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Abstract
With the ever increasing sophistication of satellites in low Earth orbit the ability to precisely predict the position and velocity of the satellite is extremely important. This need has been amplified in recent years by the development of Earth observation and meteorological satellites, which need to make measurements of the Earth{u2019}s atmosphere, gravity field, sea surface height, etc. with unprecedented precision. This has led to the development of advanced numerical methods, which allow a precise orbit to be calculated on the basis of observation data from GPS Signals as measured by the LEO satellite. Additionally, the observation data is often required by the user in 2near real- time3 (for use in, for example, numerical weather prediction simulations). This means that the data must be downloaded from the satellite every orbital revolution, and must then be processed to an accuracy of less than one meter in position and distributed to users within approximately one hour of downloading. This task presents a formidable challenge, but, with the use of numerical estimation methods, accuracies of 2-3 cm in position can be achieved, well within requirements. The present thesis depicts the algorithms concerning the orbit determination process using global positioning system starting from the data contained in the navigation message broadcasted by GPS satellites through point positioning using pseudorange till the various orbit determination strategies using different types of observables or navigation solution