الفهرس 
Chapter One “IntroductionOnly 14 pages are availabe for public view
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Abstract
The Global Navigation Satellite System (GNSS) depends mainly on precise determination of signal travel time. On the other hand, the signal is permanently delayed because the atmosphere layer causes refraction and diffraction to this signal during its journey from the satellite to the receiver. This delay describes the influence of the atmosphere layer on GPS signals and considered one of the main sources of errors that affect the GPS positioning accuracy which should be mitigated in order to determine reliable positions using GPS observations. Therefore, several studies on the atmosphere modeling has been performed with the development of the GPS in order to overcome this major error.
The first goal of this thesis is developing a new code for monitoring the ionosphere effect on GPS observations using MATLAB® called “Vertical Ionosphere Delay Estimation (VIDE)”. This code estimates the vertical ionospheric delay for any single station using a dual-frequency receiver. It utilizes a spherical ionospheric shell model which depends on the so-called geometry free linear combination of GPS observations to exploit the dispersive property of the ionosphere.
However, either carrier phase or pseudo-range measurements can be used within this model. In this code, the carrier phase observations are used in order to avoid unwanted effects of pseudo-range measurements because the noise level of carrier phase measurements is lower than the pseudo-range. A combination of dual-frequency carrier phase and code data is used to estimate the ambiguity after repairing cycle slips. Melbourne-Wübbena Linear Combination of dual-band phase and code observables is used for detecting and repairing cycle slips in the current study. Sequential least square method is applied for estimating the unknown coefficients in the proposed model.
The second goal of this research is evaluating the effectiveness of the proposed code by testing data from International GNSS Service (IGS) network. Thus, VIDE program was applied on ten IGS stations from different regions all over world in two different days. The estimated values from VIDE code show a good agreement with the results of GAPS software, which means that the ionosphere effect was successfully monitored using this proposed code. The third and the fourth goals of this dissertation, are monitoring the ionosphere effect over Egypt and studying the influence of geomagnetic storms on vertical ionospheric delay estimation, respectively. Therefore, VIDE program was applied on six stations located in Egypt during periods of stormy and quiet conditions. The results showed a big difference between stormy and quiet observations, which means that the geomagnetic storms can disturb the ionospheric activity extremely. Finally, this thesis presents a new code capable of estimating the vertical ionospheric delay for any dual frequency GPS receiver which can be used by GPS users.