Research On The Key Algorithms Of BDS Triple-frequency Data Preprocessing Based On Additional Information

Multi-frequency signal is the development trend of GNSS technology in the future.As BDS has fully entered the new stage of global service,it has become the first satellite navigation system in the world that can broadcast triple-or multi-frequency signal in the world.Giving full play to the advantages of multi-frequency signal will help BDS system further improve the quality of PNT service and expand its global market share.Based on the above analysis,this paper studies the key algorithms in BDS triple-frequency data quality-analysis and data preprocessing.(1)Quality analysis of BDS observation dataBased on the evaluation indexes(i.e.,observation data integrity rate,SRN,receiver clock jump,derivative of the ionospheric delay,pseudorange and carrier phase noise),the data quality of BDS multi-frequency observations has been analyzed by using the measured data of iGMAS that can offer continuously triple or multi-frequency observables of BDS-2 and BDS-3 satellites in high rate.(2)Three frequency cycle slip correction method based on optimized pseudorange-phase combinationsAn improved BDS triple-frequency cycle slip correction algorithm is proposed,whose effectiveness is verified by the measured data.This method uses the three different combinations(i.e.,EWL / WL / NL)to realize the real-time BDS cycle slip correction.As the wavelength of NL combination is less than the wavelength of single-frequency signal,the influence of the first-order ionospheric delay residual should be considered.The first-order ionospheric delay residual can be estimated by the original triple-frequency phase data,and then the cycle slip of NL combination can be determined.Considering that the satellite elevation can effectively reflect the quality of observation,the success-rate model of cycle slip detection is established based on the satellite elevation and the repair-threshold is verified by the measured data.(3)Improved HWM cycle slip correction method using DopplerAs Doppler does not suffer from the cycle slip,it can be used for cycle slip correction when the sampling-frequency is high.However,as the sampling-frequency decreases,the Doppler cumulative error increases gradually and then Doppler cannot be used to detect and repair cycle slips directly.In this paper,Doppler is used to modify the pseudorange observation,and an improved HWM cycle slip correction method is proposed.Firstly,the optimal EWL and WL combination are selected based on SNF and ISF values.Then,Doppler is used to smooth the pseudorange observation,and a weight factor is used to balance the weights of the original pseudorange observation and the smoothed pseudorange data,which is quite useful to determine the cycle slip of NL combination.Finally,three kinds of cycle slip in the combinations are used to repair cycle slips in the original observations based on matrix operation.(4)Multipath effect detection method considering SNR dataMultipath effect is time-varying and closely related to the measurement environment,which is difficult to be effectively suppressed by traditional methods.When the multipath effect occurs,the quality of original signal will be changed,and the corresponding SNR will also change.In this paper,the detection model is established based on the difference of triple-frequency SNR,which can detect the multipath effect in real time according to the threshold.The reliability of the method is verified by the measured data,which can be effectively used in BDS data preprocessing field.