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　　A GPS satellite constellation consisting of 21 working satellites and 3 in orbit backup satellites is referred to as the (21+3) GPS constellation. 24 satellites are evenly distributed within 6 orbital planes, with an inclination angle of 55 degrees. The distance between each orbital plane is 60 degrees, which means that the right ascension of the ascending node of the orbit is 60 degrees apart. The difference in elevation angle between satellites in each orbital plane is 90 degrees. A satellite in the orbital plane is 30 degrees ahead of the corresponding satellite in the adjacent orbital plane to the west.

　　GPS satellites at an altitude of 20000 kilometers orbit the Earth twice when the Earth rotates around a star, and the time it takes to orbit the Earth once is 12 star hours. This way, ground observers 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 station, four GPS satellites must be observed, which are called positioning constellations. The geometric position distribution of these four satellites during the observation process has a certain impact on the positioning accuracy. The time period during which precise point coordinates cannot even be measured at a certain location or time is called a "gap period". But this time gap is very short and does not affect the all-weather, high-precision, and continuous real-time navigation and positioning measurements in the vast majority of parts of the world. The number of GPS working satellites is basically the same as the test satellite.

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