The present document is written for the Master degree course in Aerospace engineering of Politecnico di Torino, during a six month thesis internship in Thales Alenia Space site of Turin (TO), Italy. Scope of the work is to build a simulation tool, in Matlab environment, of FGS attitude sensor, supporting tests on the Engineering Qualification Model of the same. The sensor is designed in the context of European Space Agency Euclid mission, whose optic scientific instruments require unprecedented pointing performances. Its main goal is to compensate thermoelastic deformations induced errors on attitude determination, being part of an innovative Attitude and Orbit Control System architecture.