Scientists, therefore, decided that the only way to provide accurate coverage for the entire world was to place high-frequency radio transmitters in space. A transmitter high above Earth would broadcast a high-frequency radio wave with a special coded signal that could cover a large area and still reach Earth far below at a useful power level. This is one of the main principles behind the GPS system. It brings together 2,000 years of advances in navigation by providing precisely located “lighthouses in space” that are all synchronized to a common time standard.

The GPS system can tell you your location anywhere on or above Earth to within about 20 to 30 feet. Even greater accuracy, usually within less than three feet, can be obtained with “differential corrections” calculated by a special GPS receiver at a known fixed location.