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Since the position of the satellite is a function of time, the GPS observations are based on precise measurements, and the information corresponding to the satellite position is transmitted to the receiver via the encoded information of the satellite signals. In GPS positioning, whether it is code phase observation or carrier phase observation, satellite clock and receiver clock are required to maintain strict synchronization. In fact, although GPS satellites are equipped with high-precision atomic clocks (é“· and é“¯), there are still unavoidable deviations and drifts between them and the ideal GPS time. The total amount of this deviation is within about 1ms.
Such deviations from the satellite clock can generally be determined by the satellite's master control station, through the continuous monitoring of the operating state of the satellite clock, and provided to the receiver through the satellite navigation message. After the clock correction, the synchronization difference between the satellites can be kept within 20ns.
In relative positioning, the satellite clock difference can be eliminated by observing differences (or differences).
2. Satellite orbit deviation
It is difficult to estimate and deal with orbital deviations of satellites. The main reason for this is that satellites are subject to the complex effects of various perturbations in their operation. It is difficult to measure these forces fully and reliably through the ground monitoring stations and to grasp their role. The law, at present, satellite orbit information is arrived through the navigation message.
It should be said that satellite orbit error is one of the major sources of error in current GPS surveys. The longer the measured baseline length, the greater the impact of this error.
In GPS positioning measurement, there are the following direct methods for processing satellite orbit error:
1) Ignoring the orbit error This method is based on the satellite orbit information obtained from the navigation message and does not consider the actual orbit error of the satellite orbit. Therefore, it is widely used in real-time single-point positioning with low precision.
2) Using the orbit improvement method to process observation data This method involves the introduction of correction parameters that characterize satellite orbital deviations in data processing, and assumes that these parameters are constant in a short time, and is solved together with other knowledge numbers.
3) The method of obtaining the difference of synchronous observations is to use the same observation value of the same satellite to obtain the difference in two or more observation stations. To reduce the impact of satellite orbital errors. Because the same satellite's position error affects the simultaneous observation of different observing stations and has the nature of system error, the above method of obtaining difference can significantly reduce the influence of satellite orbit error, especially when the baseline is shorter, its utility is even less. obvious.
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