(1) SA

　　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 (ε technology) and adding high-frequency jitter to GPS signals (currently canceled).

　　(2) Satellite ephemeris error

　　When performing GPS positioning, the satellite orbit parameters required to calculate the GPS satellite position at a certain time are provided by various types of ephemeris, but no matter which type of ephemeris is used, the calculated satellite positions will be the same as their real positions. There is a difference, which is called the ephemeris error.

　　(3) 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.

　　(4) Phase center deviation of satellite signal transmitting antenna

　　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 the propagation path

　　(1) 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.

　　(2) Tropospheric delay

　　For the refraction effect of the troposphere around the earth on electromagnetic waves, the propagation speed of the GPS signal changes, which is called tropospheric delay.

　　(3) Multipath effect

　　Due to the influence of the surrounding environment of the receiver, the satellite signal received by the receiver also includes the influence of reflected and refracted signals, which is the so-called multipath effect.

　　3. Receiver-related factors

　　(1) Receive set clock difference

　　The receiver clock error is the difference between the clock face time of the clock used by the GPS receiver and the GPS standard time.

　　(2) Receiver antenna phase center deviation

　　The receiver antenna phase center deviation is the difference between the nominal phase center of the GPS receiver antenna and its true phase center.

　　(3) Errors caused by receiver software and hardware

　　When GPS positioning is performed, the positioning results are also affected by processing and control software and hardware.

　　(4) The phase increase effect caused by the relative rotation of the antenna

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