That should also have been foreseeable.

It was foreseen. There is a huge amount of literature on hardening of GPS from the begining of the technology. Here's so random examples:

https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2161-4296....

https://apps.dtic.mil/sti/html/tr/ADA057265/

https://www.ion.org/publications/abstract.cfm?articleID=1201...

https://repository.arizona.edu/handle/10150/609916

So what was done? Has GPS been hardened, for example?

I literally provided you with sources that describe Anti-Jam GPS designs.