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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.

The lower thermosphere (between 100 and 250 km) is badly known due to the scarcity of operating satellites in this region. Nevertheless, it is very relevant for the study of Earth-Sun relations, satellite re-entry forecasting, and climate change, among other disciplines. We propose the analysis of a swarm of femtosatellites to directly analyze the thermosphere by means of a MEMS accelerometer onboard each satellite. The goal is to use the deceleration caused by drag to determine the local density, thus drawing a map of the thermosphere on multiple locations. The shape of the satellites will be spherical (with a diameter of 5 or 10 cm), thus simplifying the measurement of drag. The mass distribution will be that of a spherical top (then, no attitude control), or will have the center of mass separated from the geometric center (thus providing aerodynamic stability). Each satellite will also count with a MEMS GNSS receiver that will provide precise location and time-tagging to the measurements. A small onboard computer, mass storage system, radio transmitter, and primary battery will form the bus of the satellite. The goal of this Master Thesis is to develop a self-consistent mission analysis of the mission, with special interest in comparing the properties of the non-stabilized and aerodynamically stabilized satellites. The main objectives must be: Σ Design and analysis of the optimal orbit for these satellites. Of particular interest is the study of the different alternatives for orbital injection and dissemination. Σ Analysis of orbital decay using DRAMA. Evaluation of the risk for third-party satellites posed by a large swarm. Proposals for decreasing this risk. Σ Data gathering strategy. Σ Sources of noise for the accelerometer, both instrumental and environmental. Σ Communication strategy. Determination of the link budget, proposal for ground station typology and location. Σ Energy budget. Analysis of mission’s life limitations due to the endurance of the battery. Σ Alternative scenarios for mission success. Identify and evaluate scenarios in which the mission cannot be accomplished in the planned way. For example, analyze possible mission procedures should the accelerometer fail. This project is undertaken in collaboration with Prof. Igor Belokonov, Inter-University Department of Space Research, Samara University (Russia).