BBC News reports that a new study accuses Russia of massive GPS spoofing (see also this linked blog article). I am surprised that this is so easy¹, in particular since GPS originates as a military system. Couldn't such spoofing be quite simply prevented by signing each message with a private key? Has this been done and have those keys been stolen? Or is there some (fundamental?) reason why such a protection is not feasible for GNSS?
I would expect that if this were simple, it would be done. Why isn't it?
¹Russia is also able to pull off feats that are not easy, but I've been told that this particular one happens to be quite easy.