الفهرس 
AcknowledgementsOnly 14 pages are availabe for public view
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Abstract
Introduction: One of the most significant error sources which affect the GPS signal propagation time is the zenith tropospheric delay (ZTD). Estimating ZTD is very useful for meteorological applications and weather prediction. ZTD computation is also very important in geodetic studies to improve position accuracy. Many precise point positioning (PPP) online services developed in last years to provide GPS products such as positioning and ZTD value. Research scope: This research aims to investigate the accuracy of the precise point positioning (PPP) online services used in the study (CSRS- PPP, APPS, GAPS and MagicGNSS) in estimating the zenith tropospheric delay (ZTD) value. . Research objectives: This research evaluates the accuracy of the ZTD products from the PPP online services, introduces the best performance of different PPP online services in ZTD estimation at different study areas (Europe & Egypt) and analyzes the estimated ZTD in Europe and Egypt along the four seasons of the year using PPP online services.. Steps Study: In this thesis, firstly, the performance of the online precise point positioning services (PPP)in estimating the zenith tropospheric delay (ZTD) is tested by using observation data of four whole days from eight IGS stations in Europe. The observation time for the eight IGS stations is chosen to cover the weather conditions through the four seasons in Europe. The observation periods are selected in four days, the 22th of March, the 22th of June, the 22th of September and the 22th of December of year 2017. The IGS troposphere product is taken as a reference which the online services APPS, CSRS-PPP, GAPS and MagicGNSS ZTD products compared with. Then a case of study in Egypt is performed by using observation data of four days from six stations in Egypt. The observation time for the six stations is chosen to cover the weather conditions through the four seasons in Egypt. The observation periods are selected in four days, the 26th of January, the 20th of March, the 1st of June and the 21th of September of year 2014. Based on the first study about Europe, the APPS service is taken as a reference which the other online services compared with, because it gave themost reliable results for ZTD value, and GAPS service is eliminated because it gave the weakest accuracy for ZTD value. Summary of Study: Estimating zenith tropospheric delay (ZTD) by using PPP online services in Europe and Egypt have been extensively investigated in this thesis. This thesis analyzes the accuracy of each online service in eight IGS stations in Europe comparing to the IGS tropospheric product to judge each service, then the PPP online services are tested by applying them in six stations in Egypt and comparing their results to APPS service to judge them as APPS service gives the most reliable results in Europe.