There are many satellites that orbit the Earth for GPS. AT least three of these can pinpoint exact locations. Each satellite can detect how far a location is from it, but not where it is. By using many satellites in conjunction, where each of the satellites distances intersect, it can be determined where the location is. All satellites use radio waves to do this. Where each of these distances intersect, from the satellites, is where the location is. This is called triangulations. The distance from each satellite can be determined because the speed of light is known, so speed x time = distance. Research in this area is occurring in new technologies called Global Navigation Satellite Systems (GNSS) which are more accurate and have an extended range, useful for mining and agricultural uses where signals may be blocked.
THAT'S ALL YOU NEED TO KNOW BASICALLY.