Mission analysis tool completion: solar indices investigation for Low Earth Orbits and occultations implementation

This thesis mainly reports about two topics: the Earth's atmosphere, the empirical models that modelise it with their solar and geomagnetic indices and their forecast methods, first, and then the occultation of planets and stars by the Moon as seen from the spacecraft's telescopes. The two topics are involved in a Thales Alenia Space's internal software, named NODES. It is a numerical orbit determination software used for years inside the company in Fortran code, but recently translated into Matlab language, and used to simulate the spacecraft's orbit when it is influenced by external variable perturbations. The first topic is particularly important for a Low Earth Orbit Mission Analysis, as the drag is one of the main factors that influences the spacecraft's orbit, and causes the orbit altitude decay . The drag is strictly connected to the density of the atmosphere that varies based on the layers in which the spacecraft flies; the density is influenced by the solar activity, such as solar flares, for instance, and by the geomagnetic activity. Therefore, models such as Jacchia-Bowman 2006 and Jacchia-Bowman 2008, or NRLMSISE-00, are investigated to find which one has the best performances in modelling the atmosphere's density. In the phase of orbit propagation, it is very important to predict short-term and long-term solar and geomagnetic indices, and that is what is examined, i.e. the forecast method on which the three previous models rely on. The second topic is a completion of the NODES code in order to improve it. First, the passages of the Moon in the Field of View of the spacecraft' telescope or Star Tracker, mounted on two different satellite axes, were studied, then the occultation of planets or stars by the Moon were studied, during the motion of the satellite on its orbit. In the end, two other scripts were written, one for the observation of the sky, returning which stars and planets are occulted by the Moon during each time step, the second is to be launched after the main analysis, in a later time, but the scope is always to see if the Moon has occulted any celestial body. Moreover, a considerable part of the work was dedicated to the software testing in order to find any bugs and to improve its performances.