CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND TO THE STUDY
The first Global Navigation Satellite System (GNSS), known as Global Positioning System (GPS), was developed and maintained by US Department of Defense (DOD). In 1994 GPS achieved the full operational capability (FOC) with three segments: the space segment, the control segment and the user segment. The space segment is composed of a constellation of 24 satellites in six orbital planes at an altitude of approximately 20200 km above the earth surface. The orbiting period is about 11 hours and 56 minutes. The control segment contains a global tracking network of 16 monitoring stations and two master control stations which send commands and data to the constellation (NOAA, 2012). The user segment includes lots of applications such as positioning, navigation and timing.
The positional accuracy of GNSS is affected by several errors such as satellite and receiver clock errors, signal propagation delay errors due to ionosphere and troposphere, multipath error, receiver measurement noise and instrumental biases. Among all the error sources, ionospheric delay is the most predominant one and is of the order of 5-15m during mid-afternoon (El-Rabbany, 2002).
For the last seventy years scientists have studied the Earth’s ionosphere quite extensively through the use of multiple techniques (i.e., radiosounders, Faraday radars, top-side soundings from satellites, GPS signals, etc). Surprisingly, however, most people are unfamiliar with its existence, despite the fact that the ionosphere plays an integral role in many of their everyday activities. Extreme variations within the ionosphere induced by storm enhanced density (SED) events and responses to magnetic storms can adversely affect navigation and communication systems on Earth (Skone and Coster, 2008; Araujo-Pradere and Fuller-Rowell, 2002; Araujo-Pradere, et al., 2002).
The ionosphere is a part of the upper atmosphere, starting at height of 50 km and extending to 1000 km. In that region free electron density affects the propagation of radio frequency electromagnetic waves (Alcay et al., 2012). When solar radiation strikes the atoms and molecules of the upper atmosphere, electrons are dislodged through the process of ionization. Because of the chemical composition of the atmosphere, the shorter wavelengths of solar radiation (the extreme ultraviolet and X-Rays) are energetic enough to ionize these atoms and molecules in the Earth’s atmosphere. Depending on the energy of the incident photon, photoelectrons (i.e. - electrons emitted from an atom or molecule by an incident photon) are produced, with great enough energies to ionize other nearby neutrals or moleculars. Particle precipitation occurs when energetic particles are injected into the upper atmosphere and collide with neutrals. This process produces increased and variable levels of ionization (Bothmer and Daglis, 2007). In summary, highly variable production, loss, and conditional transport of ionization are responsible for the extremely variable ionosphere.
There are various ways to study ionospheric irregularities; one of them is the total electron content (TEC) observations derived from a network of GPS stations using dual frequency measurements (Tiwari et al., 2009). TEC is defined as the integral of the electron density along a trajectory, usually, vertical from the Earth’s surface up to a given height in the ionosphere (hence, vertical or vTEC) or the line-of-sight (LoS) from......
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