Global Positioning System
The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at
least 24 medium Earth orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its
location, speed/direction, and time.
Developed by the United States Department of Defense, it is officially named NAVSTAR GPS (Contrary to popular belief, NAVSTAR is
not an acronym, but simply a name given by Mr. John Walsh, a key decision maker when it came to the budget for the GPS program).
The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is
approximately US$750 million per year, including the replacement of aging satellites, and research and development. Despite these
costs, GPS is free for civilian use as a public good.
GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, and scientific
uses. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and
synchronization of telecommunications networks.
Simplified method of operation
A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time
delay between transmission and reception of each GPS microwave signal gives the distance to each satellite, since the signal travels at
a known speed - the speed of light. These signals also carry information about the satellites' location and general system health (known
as almanac and ephemeris data). By determining the position of, and distance to, at least three satellites, the receiver can compute its
position using trilateration. Receivers typically do not have perfectly accurate clocks and therefore track one or more additional
satellites, using their atomic clocks to correct the receiver's own clock error.
 Technical descript