We know the satellite stays in the right orbit. The correct trajectory is compiled into an ephemeris, injected into the satellite, and sent to the GPS locator via the satellite. By receiving the ephemeris of each satellite correctly, the exact position of the satellite can be ascertained.

　　The GPS system has very sophisticated atomic clocks on each satellite, which are regularly calibrated by monitoring stations. Satellites send navigational information, but also precise time information. The GPS locator receives this information and synchronizes it with its own clock to obtain an accurate time. Therefore, in addition to accurate positioning, the GPS locator can also produce accurate time information.

　　Since the position of the satellite can be accurately known, in GPS observation, we can get the distance from the satellite to the receiver, using the distance formula in three-dimensional coordinates, using three satellites, three equations can be formed to solve the position of the observation point (X, Y, Z). Considering the error between the clock of the satellite and the clock of the receiver, there are actually four unknowns, X, Y, Z and the clock difference, so it is necessary to introduce a fourth satellite to form four equations for solving, so as to obtain the longitude, latitude and elevation of the observation point.

　　From the principle of GPS use, it can be known that the GPS locator does not transmit signals, it only receives satellite transmission data for calculation to solve the positioning problem. However, if in urban high-rise areas, often faced with a small number of star search, in the actual use of the process, generally three stars can be preliminatively positioned, as shown in the following figure. From this, the lateral coordinates, longitudinal coordinates and height can be determined.

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