Galileo already contains this function for the navigation message via OSNMA, and GPS CHIMERA is soon to be operational, with the latter actually including crytographic "signatures" in the spreading code itself, so if you use these two constellations you become really harder to spoof.

Of course, they dont protect against jamming.

Because an attacker can just replay legitimate broadcasts with slightly skewed time and origin and introduce huge errors into the fix.

Why would that make spoofing impossible?