Commercial Flights Are Experiencing ‘Unthinkable’ GPS Attacks and Nobody Knows What to Do::New “spoofing” attacks resulting in total navigation failure have been occurring above the Middle East for months, which is “highly significant” for airline safety.
But if they had authentication you would know that the message doesn’t come from a legitimate satélite.
If their isn’t then there’s a big problem with implementing that now, which would require a retrofit of every single GPS system currently in use and likely a replacement of all GPS satellites
Edit: I’m slightly mistaken, the military uses encryption but they don’t have that open for public use.