My guess would be that the best solution is multi sensorial anyways. So you have GPS, but then you would have an Inertial Measurement Unit, based purely on acc

My guess would be that the best solution is multi sensorial anyways. So you have GPS, but then you would have an Inertial Measurement Unit, based purely on acceleration and inertia, radar and barometric systems and maybe optical systems for terrain identification and celestial navigation. I have recently heard an episode of Amp Hour podcast¹ where they talked about using Bluetooth from satellites as a GPS replacement for asset tags. So maybe alternatives to GPS on different bands are on the verge. Another interesting idea would be to have an antenna array on the plane that captures different radio sources and the angles from which they seem to come from and create a always updating radio activity map that maps a certain "radio landscape" to a position on the planet. AFAIK this is currently being worked on as well under the name "radio-SLAM" (with SLAM standing for Simultaneous Localization and Mapping). This means, jammers could maybe even be used as potential beacons themselves. ¹: The episode in question: https://theamphour.com/728-space-age-bluetooth-with-alex-haro/ https://theamphour.com/728-space-age-bluetooth-with-alex-har...