Research And Analysis On The Integrity Monitoring Algorithm Of BDS Satellite-based Augment System

Although the positioning accuracy of the GNSS system has been able to meet most industry needs,the simple use of GNSS still cannot meet some industry applications that require high precision and high integrity.Therefore,countries are vigorously developing enhanced technical means to ensure the accuracy and integrity of some special industries.Among them,the Satellite Based Augmentation System has the advantages of wide service range,large system capacity,and quasi-real-time information,and is widely used..On July 31,2020,China's Bei Dou-3 Global Navigation Satellite System(BDS-3)was officially announced to be put into operation.Meanwhile,the construction of the BDSBAS subsystem and the formulation of relevant international standards are also progressing in an orderly manner.Based on this,this paper designs and implements a satellite based augmentation differential and integrity monitoring system with the following main functions: data pre-processing of monitoring stations,GNSS error correction,pseudo-range multi-path and noise suppression,clock synchronization of stations,satellite differential correction information estimation,integrity parameter solving and user protection level calculation.And some of the key problems are studied and analyzed.The main work and achievements of this paper are as follows.(1)Aiming at the problem that the least square estimation cannot take into account the timing characteristics of the differential correction information,the Kalman filter and recursive least square algorithm are used to introduce more observation information to improve the accuracy of the estimation of the differential correction information.Firstly,the simulation data are used to verify the effectiveness of the two algorithms,and then the measured data are used for analysis.The results show that the equivalent distance corresponding to the difference correction information is similar to the real orbit error and clock error,that is,the correction effect is equivalent to the real error.(2)Aiming at the problem that the pseudo range residual does not reflect the orbit error and clock error accurately,the influence of tropospheric delay error on the estimation result of differential correction information in SBAS data processing is analyzed.The accuracy of the three models of Saastamoinen,MOPS,and GPT2?w are analyzed respectively,and the difference of the pseudo-range residuals calculated under different models is analyzed.Finally,the influence of the three different models on the estimation results of differential correction information is analyzed.The results show that the correction effects of the three tropospheric models are all good at high elevation angles,and there is little influence on the value of difference correction information.The differences between different models are mainly reflected in the estimation of orbital error Z direction.In the case of low elevation angle,different models have great influence on the value of correction,and all the four error components are larger than that of high elevation angle,and the Z-direction estimation of orbit error is still the most influential.(3)Aiming at the problem that the variation of the number of satellites between epochs causes the estimation of receiver clock aberration to jump,the sliding window smoothing algorithm is used to smooth the estimation of receiver clock aberration to weaken the influence of receiver clock hopping on the difference correction information.The results show that the effect of the receiver clock error on the pseudo-distance residual is about 2 decimeters,and the influence on the estimation of the difference correction information is mainly reflected in the three components of the orbit error.At the same time,considering the computer performance and computational efficiency,it is concluded that the window size of 500 should be selected to smooth the clock error estimation of the receiver.(4)The statistical verification method and geometric analysis method are used for simulation experiments using the actual measurement data of the MGEX station,and the performance and characteristics of the two integrity monitoring algorithms are compared and analyzed.The results show that the time series of GPS/BDS satellite pseudo Range residuals and User Differential Range Error(UDRE)calculated by the integrity algorithm of statistical verification using SP3 satellite instead broadcast ephemeris corrected by difference correction have the characteristics related to the operation period of the satellite.Based on this feature,a hypothesis is proposed to enhance the real-time integrity of users through ultra-short term prediction method.Compared with the statistical verification method,the UDRE and DFRE Dual Frequency Range Error(DFRE)values calculated by the geometric analytic method are smaller and more stable,which reduces the user protection level and improves the system availability.The performance of the system meets the precision approach performance requirements of the Radio Technical Commission for Aeronautics(RTCA).