Construction Of VTEC Mode Based On Spherical Harmonics And Precision Analysis

The ionosphere is in the upper layer of the atmosphere and it's an important component of the Earth's near-Earth space environment.Since the ionosphere was found,ionospheric detection technology and ionospheric research theory has been developed by leaps and bounds.The emergence of GPS technology has opened a new era of ionospheric research.In order to achieve global ionospheric monitoring and research,acquiring high-precision global VTEC and VTEC external coincidence accuracy assessment is very important.The external coincidence accuracy assessment of the VTEC modeling should pay attention to three points: the external data source should be best from different systems;external data source accuracy should be best accurate;check points should be as much as possible and should be evenly distributed.However,the above points are difficult to do in the evaluation of ionospheric model accuracy,because GNSS is considered to be the highest accuracy of the current VTEC.The accuracy of the VTEC measured by the ocean altimeter satellite ignore the plasma layer above the height of the orbit of the ocean altimeter.Compared with the data of the ocean altimeter satellite,the main coverage area of the GPS data is the mainland.When evaluating the accuracy of GNSS VTEC by VLBI,we must take care to remove VLBI receiver bias which is the same as GPS DCB.The ionospheric detector can only measure the electron density at the peak height and can not give the electronic content from the GPS satellite to the observation station as a whole.Incoherent scattering radar is expensive and suitable for small area ionospheric detection.It can not evaluate the accuracy of global ionospheric VTEC.Therefore,it is significant to seek a more effective method for assessing the accuracy of the global ionospheric VTEC.The purpose of this paper is to simulate a solar anniversary GPS data based on the IRI model.The global VTEC two dimensional model is constructed by using the simulation data.The results are compared with the true value,and the temporal and spatial characteristics of the external coincidence accuracy are investigated.We also assess the accuracy of global DCB and projection function from 2003 to 2013.The research contents and main achievements of this paper can be summarized as follows:1.This paper has introduced the basic knowledge and research progress of the ionosphere.This paper has summarized the basic principle of constructing the ionospheric two dimensional delay mode based on GNSS observation data using spherical harmonic function.2.This paper has introduced the principle of building the global ionospheric VTEC two-dimensional model based on the simulation data.Frequency-independent factors in the carrier phase observations and pseudo range observations can be eliminated by making a difference,so the simulation L,P data only consider frequency-related factors.3.The detailed analysis of the global ionospheric VTEC model based on the simulation L,P data is carried out.Global ionospheric VTEC modeling results based on measured data compared with IGS results shows that the distribution of ionospheric anomalies and trends are consistent,indicating that the modeling results are reliable.The results of the global ionospheric VTEC modeling are in contrast to the true value.The 11 years average RMS is 2.2590 TECU,which means the modeling method is reliable.When the solar activity is violent,RMS is large;when the solar activity is low,RMS is small.In the day,the maximum deviation occurs at around 14:00 normally.Absolute error is more significant on both sides of the equator,the error in the middle and high latitudes is relatively small and the accuracy of the northern hemisphere is higher than that of the southern hemisphere.4.This paper has analyzed DCB of the global ionospheric VTEC modeling.DCB estimates shows that the errors in the low latitudes are relatively large and the errors in the middle and high latitudes are relatively small,which is similar to the error distribution of VTEC.The DCB estimation error in the northern hemisphere is smaller than that in the southern hemisphere and in the western hemisphere is smaller than that in the eastern hemisphere.5.The error of the projection function of the global ionospheric VTEC model is analyzed.The larger the cutoff height of the projection function,the smaller of the projection error.The maximum deviation of the projection function occurs at sunrise.