1. Computer Vision and Aerial Robotics Group, Universidad Politécnica de Madrid (CVAR-UPM), 28040 Madrid, Spain
2. Centre for Automation and Robotics C.A.R. (UPM-CSIC), Calle Jose Gutierrez Abascal 2, 28006 Madrid, Spain
3. Department of Artificial Intelligence, Universidad Politécnica de Madrid (UPM), 28031 Madrid, Spain