الفهرس 
Chapter One “IntroductionOnly 14 pages are availabe for public view
 |  | from | 182 |  |  |
| | | from | 182 | | |
Abstract
Modeling of the ionosphere has been a highly interesting subject within the scientific
community due to its effects on the propagation of electromagnetic waves. The
development of the Global Positioning System (GPS) and creation of extensive groundbased
GPS networks started a new period in observation of the ionosphere, which
resulted in several studies on GPS-based modeling of the ionosphere.
The ionosphere reacts to geophysical events, such as earthquakes, tsunamis, surface
explosions, Underground Nuclear Explosions, etc. The (GPS) enables monitoring of the
ionospheric disturbances excited by these events. The purpose of this dissertation is to
use GPS observation to estimate Differential Code Bias (DCB) for satellites and different
types of receivers and calculating Vertical Total Electron Content VTEC maps.
The DCB is the differential hardware (e.g., the satellite or receiver) delay that occurs
between two different observations obtained at the same or two different frequencies.
Three receiver types based on the pseudo-range observables were used to collect the GPS
data: Codeless Tracking, Cross Correlation and Non-Cross Correlation styles. According
to its types, GPS receivers have been responded to restrictions on the GPS signal
structure in different ways. There are two approaches used to estimate Differential Code
Biases (DCBs) for receivers and satellites: the relative and absolute methods. The relative
method utilizes a GPS network, while the absolute method determines DCBs from a
single station (Zero Difference).
The main goal of the current research is providing a method to determine the DCBs of
GPS satellites and dual frequency receivers. The developed mathematical model was
based on spherical harmonic functions and geometry free combination of pseudo-range
observables (C/A or/and P-code) according to receiver type. A new elevation-dependent
weighting function with respect to GPS satellites in our algorithm was applied. The
applied weighting function was used to consider the variation quality of satellite DCBs,
which is caused by pseudo-range measurement errors. Ionosphere modeling software was developed within the study using MATLAB®, the
code of the proposed mathema
The software uses two different algorithms for the
modeling of the VTEC of the ionosphere and DCBs for satellites and receiver, namely,
Spherical Harmonic Analysis (SHA) and Spherical Harmonic Cap analysis (SHCA)
models. To evaluate the performance of the developed software, data from International
GNSS Service for Geodynamics (IGS) GNSS stations and different types of GPS stations
out of IGS network installed in Egypt and Saudi Arabia were tested. The estimated values
from ZDDCBE code show a good agreement with the IGS analysis centers with a mean
error of estimation for the receiver DCB not more than 8%. Therefore, ZDDCBE code
can be used to estimate the DCB for any type of receiver while the receiver from IGS
network or not.
Also, the influence of the quality of ionospheric model and DCBs of satellites and
receiver on the accuracy of Precise Point Positioning (PPP) was investigated. The results
of analysis for several data of GPS stations showed that using estimated DCB by
ZDDCBE program gave a precise point position more than IGS DCB products.
Finally, the new proposed MATLAB code was applied on a GPS station installed in
HAIL (Saudi Arabia) to estimate VTEC values. And so the new proposed MATLAB
code was capable on modeling VTEC on regional and global scale.