After the Soviet Union launched the first artificial satellite, researchers at the Applied Physics Laboratory of John Hobbs King University in the United States proposed that since the position of the observation station could be known, if the satellite position was known, it should also be possible to measure the recipient's location. This is the basic idea of navigation satellites. The basic principle of a GPS navigation system is to measure the distance between a satellite with a known position and the user's receiver, and then combine the data of multiple satellites to determine the specific position of the receiver. To achieve this goal, the position of the satellite can be found in the satellite ephemeris based on the time recorded by the onboard clock. The distance from the user to the satellite is obtained by recording the time it takes for the satellite signal to propagate to the user, and then multiplying it by the speed of light (due to interference from the atmospheric ionosphere, this distance is not the true distance between the user and the satellite, but pseudo-random code (PR): When the GPS satellite is operating normally, it will continuously transmit navigation messages using a pseudo-random code composed of 1 and 0 binary symbols (referred to as pseudo-random code). There are two types of pseudo codes used in GPS systems, namely civilian C/A codes and military P (Y) codes. The C/A code frequency is 1.023MHz, with a repetition period of one millisecond and a code spacing of 1 microsecond, equivalent to 300m; The P-code frequency is 10.23MHz, with a repetition period of 266.4 days and a code spacing of 0.1 microseconds, equivalent to 30m. The Y code is formed on the basis of the P code, which has better confidentiality performance. Navigation messages include satellite ephemeris, operating conditions, clock correction, ionospheric delay correction, atmospheric refraction correction, and other information. It is unmodulated from satellite signals and transmitted on the carrier frequency with a 50b/s modulation. The navigation message contains 5 subframes in each main frame, with a frame length of 6 seconds. The first three frames each have 10 character codes; Repeat every 30 seconds and update every hour. The last two frames total 15000b. The content in navigation messages mainly includes telemetry codes, conversion codes, 1st, 2nd, and 3rd data blocks, with the most important being ephemeris data. When a user receives a navigation message, they can extract the satellite time and compare it with their own clock to determine the distance between the satellite and the user. Then, they can use the satellite ephemeris data in the navigation message to calculate the position of the satellite when transmitting the message, and obtain information such as the user's position and velocity in the WGS-84 geodetic coordinate system. It can be seen that the function of the satellite part of the GPS navigation system is to continuously transmit navigation messages. However, since the clock used by the user's receiver cannot always be synchronized with the satellite's onboard clock, in addition to the user's three-dimensional coordinates x, y, and z, it is also necessary to introduce a Δ The time difference between the satellite and the receiver is taken as an unknown number, and these four unknowns are solved using four equations. So if you want to know the location of the receiver, you need to be able to receive signals from at least four satellites.

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