Global Positioning System Tutorial
                        The Global Positioning System (GPS) is a worldwide
Microwaves 101                        radio-navigation system formed from a constellation of 24
                        satellites and their ground stations. GPS uses these
                        "man-made stars" as reference points to calculate positions
                        accurate to a matter of meters. In fact, with advanced
                        forms of GPS you can make measurements to better than a
                        centimeter!
                        http://www.trimble.com/gps/howgps/gpsfram1.htm