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.