Drones are usually designed to navigate outdoor spaces, they are often equipped with cameras and other sensors, making them a powerful tool for surveying large areas. In these environments a Global Navigation Satellite System (GNSS) is combined with an Inertial Measurement Unit (IMU) to achieve precise positioning, allowing successful navigation in a 3D open space. During the past years users started to use small size drones in challenging environments, indoor places, inside caves or near bridges, where a GNSS is not always reliable or reachable. Classical positioning techniques are no longer efficient in these cases, so it is necessary to develop new systems to adapt in these situations.