One of the key features of an aircraft is the Navigation System, which is responsible for the evaluation of the actual localization during a mission. Moreover, the Navigation System is the interface of determination of position and velocity, and all the aircraft parameters thanks to the on-board sensors. Navigation is usually carried out thanks to the integration of several sensors and technologies. The Global Navigation Satellite System (GNSS) and the Inertial Navigation System (INS) are widely used, combining the precision in estimating the position and velocity of the former with the high update rate of the latter. The integration of GNSS and INS yields high performance and accuracy in a nominal scenario, under ideal conditions.