Is military GPS more accurate than civilian GPS?
GPS offers two services -- the Precise Positioning Service (PPS) for U.S. and Allied military users, and the Standard Positioning Service (SPS) for worldwide civilian users. PPS is a more robust form of GPS that is encrypted and jam-resistant. It employs two signals to reduce radio transmission errors caused by the atmosphere, thus improving accuracy. SPS uses only one signal, but the ongoing GPS modernization program is adding several more civilian signals over the coming years. Detailed descriptions of PPS and SPS are available here.
Eventually, the accuracy difference between military and civilian GPS services will disappear. Using local or regional augmentations, today's civilian GPS users can already achieve much higher accuracy than PPS.
http://pnt.gov/public/faq.shtml#difference