The CA-code is a short sequence that repeats itself every millisecond, is different for every satellite, and is known and open to anyone who wishes to receive and decode it. 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, a good signal from three satellites is required to determine current time, latitude, and longitude, and a fourth to also determine altitude.
Any additional signals increase accuracy.
P-code enabled receivers are able to benefit from having two different frequencies to lock onto. This is used to 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 indoors is impossible. Quality of position determination can also be affected by hills, buildings, and trees.