gps service standard positioning service sps and precise positioning system pps gps satellites transmit two different signals the first signal is the precision or p code and the second signal is the coarse acquisition or c a code the p code is designed for authorized military users and provides pps service the military engages an encryption segment on the p code called anti spoofing as to limit access to the p code to authorized users the c a code is designed for use by nonmilitary users and provides sps service the c a code is less accurate and easier to jam than the p code it is also easier to acquire selective availability is achieved by degrading the accuracy of the c a code however by 2006 the selective availability error is scheduled to be set to zero other than intentional errors inserted by the dod that are being removed from the gps system there are a variety of other error sources that vary with terrain and other factors gps satellite signals can he blocked by most materials gps signals have difficulty passing through buildings metal mountains or trees leaves and jungle canopy can attenuate gps signals so that they become unusable in locations where at least four satellite signals with good geometry cannot be tracked with sufficient accuracy gps is almost unusable differential gps was developed in order to compensate for the inaccuracy of cps readings a high performance cps receiver is placed.