Research On GNSS Ionospheric Delay Modeling In China

With the development of GNSS(Global Navigation Satellite Systems),the accuracy requirements of navigation and positioning are getting higher and higher.The ionospheric delay error is the main source of error that affects high-precision navigation and positioning.How to effectively correct the ionospheric delay error has become a hot spot in ionospheric research.Based on this,in this paper,we studied the factors affecting the change of the ionospheric TEC,the accuracy characteristics of the Klobuchar model in different solar activity years,the improvement of the Klobuchar model and its application in navigation and positioning.The main research and conclusions as follows:1.The basic principles and methods of ionospheric monitoring are elaborated in detail.The main research progress of the current Klobuchar model is summarized.The method of obtaining the ionospheric VTEC is given.Finally,the correlations between the ionospheric TEC and solar activity F10.7 index,geomagnetic Dst index and Kp index of grid points in China from 2008 to 2018 are analyzed.2.Using the GIM(Global Ionospheric Map)provided by the IGS Center to study the variation of daily average,monthly average,seasonal average of the relative accuracy and residual with the Klobuchar model in different ionospheric activity years.The result show that the residual and maximum underestimation will change with the change of solar activity.The residual and maximum underestimation are larger in the high solar activity year and daytime;in the high solar activity year,the average residual and maximum underestimation in daytime have a significant seasonal variation characteristic in low latitudes.But they only show seasonality at the twtf station at night.(2)The relative accuracy of the model does not change with the solar activity changes,and the accuracy is lower at night than that in daytime.(3)The bias of the model has the characteristics of changing with the diurnal change.The amplitude of the fluctuation in daytime is greater than that at night,and the amplitude of the fluctuation is not affected with the latitude.In addition,the bias of the model at night is negative in the high solar years.3.A method for improving the Klobuchar model based on time series short-term prediction is proposed.First,we obtain the ionospheric TEC deviation between the Klobuchar model and the dual-frequency correction model.Then,we use the TEC deviation of the previous 5 days to make a 1-day deviation prediction.Finally,we use the predicted deviations to improve the Klobuchar model.The results show that:(1)During the ionospheric quiet period and the ionospheric active period,the improved Klobuchar model and the dual-frequency correction model have a higher degree of conformity and can better describe the diurnal characteristics of the ionosphere,especially at night.(2)The relative accuracy of the improved Klobuchar model has been significantly improved,and the absolute deviation and root mean square error have been significantly weakened.(3)Compared with the 1-day prediction results of the ionospheric TEC calculated directly based on the dual-frequency correction model,the improved Klobuchar model is better for correcting the ionospheric delay error.4.Using standard single-point positioning technology to evaluate the practicability of the ionospheric model.We conducted navigation and positioning experiments at 9 IGS stations in China and abroad with the ionospheric quiet period and ionospheric active period,respectively.The results showing that: when using the improved Klobuchar model as the ionospheric correction mode,the average three-dimensional positioning accuracy is 3.12 meters and 2.28 meters in the active period of the ionosphere and the quiet period of the ionosphere,respectively.Compared with the uncorrected Klobuchar model for positioning,the overall positioning errors are reduced by 1.21 meters and 0.60 meters,respectively.