Europe’s aviation safety body is working with the airline industry to counter a danger posed by interference with GPS signals - now seen as a growing threat to the safety of air travel.
Interference with global navigation systems can take one of two forms: jamming requires nothing more than transmitting a radio signal strong enough to drown out those from GPS satellites, while spoofing is more insidious and involves transmitting fake signals that fool the receiver into calculating its position incorrectly.
According to EASA, jamming and spoofing incidents have increasingly threatened the integrity of location services across Eastern Europe and the Middle East in recent years.
For Galileo they have something where basically each message contain something to authenticate the previous one. So this could be fully based on ground segment. Anyway, they have probably countless reasons to update software anyway considering the many services that were added after their launches years ago.
What GNSS satellites do is (approximately) timestamp a message they receive from ground. They don’t really know their position by themselves, they are clocks in orbit.