there is quite a lot of research on purely camera-based localization of drones. have a look at this video for example: http://www.youtube.com/watch?v=3mNY9-DSUDk
there is quite a lot of research on purely camera-based localization of drones. have a look at this video for example: http://www.youtube.com/watch?v=3mNY9-DSUDk