The GPS global positioning system adopts a multi -star high -track ranging system, with a distance as a basic observation capacity, and a pseudo -distance measurement of the four satellites can be calculated at the same time. Because the ranging can be completed in a very short time, that is, the positioning is completed in a very short time, so it can be used for dynamic users.

　　Modern ranging is essentially using a wireless telecommunications signal to measure its propagation time to calculate the distance. You can measure the delay of circuit transmission, or you can measure the delay in one -way transmission. Tour -to -and -firing ranging is active ranging, which requires satellites and users to have the ability to send and receive. For users, this not only greatly increases the complexity of the instrument, but also is also very unfavorable from the perspective of concealment, because the emission signal is easily exposed. One -way ranging (that is, passive range) to a large extent avoids the shortcomings of the above. However, the one -way ranging requires that the clock of the satellite and the user receiver is synchronized. If the two clocks are not synchronized, in the time of the measured delay, in addition to the delay caused by the distance between the distance between the satellite to the user receiver, it also contains the clock difference of the two clocks. To reach the synchronization of satellites and user clocks, it is difficult to do in actual work, but it can be solved by appropriate methods

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