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Signed transmissions help with most spoofing, but it is still possible to replay (slightly out of date) signals received from the satellites, isn't it? Receivers with good internal clocks might be able to detect the delays, but they'd still be jammed.

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> isn't it?

No, it isn't. Not unless you intentionally design the protocol to be vulnerable to replay attacks.

It's not really a replay attack. GPS is unique in that it's purely time sensitive, so unlike traditional cryptography you're not trying to replay the signal a second time, you're trying to delay the signal. Replay protection won't work - what an attacker would do is jam the original signal first for some minutes to introduce uncertainty, then replay with a slight delay. Now with experience from traditional cryptography we could think this is avoidable - you just have to detect this new delay. But relativity says you can't. The shift is fully consistent with you having moved, unless you know your position independently it's completely impossible to know if the relative delay changed due to a position change or due to an attack. This attack is pretty difficult to pull off, but it has been done. You can't protect against it cryptographically, but if you have a better INS and onboard clock with less drift you can make it harder.

How could you "jam and replay". It seems like you'd have to choose one or the other.

Why would you have to? You jam the target, not yourself, then you replay the signal louder (you could also be smarter in how you jam so your signal gets through)

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