Beidou Navigation system-GPS system composition and working principle

　　The system composition of the Beidou Navigation and Positioning system is as follows: it consists of two geostationary satellites (800E and 1400E), one in-orbit backup satellite (110.50E), the central control system, the standard calibration system and various user machines, etc.

　　The working process of the system is as follows: First, the central control system sends inquiry signals to satellite I and satellite II simultaneously, and broadcasts to users in the service area of the satellite forwarder. The user responds to the inquiry signal from one of the satellites and simultaneously sends the response signal to both satellites, which is forwarded back to the central control system. The central control system receives and demodulate the signal sent by the user, and then carries on the corresponding data processing according to the applied service content. For the positioning application, the central control system measured two time delays: namely, the inquiry signal sent from the central control system was forwarded to the user by a certain satellite, and the user sent the positioning response signal was forwarded back to the central control system by the same satellite. And the delay of sending an inquiry signal from the central control, reaching the user via the same satellite mentioned above, and the user sending a response signal and forwarding it back to the central control system via another satellite. As a result of the position of the center control system and the two satellites were known, and thus can calculate the amount of users by the above two delay, the distance to the first satellite and the user to the sum of two satellites distance, so they know the user in a first satellite as a sphere, its center and between two satellites as the focus of the ellipsoid to pay online. In addition, the central control system can find the user's elevation value from the digital topographic map stored in the computer and know that the user is on an ellipsoid parallel to the earth reference ellipsoid. Thus, the central control system can finally calculate the three-dimensional coordinates of the user's point, which are encrypted and sent to the user by the outbound signal.

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