Modifying a scientific flight control system for balloon launched UAV missions

In this paper we present our work on enabling Balloon launched high altitude UAV Missions for an autopilot system previously used only at lower levels in visual line of sight conditions. One field of our research in the context of flying robotics is focused on high altitude pseudo satellites (HAPS). To gain operational experience in high altitude flying and for system and payload testing, a balloon launched small UAV (sub 10kg) system was designed including building an optimized airframe. Balloon launching was chosen because it offers fast and clearly regulated access to the desired altitudes. Our autopilot system has proven its capabilities in many years of flight experiments with different platforms (helicopter and fixed wing). The main characteristics are modularity and easy use for scientists. On the hardware level the task was to integrate the existing segmented systems of the research autopilot in a compact form factor, with the possible use in larger platforms in mind. The design was driven by the special thermal requirements resulting from flying in stratospheric conditions. In the autopilot software, several mission specific functions had to be added, which only required moderate effort due to the modular system design. Major changes included adding a flight termination manager. A launch routine was developed allowing a safe transition from free-fall to stable horizontal flight in thin air after being dropped from the balloon. Extensive testing was performed to validate the design. Simulating the mission, including balloon ascend, was used to check the mission software. Thermal and pressure conditions at altitude were replicated in a thermal vacuum chamber with additional sensors applied to identify problems. The simulation and control laws were verified by means of low altitude test flights.