China Beidou satellite positioning and navigation system, through which three ways for interactive positioning and communication?

Beidou satellite navigation system consists of three parts: space terminal, ground terminal and user terminal. The space end includes 5 geostationary orbit satellites and 30 non-geostationary orbit satellites. The ground end includes several ground stations such as main control station, injection station and monitoring station. The client consists of Beidou user terminals and terminals compatible with other satellite navigation systems such as American GPS, Russian GLONASS and European Galileo. The working process of Beidou satellite navigation system is as follows: first, the central control system sends inquiry signals to satellites 1 and 2 at the same time, and broadcasts them to users in the service area through satellite transponders. Users respond to the query signal of one of the satellites, and send response signals to both satellites at the same time, which are forwarded back to the central control system through the satellites. The central control system receives and demodulates the signals sent by users, and then carries out corresponding data processing according to the service content applied by users. For positioning applications, the central control system measures two time delays: the delay of sending an inquiry signal from the central control system and forwarding it to the user through the satellite, and the delay of sending a positioning response signal by the user and forwarding it back to the central control system through the same satellite; The inquiry signal sent by the central control reaches the user through the same satellite, and the user sends a reply signal, which is forwarded back to the satellite navigation system diagram of the central control system through another satellite. Since the positions of the central control system and the two satellites are known, the sum of the distances between the user and the first satellite and between the user and the two satellites can be calculated from the above two delays, thus knowing that the user is at the intersection of a sphere centered on the first satellite and an ellipsoid focused on the two satellites. In addition, the central control system can find the user's elevation value from the digital topographic map stored in the computer, and can know that the user is on an ellipsoid parallel to the earth reference ellipsoid. Therefore, the central control system can finally calculate the three-dimensional coordinates of the user's point, which is encrypted and sent to the user through the outbound signal.