The Global Positioning System (GPS) is a satellite-based locating and navigating utility that determines a user's precise latitude, longitude and altitude by tracking signals from satellites. Depending on the type of receiver and certain other conditions, it is possible to achieve real-time position accuracies within meters or even centimeters, with position calculations several times per second.
The GPS constellation of satellites was declared operational in late 1995 and consists of 24 satellites orbiting Earth to provide worldwide coverage, 24 hours a day, free of charge. GPS receivers are passive, so the system can serve an unlimited number of users.
GPS satellites send two signals: a carrier and a pseudo-random code. The signals are timed by an atomic clock in the satellite, and the GPS receiver generates a matching code timed by its own synchronized clock. The time it takes for the signals to reach the receiver indicates how far away the satellite is. This calculation is generally performed using the pseudo-random code signal, but for better precision, the carrier signal can be used instead. To make position calculations, GPS receivers use signals from four or more GPS satellites. The first three satellites are used to triangulate a position. The fourth is used to improve the position's accuracy by factoring in the time offset between the satellite system's clock and the GPS receiver's clock.
Receivers vary in the precision of their data and in their ability to overcome adverse signal conditions. Some can process their readings instantaneously, while others merely collect data for post-processing. A good quality GPS receiver delivers data that is robust, reliable and repeatable.
GPS is a fast growing, technologically sophisticated field, with potential applications in many different industries. The most common civilian applications to date have been land, air and marine navigation, and surveying. More recent applications include aircraft precision approach, robotics, IVHS (intelligent vehicle highway systems), construction, resource extraction, and geographic information systems (GIS). Developments in differential GPS (ground-based transmitters providing additional reference points) mean increased reliability and even more widespread possibilities.