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How did they do it? In normal operation, GPS receivers deduce their position by calculating their distance from several satellites at once. Each satellite carries an atomic clock and broadcasts its location, the time, and a signature pattern of 1,023 plus and minus signs known as a pseudorandom noise code (or PRN code). These codes identify a signal as originating from, say, satellite A versus satellite B, which is necessary because all GPS satellites broadcast civilian signals on the same frequency.
Public-key systems have one significant limitation. They rely on knowing, or trusting, that the public key which will be used in communications with a person or organization really is the public key of the person or organization and has not been spoofed by a malicious third party. There are two broad methods by which this is usually accomplished. A Public Key Infrastructure (PKI), or more commonly by the use of a trusted third party. The third party securely manages, and attests to the authenticity of, public keys. If the third party (. a Certificate Authority (CA) in the context of certificates) is requested to provide the public key of X, they are trusted to provide the correct public key. The third party is trusted to have satisfied themselves by some process - attestation, notarization, or another process - that X is the one and only, or globally unique, X. The most common method for making available public keys that have been verified by a third party is to embed them in an (or SSL) certificate which has been digitally signed by the the issuer (typically a Certificate Authority).