Research On Elaboration Of GNSS Navigation Ionospheric Model

Along with the development of GNSS(Global Navigation Satellite System)and the increasing higher accuracy requirement of navigation,ionosphere delay error,one of the main error sources in navigation and positioning,has been constantly paid extensive attention and in depth study.Paying attention to the ionosphere model refinements and their impacts on navigation accuracy as far as possible,ionosphere model factors,including mapping functions and thin layer height,and the applications of long time series GIMs(Global Ionospheric Maps)in navigation ionospheric model refinements have been deeply studied and fully explored,and Klobuchar-like ionospheric model has been proposed and its application impacts on navigation have been compared with other different ionospheric models in this paper.The concrete works,innovative ideas and main conclusions of this paper are listed as follows:1.The existing ionosphere mapping functions are systemically summarized,and the impacts of the broadcast ephemeris mapping function,projection mapping function,geometric mapping function and ellipsoid mapping function on navigation precision based on Klobuchar ionosphere model are compared by self-designed software.The results show that the different ionospheric mapping functions have some influences on navigation precision with a relatively small magnitude,but there are different impacts on different periods of solar activity.The shell height variations in time and space were analyzed by utilizing IRI2012 model.The method for precision evaluation of the ionosphere using STEC(Slant Total Electron Content)differences in applications was put forward,and the influences of shell height on the positions of IPP(Ionospheric Pierce Point),mapping function,ionospheric modeling,ionospheric elaboration and ionospheric accuracy evaluation were discussed in theory and data.The results show that the shell height has important impacts on them,which can not be ignored.2.Data quantitative analysis method based on EMD(Empirical Mode Decomposition)was proposed and put in investigating the global mean nighttime ionospheric VTEC(Vertical Total Electron Content)qualitatively.The quantitative analysis result is reliable to describe and forecast the trend of the mean nighttime VTEC in a solar activity cycle.The methods of calculating sunshine duration of point in air by apparent solar time,mean solar time and approximate mean solar time,have been given and were utilized in this paper.A Klobuchar improvement model was used to calculate the ionospheric diurnal variation cycles.Study on the correlation between ionospheric diurnal variation cycle and sunshine duration has been done.3.The methodology for mosaicking ionospheric grid maps and a methodology for solving global ionosphere VTEC/RMS eigenvalues are presented in this paper.On the basis of extracted VTEC and RMS maximums,the long time series products furnished by GIMs are assessed for their data quality.Daily global VTEC and RMS maximums,minimums and means are evaluated by experimental data of 2005 and onwards.These extreme values are investigated for their spatial and temporal distribution patterns,and an exploration is made into the correlation between the eigenvalues and solar activity.The statistical analysis of large sample GIMs eigenvalues substantiates some reasonableness of IGS ionosphere products,and this study offers some guidance to ionosphere modeling by VTEC data and contributes to GIMs application popularization.4.The applications and differences of Klobuchar model in GPS and BDS have been introduced,and the pervious researches on parameter and model refinements of Klobuchar ionospheric model have been systemically reviewed.The spatial and temporal characteristics of ionospheric nighttime VTEC and diurnal variation are analyzed based on GIMs,and the shortcomings of the classical Klobuchar model in describing those characteristics were discussed.On the basis of the study above,the Klobuchar-like ionospheric model is proposed.5.The fitting methods of the ionospheric model parameters,including least squares estimations and robust estimations,are given.Aimed at the uneven phenomenon at the ±k0 point,two improved schemes of IGG? scheme in robust estimations have been proposed in robust estimation,making full use of the available information near the ±k0.The least squares estimations and robust estimations of model parameters with equivalent initial weight and initial weight of 1/RMS were carried out by self-designed programs.The robust estimations with equivalent initial weight perform better no matter there are errors or not in observed data.6.The Klobuchar-like,14-paramaters Klobuchar and 8-paramaters Klobuchar models were established for the small,large and global regions by GIMs in different solar activity periods and seasons,respectively.Klobuchar-like models have better performances in Bias,RMS and correction rates,than 14-paramaters Klobuchar,8-paramaters Klobuchar and GPS Klobuchar models,which have verified the superiority of Klobuchar-like model.The evaluation method of ionospheric model by standard point positioning is proposed.The standard point positioning experiments in different solar activity periods and seasons have been conducted to evaluate ionospheric models above.The 3D positioning accuracy of Klobuchar-like model,14-paramaters Klobuchar model,8-paramaters Klobuchar model,GPS Klobuchar model and ionosphere-free model are 2.69 meters,2.87 meters,3.02 meters,3.45 meters,5.40 meters respectively,verifying the practicability and effectiveness of the Klobuchar-like ionospheric model.