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GPS stands for Global Positioning System.

GPS is a constellation of 27 earth orbiting satellites. They are developed and operated by the U.S. Department of Defense (DOD) as a worldwide military position and navigation system. It permits land, sea, and airborne users to determine their three-dimensional position, velocity, and time 24 hours a day, in all weather, anywhere in the world with a precision and accuracy far better than other navigational systems available today. The GPS reach Full Operational capabilities on July 17,1995. It consists of the space, control and user segment.

The Space Segment, consists of 24 operational satellites 12,000 miles above earth at an inclination of 55 degrees. They are constantly moving, making two complete orbits in less than 24 hours. There are currently 27 satellites in orbit, with 3 spare satellites. The solar powered satellites are spaced in orbit so that at any time a minimum of 4 satellites will be in view to users anywhere in the world. The satellites continuously broadcast position and time data to users throughout the world.

The Control Segment consists of a master control station in Colorado Springs, Colorado, with five monitor stations and three ground antennas located throughout the world. The monitor stations track all GPS satellites in view and collect ranging information from the satellite broadcasts. The monitor stations send the information they collect from each of the satellites back to the master control station, which computes extremely precise satellite orbits. The information is then formatted into updated navigation messages for each satellite. The updated information is transmitted to each satellite via the ground antennas, which also transmit and receive satellite control and monitoring signals.

The User Segment consists of the receivers, processors, and antennas that allow land, sea, or airborne operators to receive the GPS satellite broadcasts and compute their precise position, velocity and time. A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration.
How is GPS used?

GPS receivers collect signals from satellites in view. They display the user's position, velocity, and time, as needed for their marine, terrestrial, or aeronautical applications. Some display additional data, such as distance and bearing to selected waypoints or digital charts.
Who uses GPS?

GPS is used to support land, sea, and airborne navigation, surveying, geophysical exploration, mapping and geodesy, vehicle location systems, farming, transportation systems, and a wide variety of other additional applications. Both military and civilians use this navigational technology.