The Brits did a test quite a few years ago when someone said its easy to spoof GPS so they got a few navy boats in the Irish sea and a guy on the shore blocked the gps then sent wrong data.
Hence all EU govs kept investing in E-Loran
Doesnt matter whose GNSS you use they all suffer the same as they are such low power transmissions.
The US gov clearly knew this when the built it so they send the encrypted code down ( which is just same data we all get) and we assume they get almanac updates from shore to compare with the sat signal to know they have their own data??
PS if you work in high latitudes you should be getting updates from iridium sats, every one does that arent you?