Orbit reconstruction using GNSS-derived state vectors

The lower thermosphere (between 100 and 550 km of height) is known with lesser accuracy than desired. The use of spherical satellites to determine the density could result in a substantial improvement of the present situation. The orbits of satellites at these heights are affected by drag and the inhomogeneities of the terrestrial gravitational potential (among other perturbations). This projects has as main goal to analyse how the atmospheric drag affects the orbit of a real satellite, and then, determine the thermospheric density. Hence, it will be necessary to reconstruct the orbit accounting for the perturbations in the most accurate way possible. Regarding the gravitational potential of the Earth, we have employed three approximations: spherical symmetry, a zonal model truncated at sixth order and a tesseral model truncated at tenth order. This last model has revealed as the most accurate. To achieve our goal, we have analysed the telemetry gathered by one of the satellites of the FSSCat mission. Then, we have determined its orbital elements. Afterwards, we have considered two possible methods for orbit reconstruction: Non-Linear Least Squares and Extended Kalman Filter. After this analysis, we have chosen the Non-Linear Least Squares method for the purpose of orbit reconstruction. The orbital elements determined from a physical analysis of the telemetry and the ones resulting from the non-linear least square method coincide with a remarkable accuracy.