It's on the high end of hardware for amateur projects, but it's incredibly cheap as differential GPS Real Time Kinematics solutions cost: the Piksi. http://www.swiftnav.com/piksi.html $2000 for a pair. No data on its response to high acceleration, but it could be made to work with a hovering rocket. It is still definitely in development though. Not to be confused with the Pixy, which is a different microcontroller project that does fast machine vision: http://charmedlabs.com/default/?page_id=211 Could probably be used as a highly effective sun tracker or in a rocket launch camera tracker.