Research On The Method Of Ionospheric Monitoring And Evaluation Based On IGMAS

The influence of the ionosphere on satellite signals has always been one of the major sources of error in the data processing of the Global Navigation Satellite System(GNSS).GNSS-based ionospheric research primarily includes ionospheric delay monitoring,modeling,forecasting,and application of ionospheric products.With the completion of the global networking of Bei Dou Navigation Satellite System(BDS),GNSS-based ionospheric research has more opportunities and possibilities.On the one hand,the constellation of the BDS is unlike those of the other satellite navigation systems;it contains special Geostationary Earth Orbit(GEO)satellites over the equator,which can realize high-precision ionospheric delay monitoring.On the other hand,China has established an independent international GNSS Monitoring and Assessment System(i GMAS),it provides reliable data support and analytical basis for the study of the ionosphere.Therefore,this study is based on i GMAS to focus on the BDS GEO satellites ionospheric monitoring,Bei Dou Global broadcast Ionospheric delay correction Model(BDGIM)evaluation,and i GMAS ionospheric product long-term prediction research.The results of this study are expected to promote development of the i GMAS as well as provide support for the development and application of the BDS and ionospheric-related technologies.The results and innovations of this research are as follows:(1)Using the geostationary characteristic of the BDS GEO satellites and based on the recent observation data and frequency deviation products,a continuous monitoring experiment of the ionospheric TEC at a fixed ionospheric pierce points(IPPs)were conducted.The BDS-specific GEO satellite and corresponding ground station have a relative fixed position,and its IPPs are almost constant,which can perform continuous and uninterrupted ionospheric monitoring at the fixed IPP.Therefore,this study proposes a method of using the GEO satellite dual-frequency observations to monitor the ionospheric TEC at a fixed IPP.By comparing the different combinations of the BDS code pseudoranges and carrier phase observations,the analysis reveals that B1&B2 dual-frequency combination is optimal for calculating the ionospheric TEC.The carrier phase smoothing pseudorange(CSP)method is used to calculate the ionospheric delay TEC,compared with other ionospheric mathematical models,the advantage of the CSP method is that it does not introduce model errors,and the comparison error between the monitoring results and IGS grid products is less than 2 TECU.Finally,the ionospheric response characteristics of solar activity are analyzed using the ionospheric TEC sequenses of continuous monitoring GEO satellites.(2)Before the inauguration of the BDS-3 global system,based on the i GMAS global tracking network and other data,taken the GNSS multi-system post-precision ionospheric products and dual-frequency measured ionospheric products as references,the BDGIM was evaluated for methodological research and actual experimental evaluation,as well as comparative analyses with other broadcast ionospheric models.The evaluation results show that: a)compared with BDSKlob,the performance of the BDGIM model is significantly improved,and the accuracy of ionospheric correction has increased by approximately 20%,which has compensated for the drawbacks of the BDSKlob model at high latitudes and Polar Regions.b)compared with the GPSKlob model,the BDGIM parameter update rate is faster,and the ionospheric delay description on a global scale is more accurate.The ionospheric correction advantages in the northern hemisphere and equatorial regions are evident.However,the appearance accuracy is slightly inferior to the GPSKlob model in the southern hemisphere.c)the BDGIM model performs better than the BDSKlob and GPSKlob models during the ionospheric calm and spring abnormal periods.Moreover,the BDGIM model is reliable for a longer time scale.d)By comparing with the dual-frequency measured ionosphere TEC,the difference in STD of BDGIM is approximately 1–2.5 TECU,that of BDSKlob is approximately 2–3 TECU,and that of GPSKlob is approximately 1.7–6.8 TECU.(3)Based on the i GMAS ionospheric product,the long-term forecasting method of ionospheric TEC was studied;the Direct Sequence Method(DSM)and Indirect Coefficient Method(ICM)of ionospheric TEC were proposed,and the actual forecasting effect was verified.The study of long-term prediction methods for ionospheric TEC is essential for the autonomous operation of satellite navigation systems and related scientific research.The DSM uses the autoregressive moving average ARMA(p,q)model to directly predict the ionospheric VTEC sequence at each grid point,while the ICM uses the ARMA(p,q)model to predict spherical harmonic coefficients after convert the ionospheric VTEC into spherical harmonic coefficients.When using i GMAS ionospheric products to test and compare the two proposed methods,the results show that: within 15 days,the prediction results of two methods are better,and they conform to the reference value.The difference between the predicted and reference values of more than 75% grid points are less than 3 TECU.The difference between the predicted and reference values are slightly larger(about within 4 TECU)during the sun direct irradiation stage.More than 15 days,the accuracy of the ICM is slightly higher than that of the DSM.The results of the second half of 2019 obtained through six 30-day forecasts indicate that the ionospheric forecast accuracies of the two methods are greater than 80%.In addition,the DSM is suitable for regional forecasting,and the ICM is suitable for global forecasting;the DSM for temporary forecasting is more time-efficient,while the ICM for continuous automatic forecasting saves time and storage space.