Yes that's what spoofing is, but why wouldn't they be able to?
(EDIT: I see the other reply thread is already asking the same thing, didn't intend to ask about the same thing)
Because, due to how cryptography works, nobody other than the entity holding the signing key (ie the one that deployed the satellites) can produce valid signatures for that key.
Because, due to how cryptography works, nobody other than the entity holding the signing key (ie the one that deployed the satellites) can produce valid signatures for that key.