If the thing is flying fast then no problem because the mission does not take a long time. In a more useful application (e.g. a ship) it's difficult to reach true autonomy, because of number of factors. Periodic correction seems more practical approach. That again brings us to a global navigation system. GNSS works fine but it can't be the only option because who's said the nature will always be on the our side? Everyone knows that but little is achieved so far because of no commercial interest.
AFAIK in the Trump era, the government program to develop a backup navigation system was announced and the three companies were asked for proposals. No idea about any real outcome from that. Anyway it's not about new technology. Such systems were in use well before satellites. They all were using low frequencies so the antennas were bulky. But it's not inevitable. Seen a nice tiny part that combines GPS patch antenna and H-field e-LORAN antenna. Designed specifically for mobile phones. So the only problem is commercialization. No enemy no funding. Seems not any more.