The basic principle of GPS positioning is to use the instantaneous position of high-speed moving satellites as known starting data, and use the method of spatial distance resection to determine the position of the test point.

　　Assuming that a GPS receiver is placed at the ground test point at time t, the time △ t of the GPS signal reaching the receiver can be determined. In addition, the satellite ephemeris and other data received by the receiver can determine the following four equations:

　　The meanings of each parameter in the four equations are as follows:

　　x. Y and z are the spatial Cartesian coordinates of the coordinates of the point to be measured.

　　Vto is the clock deviation of the receiver.

　　C is the propagation speed of GPS signals, that is, the speed of light.

　　Di (i=1, 2, 3, 4) is the distance between satellite 1, satellite 2, satellite 3, and satellite 4 to the receiver.

　　△ ti (i=1, 2, 3, 4) is the time it takes for signals from satellites 1, 2, 3, and 4 to reach the receiver.

　　Di=c △ ti (i=1, 2, 3, 4)

　　Xi, yi, zi (i=1, 2, 3, 4) are the spatial Cartesian coordinates of satellite 1, satellite 2, satellite 3, and satellite 4 at time t, obtained from satellite navigation messages.

　　Vti (i=1, 2, 3, 4) is the clock deviation of satellite clocks for satellites 1, 2, 3, and 4, provided by the satellite ephemeris.

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