GPS (Global Positioning System) is satellite-based global navigation system which provides accurate location information anywhere on Earth in all weather conditions. The only condition for accurate positioning that has to be met is that the location has unobstructed line of sight to at least four GPS satellites.
The GPS is a "constellation" of 24 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location. The location accuracy is anywhere from 100 to 10 meters for most equipment. GPS equipment is widely used in science and has now become sufficiently low-cost so that almost anyone can own a GPS receiver.
21 GPS satellites and three spare satellites are in orbit at 20200 km above the sea level on earth and are moving at 3.9km/s. In such way, each satellite circles the Earth twice a day. The satellites are spaced so that from any point on Earth, four satellites will be above the horizon.
Each satellite contains a computer, an atomic clock, and a radio. With an understanding of its own orbit and the clock, it continually broadcasts its changing position and time.
On the ground, any GPS receiver contains a computer that "triangulates" its own position by getting bearings from three of the four satellites. The result is provided in the form of a geographic position - longitude and latitude - to, for most receivers, within 100 meters.
GPS receivers are becoming consumer products. In addition to their outdoor use (hiking, cross-country skiing, mountain biking, ballooning, flying, hunting and sailing), receivers can be used in cars to relate the driver's location with traffic and weather information.
GPS boosts productivity across a wide swath of the economy, to include farming, construction, mining, surveying, package delivery, track and trace systems, and logistical supply chain management. Major communications networks, banking systems, financial markets, and power grids depend heavily on GPS for precise time synchronization. Some wireless services cannot operate without it.
GPS saves lives by preventing transportation accidents, aiding search and rescue efforts, and speeding the delivery of emergency services and disaster relief. GPS is vital to the Next Generation Air Transportation System (NextGen) that will enhance flight safety while increasing airspace capacity. GPS also advances scientific aims such as weather forecasting, earthquake monitoring, and environmental protection.