The Global Positioning System, usually called GPS,
The Global Positioning System, usually called GPS, is the only fully-functional satellite navigation system. A constellation of more than two dozen GPS satellites broadcasts precise timing signals by radio to GPS receivers, allowing them to accurately determine their location (longitude, latitude, and altitude) in any weather, day or night, anywhere on Earth.
United States Department of Defense developed the system, officially named NAVSTAR GPS (Navigation Signal Timing and Ranging GPS), and the satellite constellation is managed by the 50th Space Wing at Schriever Air Force Base. Although the cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites, GPS is available for free use in civilian applications as a public good.
GPS has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world, as well as an important tool for map-making, and land surveying. GPS also provides an extremely precise time reference, required for telecommunications and some scientific research, including the study of earthquakes.
In late 2005, the first in a series of next-generation GPS satellites was added to the constellation, offering several new capabilities, including a second civilian GPS signal called L2C for enhanced accuracy and reliability. In the coming years, additional next-generation satellites will increase coverage of L2C and add a third and fourth civilian signal to the system, as well as advanced military capabilities.
The Wide-Area Augmentation System (WAAS), available since August 2000, increases the accuracy of GPS signals to within 2 meters (6 ft)  for compatible receivers. GPS accuracy can be improved further, to about 1 cm (half an inch) over short distances, using techniques such as Differential GPS (DGPS).
Global Positioning System) A satellite-based radio navigation system run by the U.S. Department of Defense. It was designed so that signals from at least four satellites would be on the horizon at all times, which is sufficient to compute the current latitude, longitude and elevation of a GPS receiver anywhere on earth to within a few meters. The first GPS satellite was launched in 1978.
In six different orbits approximately 12,500 miles above the earth, the system's 24 MEO satellites circle the earth every 12 hours. The satellites do nothing more than constantly transmit their current time based on atomic clocks and current location. A monitoring network on the ground tracks the satellites and uplinks data for synchronization. The system uses four frequencies in the L-band from 1.2 to 1.6GHz .
Whether installed in vehicles or carried by hand, a GPS receiver calculates the distance to the satellites by comparing the times the transmitted signals were sent with the times they were received. By knowing the precise locations of the satellites at a given moment, the receiver uses triangulation, the navigation technique of ship captains for centuries, to pinpoint its own location.