Ionospheric Effect On Atmospheric Parameters Retrieval Of COSMIC Occultation

GNSS occultation atmospheric sounding technology has the advantages of high accuracy,high vertical resolution and global coverage,which plays an important role in climate research,weather monitoring and weather forecast.Ionospheric error is an important source of errors in GNSS occultation atmospheric sounding,and there is still residual error after correction due to the influence of higher-order ionospheric terms.It is of great significance to seek effective ionospheric correction methods and to explore the characteristics of ionospheric residual error and its influence on atmospheric parameter inversion.Based on COSMIC occultation data and geometric optics inversion method,the ionospheric correction methods and the characteristics of ionospheric residual error are analysed and studied in this paper.(1)Analysis of main error sources in COSMIC occultation atmospheric parameter inversion.The error sources such as doppler deviation,doppler frequency shift,receiver noise,local multipath effect and clock stability are analysed.The results show that clock stability,receiver noise,doppler deviation and ancillary period amplitude have great influence on atmospheric parameter inversion.(2)Comparison of correction methods for ionospheric error of COSMIC occultation.The results show that both methods have certain effects,and the effect of bending angle linear combination method is better than that of phase linear combination method above 35 km.(3)The influence of ionospheric residual error on COSMIC occultation events.The data of COSMIC occultation on January 5,2009 and January 12,2011 are analysed.The results show that the ionospheric residual error is positively correlated with the intensity of solar activity,negatively correlated with latitude in the middle and high latitudes.(4)The simulation analysis of ionospheric residual error effect of COSMIC occultation.The MSIS90 atmospheric model and 3D NeUoG ionospheric model are used to simulate and analyse the results.(1)The ionospheric error of L2 carrier is larger than L1.The maximum ionospheric error is 0.1 mrad at the bottom of stratosphere.Compared with the spring and autumn,the ionospheric errors are larger in summer and winter,and the bending angle error can reach 0.15 mrad in the range of 0-65 km.(2)Bending angle residual ionospheric error increases with altitude,at the top of the stratosphere,the relative errors of the bending angle caused by ionospheric residual error are mostly within ±10% and the maximum is 20%.At the bottom of the mesosphere,the relative errors of the bending angle caused by ionospheric residual error are between-10% and 40%.(3)The influence of ionospheric residual error on atmospheric parameter inversion is time-dependence and seasonal.(4)Statistical optimization can reduce the influence of ionospheric residual error on the accuracy of refractivity and temperature inversion,and the statistical optimization effect is more obvious at the top of stratosphere and the bottom of mesosphere.