What is GPS?

GPS is a navigational system, operated and controlled by the U.S. Department of Defense, that allows users to determine position, velocity, and time, using GPS receivers and radio signals broadcast from GPS satellites. When operating in its full configuration, GPS is a constellation of six orbital planes of four satellites each. This makes a total of 24 satellites, plus orbital spares. The satellites orbit Earth roughly every 12 hours, at an altitude of 20,200 kilometers, covering the entire planet 24 hours a day, seven days a week. To determine its position in X, Y, and Z coordinates, a GPS receiver requires an unobstructed view of at least four GPS satellites.