Research On Time And Frequency Characterization And Prediction Algorithm Of GNSS Satellite Clock

In GNSS navigation and positioning,accurate time criterion is the key to realize high precision positioning.As a core equipment of high precision and high reliability navigation system,the performance of the on-borne atomic clock directly determines the service performance of the navigation system,which provides important guarantee for the user's navigation and positioning.In recent years,since BDS and Galileo are aiming at globalization and have achieved great development,satellite clocks of GPS and GLONASS are gradually aging due to their long time in orbit and the replacement of the clock of GPS BLOCK IIF,GLONASS-M and GLONASS-K1.The analysis of time-frequency characteristics of satellite clock is of great importance to get comprehensive understanding of the service performance about GNSS positioning and timing.In this paper,the GNSS clock performance is analyzed form two aspects as time domain and frequency domain.The main research and results are as follows:(1)As for the in-orbit performance of the four main GNSS systems,one year clock products are used to analyze the data quality.The stability of clock in time domain is analyzed by using hadamard variance.The results show that the atomic clocks of GPS were replaced frequently in the recent two years,the IIA Rb clock and IIA Cs clock were replaced by BLOCK IIF Rb clock,which greatly improved the navigation service performance of GPS.The number of Galileo satellites continues to increase in the past two years,which makes it towards a rapid global networking stage.There are data losses,frequency and phase modulation phenomenon among these four systems while the most palpable is BDS,which exists a batch of clock products absence,and the jump of clock bias is more frequent than any other navigation system.(2)It is found that the stability of GPS and Galileo is obviously higher than those of BDS and GLONASS from the results of time domain stability analysis.For the GPS,the daily stability of BLOCK IIF Rb clock is one order higher than that of BLOCK IIF Cs clock.The stability of the lately launched BLOCK IIF Rb is higher than the other two types of Rb clock,but with an obvious frequency drift.The number of BLOCK IIF Rb clock gradually increased,indicating that the performance of the GPS satellite clock is steadily improving.The current performance of the GLONASS-M clock is stable with its daily stability of 3~6×10-14,which is comparable with the GPS BLOCK IIF Cs clock.The satellite C06 of BDS gets a poor performance which is of obvious frequency modulation in every month.The stability of MEO is equivalent to IGSO,and the performance of these two kinds of atomic clocks is much better than that of GEO clocks and is comparable with that of the GLONASS.As for Galileo,the stability of PHM clock is in between of the GPS IIR M Rb clock and IIF Rb clock;while the stability of RAFS clock is comparable with that of the BLOCK IIF Cs clock.(3)Based on the noise analysis of the full GNSS clocks,it can be found that the clocks of GPS and GLONASS are mainly affected by White PM,Flicker PM and White FM.The frequency sequence of the BLOCK IF Rb clock has a periodic fluctuation and obvious frequency drift phenomenon,which results in the instability of the noise identification.The clocks of BDS and Galileo are mainly affected by the three types of frequency noise.The accurate identification of the noise is affected due to the existence of local jump and frequently fluctuation of the regular non overlapping sampling in the recognition of the noise.Since the sampling points of lag1 have to be more than 30,a new method of self-correlation based on the overlapping sampling is proposed.For GPS,the frequency drift has a relatively small effect on noise analysis in terms of three atomic clocks except for BLOCK IIF Rb clocks.As for GLONASS,the frequency drift has no impact on it since the Cs clock is used,the noise identification results of the two kinds of self-correlation methods is consistent in general.The clock frequency drift of BDS and Galileo is more obvious,and there are differences between the results of these two kinds of self-correlation methods while the results of these two are same when the influence of the frequency drift on noise analysis is removed.(4)As for the prediction of satellite clock bias,considering the periodic characteristic of in-orbit satellite clock,nearly one year of clock bias sequence is used to spectrum analysis based on Fourier transform.It is found that the first or the second main periods of various kinds of clock is approximately equal to the orbit main period,which means that part of orbit error is fused in the clock bias in the precise orbit determination.Py putting periodic term correction in clock bias prediction,and considering the impact of the deviation of the starting point between adjacent day in the clock product,a modified prediction model of clock bias is developed,The real examples show that the modified model has an accuracy improvement of 17%~26%.(5)The system noise of clock bias calculated in the precise orbit determination and time synchronization(ODTS)is analyzed.By using the clock product provided by IGS and IAC,the system noise based on ODTS can evaluate the clock performance of GPS and GLONASS effectively.As for the clock products of GBM,the system noise calculated by ODTS can evaluate the Rb clock performance of BDS on the daily stability of Hydrogen clock.