Global Positioning System or Satellite navigation system was developed by United State which provides users with positioning, navigation, and timing services. Global Positioning system consists of three segments: the space segment, the control segment, and the user segment. The U.S. Air Force extend, keeps, and drives the space segment and control segment.
Space segment consists of a nominal constellation of 24 operating GPS satellites that transmit one-way signals that give the current GPS satellite position and time.
GPS control segment consists of worldwide check and control stations that maintain the GPS satellites in their proper orbits through occasional command manoeuvres, and adjust GPS satellite clocks. GPS control segment tracks the GPS satellites, uploads updated navigational data, and maintains health and status of GPS satellite constellation.
GPS user segment consists of the GPS receiver equipment, which receives the signals from the GPS satellites and uses transmitted information to compute three-dimensional position and time.
GPS satellites provide two type user services : civilian user and military user. The civilian service is freely available to all users on a continuous, worldwide basis. The military service is available to United State and allied armed forces as well as approved Government agencies.
A variety of GPS augmentation systems and techniques are available to enhance system performance to meet specific user requirements. These improve GPS signal availability, accuracy, and integrity, allowing even better performance than is possible using the basic GPS civilian service.
The outstanding performance of GPS over many years has earned the confidence of millions of civil users worldwide. United State always improves GPS system for the future. The United States is committed to an extensive modernization program, including the implementation of a second and a third civil signal on GPS satellites. The second civil signal will improve the accuracy of the civilian service and supports some safety-of-life applications. The third signal will further enhance civilian capability and is primarily designed for safety-of-life applications, such as aviation.
Using GPS receiver, the user should to know about positioning, navigation, and timing Policy. U.S. law and policy on GPS emphasize continuity of service, unlock access to civil signals, and technological leadership. In 1996, the United States issued a national policy statement on the management and use of space-based positioning, navigation and timing services, which include GPS and augmentations. It underscored the dual use (civilian-military) nature of GPS and established a joint civil-military national management structure to oversee its operation.
U.S. policy was expanded in 2004 in response to changing international conditions and the amazing growth in the types and complexities of GPS applications. This policy reaffirms the United States commitment to provide reliable civil space-based positioning, navigation, and timing services through GPS on a continuous, worldwide basis generously available to all. The guidelines also calls for improving the performance of Nav sat.

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