Anti-Spoofing

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Anti-Spoofing (abbrevated A-S or AS) is a method implemented in GPS technology to deny non-U.S. military users signal's P-code, which is modulated on both the L2 and L5 frequencies.

A-S was implemented on January 31, 1994, by the secret encryption called the "W-code" onto the P-code. By keeping the military PRN code secret, an enemy of the U.S. could not jam the satellite signal through ground-based transmitters, nor "spoof" military GPS receivers by transmitting a false P-code signal from a satellite. However, several GPS receiver manufacturers have developed proprietary techniques to make dual-frequency measurements even when A-S is activated.