When we use GPS for positioning, we will be affected by various factors. The factors that affect GPS positioning accuracy can be divided into the following four categories:

　　(1) Factors related to GPS satellites

　　SA Policy

　　Starting from its national interests, the U.S. government artificially reduces the accuracy of ordinary users using GPS for navigation and positioning by reducing the accuracy of broadcast ephemeris (technology) and adding high-frequency jitter to the GPS reference signal (technology).

　　satellite ephemeris error

　　When GPS positioning is performed, the satellite orbit parameters required to calculate the GPS satellite position at a certain time are provided by various types of ephemeris [7], but no matter which type of ephemeris is used, the calculated satellite position will be The difference from its true position is called ephemeris error.

　　Satellite clock difference

　　The satellite clock error is the error between the clock face time of the atomic clock installed on the GPS satellite and the GPS standard time.

　　Satellite Signal Transmitting Antenna Phase Center Deviation

　　The satellite signal transmitting antenna phase center deviation is the difference between the nominal phase center of the signal transmitting antenna on the GPS satellite and its true phase center.

　　(2) Factors related to transmission routes

　　ionospheric delay

　　Due to the refraction effect of the ionosphere around the earth on electromagnetic waves, the propagation speed of the GPS signal changes, which is called ionospheric delay. The influence of ionospheric refraction on electromagnetic waves is related to the frequency of electromagnetic waves and the total content of electrons in the propagation paths of electromagnetic waves.

　　Tropospheric delay

　　Due to the refraction effect of electromagnetic waves by the troposphere around the earth, the propagation speed of GPS signals changes, which is called tropospheric delay. The influence of tropospheric refraction on electromagnetic waves is related to the temperature, humidity and air pressure on the propagation path of electromagnetic waves.

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