The article about astro-inertial navigation systems, as found on the SR-71, is interesting as well: http://www.thedrive.com/the-war-zone/17207/sr-71s-r2-d2-cou

The article about astro-inertial navigation systems, as found on the SR-71, is interesting as well: http://www.thedrive.com/the-war-zone/17207/sr-71s-r2-d2-could-be-the-key-to-winning-future-fights-in-gps-denied-environments http://www.thedrive.com/the-war-zone/17207/sr-71s-r2-d2-coul... I imagine the military has also developed downward-looking computer-vision and/or radar based positioning systems.