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Global Navigation Satellite Systems (GNSS) have an endless number of applications in industry, science, military, transportation and recreation & sports. Two systems are currently in operation namely GPS (the USA Global Positioning System) and GLONASS (the Russian GLObal NAvigation Satellite System), and a third is planned, the European satellite navigation system GALILEO. The potential performance improvements achievable through combining these systems could be significant and expectations are high. Computer software can be used to simulate the overall process of GNSS (signal transmission and reception) and produce realistic simulated GNSS measurements. Using such simulated measurements, current and future GNSS systems and possible new applications of GNSS can be investigated. The potential impact of using GPS and GALILEO on LEO satellite orbit determination could be investigated on Topex/Poseidon mission which real GPS data was available from the beginning of this study. The performance of GPS, GALILEO, combined GPS/GALILEO and combined GPS modernised/GALILEO constellations have been investigated in relation to the reduced dynamic LEO satellite orbit determination.

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Dr. Ashraf Farah is an associate professor (Space Geodesy)at Faculty of Engineering, Aswan university, Egypt.Ph.D. degree from the university of Nottingham, United Kingdom (2004). Published over 45 research papers. Participated in early studies for GALILEO (European satellite navigation system). visiting scholar; Darmstadt univ., Germany.

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