The project aims at the investigation of a modular, open-source system architecture of avionics especially for the challenges of small unmanned aerial vehicles. Using modular hardware and open-source software, an aircraft electronic device shall be set up and used for flight control. In addition, a navigation system utilizing GNSS, inertial sensors, barometer and magnetometers will be implemented. To complete the system, a C2 link and aircraft antennas integrated into the fuselage are considered. For small unmanned aerial vehicles with limited transport capacity only light-weight low-cost sensors with reduced quality are eligible for the navigation module. On the other hand, existing autopilot systems usually do not provide the required accuracy and do not provide any quality information of the navigation parameters. As a result, the optimal solution concerning accuracy and reliability will be achieved utilizing a self-implemented and tailored loosely coupled extended Kalman filter to integrate the different sensors (GNSS, IMU, barometer and magnetometers). To increase the safety margins for unmanned aerial vehicles, one focus of the research is to find out how to define and calculate reliable quality measures for the outputs of the navigation module.