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Only against imperfect spoofing.

A GPS receiver in Los Angeles sees the same set of signals as a receiver in San Francisco - just the relative arrival times of the signals are different by, at most, a single-digit number of milliseconds.

So an attacker just needs to do a replay attack, and all the almanac stuff checks out fine because it's a perfect replica of the authentic signal.

Most phones also have a GLONASS receiver, which is Russia's GNSS.

Yes.