The Global Positioning System is a satellite-based radionavigation system provided by the US Department of Defense. It permits users with suitable receivers to establish their position, speed and time on land, sea or in the air, at any time of the day or night and in any weather condition. The System is accurate to within 30 metres, which is equal to or better than any other radionavigation system available today.
A GPS receiver can obtain a position fix anywhere in the world.
The GPS system has 3 major segments: space, control and user.
The Space Segment consists of a constellation of 24 satellites orbiting the earth at an altitude of 20,000 kms. These satellites act as reference points from which receivers on the ground determine their position.
The Control Segment consists of 5 stations here on earth which track the satellites.
The User Segment consists of antennas and receiver-processors.