GPS is short for Global Positioning System, and it is quickly becoming an integrative part of our society. It is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these satellites as reference points to calculate positions accurate to a matter of meters. In a sense it's like giving every square meter on the planet a unique address. GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone. In order to find the its position, the GPS unit must complete a process called triangulation.
To "triangulate," a GPS receiver measures distance using the travel time of radio signals. To measure travel time, GPS needs very accurate timing which it achieves with some tricks. Along with distance, the unit needs to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. Finally the unit corrects for any delays the signal experiences as it travels through the atmosphere. For the unit to gather most of the information it needs, about 3 satellites are required. However, four or more is optimal.