Roughly speaking, the ground coordinates and their conventional geographic features (i.e. spatial resection) are obtained by measuring the distance from an in orbit satellite (which determines it to be a known data) to a ground point. Because the position variables in the equation include XYZ and clock deviation T, the number of accepted satellites must be greater than or equal to 4.

　　In the positioning process, there are two methods to obtain the pseudorange (i.e. the distance with errors) from the satellite to the ground: 1. ranging code; 2. Carrier phase.

　　1. The ranging code belongs to pseudo-random noise code, which is a binary code sequence. The key is to measure the time difference between the transmitted signal and the received signal, which is obtained through atomic clock synchronization. Due to the refraction of the carrier wave carrying the ranging code when passing through the current layer, ionospheric errors are generated. Solar activity has an impact on the total electron density of the ionosphere, so it can also have an impact on GPS positioning. According to our teacher, there was a period of time when GPS could not be used due to solar flares.

　　2. If a satellite S emits a carrier model (which is used as a ranging signal here), the signal propagates to various places. At a certain moment, the phase of the signal at receiver R is ψ R. At satellite S ψ S. Note that what is mentioned here is ψ R, ψ S is the complete carrier phase, including the entire number of cycles, calculated from the same starting point. The distance between the satellite and the ground ρ=λ ( ψ S- ψ R) .

　　As can be seen from the above, if the corresponding phase number is inaccurate or cannot be directly obtained, then GPS will not work. In addition, during the measurement process, if the altitude angle of the satellite (the angle between the satellite and the ground level) is too small, it is also impossible to perform measurement and positioning. In summary, during the measurement period, there were no four or more satellites, the altitude angle was too small, there were obstructions such as high-rise buildings, intense solar activity, the position and operation of satellites in orbit, and the condition of the receiver may all cause GPS to be unable to locate or inaccurate.

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It is recommended to install at least two GPS locators for risk control practice