The Global Positioning System is based on a constellation of 24 satellites orbiting the earth at an altitude of 10,900 nautical miles. In a way, you can think of these satellites as "man-made stars" that replace the stars navigators used in the past to find their way. The handsheld GPS receiver that you will be using to navigate your way around the NOAA campus takes measurements from at least 4 of the orbiting satellites and computes your position on the surface of the earth.