الفهرس 
ACKNOWLEDGMENTOnly 14 pages are availabe for public view
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Abstract
Nowadays, monitoring the ionosphere layer density and variation became
highly demanded by the scientific community because of its wide applications in radio and telecommunications, satellite tracking, earth observation from space, and satellite navigation. Reliable Global Ionosphere Maps (GIMs) and accurate Differential Code Biases (DCBs) of satellites and the International
Global Navigation Satellite Systems (GNSS) Service (IGS) stations can be
obtained from Ionosphere Associated Analysis Centers (IAACs). However, the
IAACs DCB receivers’ values are only available for IGS stations. Furthermore, some of the IGS ground receivers’ DCBs estimates are not available from all analysis centers. Also, some regions do not have any IGS ground stations, like our country Egypt, which means the Total Electron Content (TEC) values over them would be interpolated from the nearest calculated values. Unlike the interpolation process of TEC maps made by IAACs over areas have no IGS
stations, TEC mapping tool with a temporal resolution of 1 h based on dual frequency observations of Global Positioning System (GPS) network was
developed. This tool was called Global TEC Mapping (GTM) which was written under MATLAB environment and can be used globally to produce GIMs from local GPS networks in several areas with low number of IGS stations or without. In addition, the GTM can calculate satellite and receiver
DCBs. GTM is based on the Spherical Harmonic Function (SHF) and a
geometry-free combination of GPS carrier-phase and pseudorange code
observations. The weighted least squares function was applied to solve
observation equations and to improve estimation of DCBs values and the SHF
coefficients. The used weight function is dependent on the satellite elevation
angle. A global IGS network (BOGO, BRUS, GOPE, GRAS,MADR, ONSA, POTS, PTBB, SOFI and WTZA) data was used to calculate the satellites and receivers DCBs. By comparing the DCBs results of GTM to another similar programs, the receivers and satellites DCBs show a better agreement with those of IAACs than of the other programs.The results of weighted least squares show an improvement for estimated DCBs, where mean differences from the Center
for Orbit Determination in Europe (CODE) are less than 0.746 ns. DCBs
estimated from the GPS network show better agreement with IAACsthan DCBs
estimated from single station, where the mean differences are less than 0.1477 and 1.1866 ns, respectively.
Another IGS network stations (BRUX, FFMJ, GOPE, HUEG, LEIJ, OBE4, POTS, PTBB, WSRT and WTZZ) was used to validate the output TEC values of GTM. As the geomagnetic storm field affect directly on the TEC, three stormy days and other three quiet days were chosen for validating the developed model. The estimated TEC resulted from the model firstly were compared with CODE and International Reference Ionosphere (IRI) TEC results using the data of ten European IGS stations. The comparison of the results shows a convergence between CODE and GTM estimated TEC and the results of the IRI values was far from both specially in the stormy days. The maximum differences between GTM and CODE results were 2.52 TECU and
1.31 TECU for the stormy and quiet days, respectively. Then, the GTM program was used to generate TEC maps of Egypt using a regional network of seven stations with a temporal resolution of 1 h and 2 h.
To study the effect of the ionosphere error on the estimation of the Precise
Point Positioning (PPP), the output TEC values of GTM were saved in the form
of IONosphere Map EXchange Format (IONEX) file and it was used as the
ionosphere correction values of the RTKLIB post processing program. The
seven stations’ coordinates were estimated four times, the first one by using GTM IONEX file, in the second time CODE IONEX file was used, the third time by using IRI IONEX file and the last time no ionosphere correction was used. The average and standard deviation values of the output coordinates of the seven stations were computed. The results show the values of GTM were the most consistent and precise.