHAPTER 1
INTRODUCTION
1.1 Research background
The ionized part of our planet’s atmosphere called the ionosphere contains free electrons and ions in significant quantities enough to produce enormous interference on Radio Frequency (RF) signals. Fountain effect causes irregularities in plasma density (Davies, 1965; Liu et al., 2017) leading to ionospheric scintillation associated with rapid fluctuations in signal phase and amplitude when the RF signals traverse the non-uniform parts of the ionosphere. It is well established that geomagnetic activities and the sun modulate scintillation (Fallows et al., 2016; Guo et al., 2017).
In the solar cycles besides active geomagnetic storms, ionospheric turbulence increases hence affecting how radio signals propagate through space. The global distribution of ionospheric scintillation is such that more activities are prevalent in the polar and equatorial regions (Aarons, 1982; Basu et al., 1988).
The Ionospheric scintillation events lead to strong spatial and temporal dependencies. During autumnal and vernal equinoxes, there is found to be a higher degree of prevalence for scintillations (Mezaoui et al., 2014; Paznukhov et al., 2012; Prikryl et al., 2016). Scintillation shows global distribution during the auroral to polar as well as in the equatorial regions (Aarons, 1982; Basu et al., 1988).
The effect on scintillation on GNSS intensity fading on signals is a subject under extensive research. The relationship between the likelihood of cycle clips occurrence and fading duration was developed by (Oliveira et al., 2014) after they studied characteristic of fading from scintillation data ranging for a period of one month. They arrived at a finding that scintillation can severely lower the performance of GPS receiver with Carrier Power to Noise (C/N0) power threshold or above 30 dB Hz can be uncovered due to deep fading. For the scintillation intensity amplitude fading features on GPS L5, L1 and L2C signals at the regions around the equatorial zones, fading does not affect all the GPS bands simultaneously (Jiao et al., 2018). Seo et al. (2016) analyzed and characterized data based on commercial software receiver for 45 minutes and 50 Hz C/N0.
Moreover, a fading duration model beneficial for the design of aviation receivers having small reacquisition interval to reverse the hostile effects was built. The data however, were collected within a short time and not based on sets with severe scintillation events. These studies fell short of tackling the issues related to the challenges that signal fading has on the performance of receivers and how tracking loop performance is interrelated to fading.
1.2 Statement of the Problem
Is it possible to predict scintillations? In communication that relies on links between ground stations and satellites, scintillation happens to bear the greatest effect on trans-ionospheric radio signals. Most equipment for communication on the Earth surface depend on data that is sent from navigation satellites located in space exposed to adverse solar activities. Since these equipment send radio frequency signals through the irregular ionospheric layers, any amount of scintillation may end up degrading their operations. It is therefore crucial that the extent of scintillation and its effects on the GPS data be analyzed and then the answer to the question on what can be done to mitigate such effects can be developed. This research is intended to assess the extent to which ionospheric scintillations occur at low latitudes and its effects on the GPS data.
1.3 Research objectives
1.3.1 Main objective
The main objective of this research is to study the effects of ionospheric scintillation using GPS receivers located at low latitudes.
1.3.2 Specific objectives
The specific objectives of this research are as follows:
i. To acquire the GPS datasets comprising of S4 and sigma60 for the period ranging from January 2020 – February 2021 for low latitude area of Darwin from https://www.sws.bom.gov.au/World_Data_Centre/1/11/.
ii. To import this data into python and Gnuplot or Matlab and analyze the scintillation intensity.
iii. To model scintillation effect during the period of study.
iv. To provide recommendations on mitigation of the resulting scintillation effects at the end of this project
1.4 Research justification and significance
Satellites in space play an important role in collecting and relaying information for a range of uses including weather forecasting, global positioning and entertaining among others. Examples of areas where such data is used include phone towers, airports and marine navigation systems.
The security of these equipment in relation to the data received rely on accurate relay of information which can be compromised if there is degradation or interferences. Hence in cases of irregularities in the ionosphere, data received by ground receivers in ships, data centers would be compromised and the accuracy of information won’t be reliable. A study of low latitude ionospheric scintillations is therefore an important undertaking to provide not only an understanding of what goes on in the space, but also how the activities would affect communication. Once this is clearly understood, ways of mitigating the effects would be possibly laid out or a research on how to prevent the changes will then take place.
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