GPS (Global Positioning System) is the second generation Satellite navigation of the United States, which is a new generation of satellite navigation and positioning system with the capability of real-time three-dimensional navigation and positioning in all directions at sea, land and air. The system consists of three parts:

　　Space part - GPS Satellite constellation

　　Ground Control Part - Ground Monitoring System

　　User Equipment Part - GPS Signal Receiver

　　Space part - GPS Satellite constellation

　　The GPS Satellite constellation consists of 21 working satellites and 3 on orbit standby satellites, which is recorded as the (21+3) GPS constellation. The 24 satellites are evenly distributed in 6 Orbital plane, the Orbital inclination is 55 degrees, and the Orbital plane are 60 degrees apart. The elevation angular distance between each satellite in each Orbital plane is 90 degrees, and the satellite on the Orbital plane is 30 degrees ahead of the corresponding satellite on the west adjacent Orbital plane. GPS satellites at an altitude of 20000 kilometers orbit the Earth twice when the Earth rotates one revolution towards the star. For ground observers, they will see the same GPS satellite 4 minutes in advance every day. The number of satellites located above the horizon varies with time and location, with a minimum of 4 visible and a maximum of 11 visible. When using GPS signal navigation and positioning, in order to calculate the three-dimensional coordinates of the measured point, it is necessary to observe four GPS satellites, called positioning constellations. The geometric position distribution of these four satellites during the observation process has a certain impact on the positioning accuracy.

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