Roughly, the ground coordinates and its regular geographic elements (that is, space rendezvous) are obtained by measuring the distance from the on-orbit satellite (which determines it is a known data in orbit) to the ground point. Because there are XYZ and the clock difference T in the solution equation, the number of receiving satellites must be greater than or equal to four.
In the positioning process, there are two methods to obtain the pseudo-range from the satellite to the ground (that is, the distance containing the error): 1, ranging code; 2, carrier phase.
1. The ranging code is a pseudo-random noise code and is a binary code sequence. The key is to measure the time difference between the transmitted signal and the receiving signal, which is obtained by the atomic clock synchronization. Since the carrier carrying the ranging code is refracted through the current layer, ionospheric errors occur. Solar activity has an impact on the total electron density of the ionosphere, so this will also have an impact on GPS positioning. According to our teacher, there was a time before the GPS could not be used due to solar flares.
2. If a satellite S sends out a carrier model (where the carrier is used as a ranging signal), the signal is propagated to various places. At a certain moment, the signal has a phase of ψR at the receiver R and ψS at the satellite S. Note that ψR, ψS mentioned here are the complete carrier phases including the entire number of cycles that are calculated from the same starting point. The guard distance is ρ=λ (ψS-ψR).
From the above we can see that if the corresponding phase number is inaccurate, or can not be obtained directly, then GPS will not work. In addition, if the altitude angle of the satellite (the angle between the satellite and the ground level) is too small during the measurement process, measurement positioning cannot be performed. In summary, there are no four or more satellites during the measurement period, the elevation angle is too small, there are obstructions such as high-rise buildings, solar activity is intense, the orbiting position and operating status of the satellite in the sky, and the condition of the receiver may all cause the GPS to be unable to locate. Or positioning is not accurate.