| The GNSS are widely applied in many areas such as high precision and high sample rate navigation and positioning,atmosphere monitoring,military and civilian timing.The GNSS satellite orbit and clock offset are two most core products,which directly determine the service performance level of GNSS.At present,the post-process satellite clock offset products technology is mature,while the requirements of real-time satellite clock offset are wider,many difficult problems are existed such as datum instability,computation efficiency is low,quality control is difficult,the accuracy of estimated clock offset is poor,which cannot meet the requirement of real-time centimeter-level and stable precise positioning and timing service.Aiming at above problems,the key technologies of GNSS real-time satellite clock offset estimation is investigated.The impact of clock offset datum on clock offset,rapid estimation of GNSS real-time satellite clock offset,GNSS real-time satellite clock offset estimation with quality control,clock offset with AR for rapid re-convergence,the main work and contributions are as follows:(1)For the problem of the difference among different datum,the transformation relationship among three datums is derived,the characteristics and application conditions for different datums are analyzed.Taking BDS-2 as an example,the receiver clock,satellite clock and zero-mean condition are selected as datum to estimate real-time satellite clock offset,respectively.The results show that: the estimated clock offset products using three different datum is incomplete equivalence,the clock offset STD are 0.118,0.124 and 0.101 ns,respectively;The clock offset products are evaluated in terms of clock offset accuracy,modeling fitting precision and frequency stability.When using zero-mean condition,the clock offset performance is better,all satellite and receiver clock offset can be estimated.(2)For the problem of the observation data interruption of clock offset datum receiver,a datum switching and compensation method for real-time satellite clock offset estimation is proposed,in this method,the satellite clock offset jump and receiver clock drift difference can be real-time detected and compensated before and after switching datum.The datum switching,datum switching and clock offset jump compensation,datum switching and clock offset jump,receiver clock drift difference compensation(the proposed method),the datum is not switching are applied to estimate satellite clock offset,respectively.The clock offset STD are 0.184,0.183,0.183 and 0.150 ns,respectively;the clock offset model precision are 70907.344,0.731,0.502 and 0.543 ns.The result of the proposed method is better.Moreover,the frequency stability evaluation of satellite clock is also better.The coordinates jump can be eliminated when using this method to estimate satellite clock offset in PPP experiments.(3)For the problem of the slow speed of clock offset estimation.The elevation cutoff observation based on the number of normal equation constraint preferred strategy is designed.In this method,some observations are removed by increasing the elevation mask.Compared to traditional clock offset estimation using all observations,the elapsed time of 60,70 and 80 MGEX stations can be reduced from 6.80,7.93 and 12.04 seconds to 3.10,2.97,3.14 seconds,respectively.The reduction rates are 54.41%,62.55% and 73.92%,respectively.The clock offset estimation can be updated within 5 seconds.The clock offset accuracy of GPS,BDS-3,Galileo and GLONASS is better than 0.04,0.05,0.03 and 0.16 ns,and the clock offset accuracy is almost the same as the as clock offset estimated from all observations.The kinematic PPP experiment is conducted using estimated clock offset,the positioning accuracy can be better than 2.7,2.2 and 5.6 cm in E,N and U directions at 95% confidence level,respectively.(4)For the problem of the outliers in observations of real-time satellite clock offset estimation,GNSS real-time satellite clock offset estimation quality control based on an improved IQR method is proposed.In this method,the posterior residuals are detected,and when the posterior residuals are beyond limitation,the corresponding observations are removed.50 MGEX stations is used to conduct the experiments.The GPS,Galileo,BDS-3 and GLONASS satellite clock offset STD are 0.055,0.072,0.083 and 0.100 ns.Compared to the clock offset estimation without quality control,the improvements are 32.93%,32.07%,53.37%and 31.51%.Compared to the clock offset estimation with traditional quality control method,the estimated satellite clock offset using this method is more stable.For multi-GNSS kinematic PPP using estimated clock offset,both the convergence time and positioning accuracy are improved.(5)For the problem of the difference between different NL AR in real-time satellite clock offset estimation,the characteristics and difference for LAMBDA and Rounding method in the daily and weekly clock offset estimation is compared.The theory and experimental show that:For daily solution,the fixed rate,convergence time and clock offset accuracy of GPS,BDS-3and Galileo satellite using LAMBDA are better.In terms of weekly solution,the difference of fixed rate and clock offset accuracy using Rounding and LAMBDA is marginal.But the LAMBDA is time-consuming.Based on this,the combined LAMBDA and Rounding method are proposed to perform NL AR,in this method,the NL AR method can be adaptively switched from LAMBDA to Rounding according to fixed rate and clock offset difference.Both the advantages of LAMBDA and Rounding methods can be considered.(6)For the problem of long re-convergence of clock offset estimation,a rapid reconvergence method by using dual-thread and AR for GNSS real-time satellite clock offset estimation is proposed.In this method,all parameters are estimated in slow thread,and the troposphere,ambiguities and inter-system bias from slow thread are applied to quick thread,the estimated parameters are only satellite and receiver clock offset.The experiments show that:For short-term interruption,compared to float solutions,the re-convergence time can be improved from 27.21,19.58,41.69,32.80 and 32.80 to 0.12,0.17,0.94,0.36 and 1.74 minutes for GPS,Galileo,BDS-2,BDS-3 and GLONASS satellite clock,respectively.The clock offset accuracy are 0.028,0.024,0.069,0.043 and 0.125 ns,respectively.The elapsed time for fast and slow thread are 21.4 and 1.7 seconds,respectively.For the interruption from several to tens of minutes,the troposphere,ambiguities and inter-system bias before interruption and used after interruption,a strong constraint is imposed.The rapid re-convergence of GPS and Galileo satellite clock offset can be achieved when the interruption of ten minutes,whereas the BDS and GLONASS cannot. |
