Each satellite transmits a NAV message superimposed on both the CA and P codes. Position determination works by simple triangulation between a receiver and three or more satellites visible that moment. Those in northern or southern locations enjoy greater satellite coverage. Drivers, boaters and pilots can know where they are, and where they've been. Because of this, it requires a good signal from three satellites in order to determine current time, latitude and longitude, and a forth to also determine altitude.
Any additional signals increases accuracy.
P-code enabled receivers are able to benefit from having two different frequencies to lock onto. This is used the measure the effect the ionosphere is having on the signals, and helps improve accuracy even further. Since the CA-code is only carried on one frequency, such measurements are not possible so an estimate provided by the satellite is used. Thus, reception in doors is impossible. Quality of position determination can also be effected by hills, buildings and trees.