The Study Of The GNSS Tropospheric Zenith Delay Model And Mapping Function

The troposhpheric delay refers to the delay effect when the electromagnetic wave travelling through the neutral atmosphere,in which condition the speed and path of the electromagnetic wave may be both changed significantly.Due to its non-dispersive characteristic,this effect can not be eliminated by combination of dual-frequency observations.Furthermore,the nonhrdrostatic component of the troposoheric delay is difficult to be modeled,since the distribution of the water vapor is with great randomness and strong uncertainty.Therefore,the tropospheric delay is regarded as one of the most important error source for the GNSS positioning.Since the slant delay on any elevation always be expressed as the product of a zenith delay and a mapping function,and the zenith delay can be estimated during GNSS data processing if necessary,this dissertation mainly studies the research history,status,existing problems and accuracy of the hydrostatic zenith delay(ZHD)models and mapping functions.The theory and method of ray-tracing in the tropospheric refractivity field is analyzed and discussed.A ray tracing software named WHURT was developed.The applicability of three classical mapping functions in China is evaluated.The inhomogeneity of the tropospheric delay due to the anisotropy of the atmosphere is verified.Several mapping functions are fitted using the least square method.The importance of the parameter estimation of the zenith delay,as well as the advantage of the WHMF over VMF1,is revealed by several GNSS baseline experiments.The main research contents and conclusions include:1)The theory of the ray-tracing method is generalized systematically.We mainly discuss the refractivity calculation,the interpolation and extrapolation of the meteorological parameters,the transformation of the water vapor pressure,the conversion between different height systems,and the curvature radius of the earth.A ray tracing software named WHURT was developed,with the ability to calculate the slant delay for any given elevation and azimuth precisely,assuming that the meteorological data around the station is available,either from radiosonde station or the NWM.2)Using WHURT,the accuracy of two ZHD models(Saastamoinen and Hopfield),and five meteorological models(STP,UNB3m,GPT2 and GPT2w),is evaluated based on the 4 years meteorological data from over 90 radiosonde stations distributed worldwide.Errors of all models show a latitude-dependent tendency more or less.Among different meteorological models,the GPT2/GPT2w perform the best,which mainly contribute to the high temporal and spatial resolution of the meteorological data source,as well as their cute expression of the model.GPT2 is preferable since its equivalent accuracy and smaller size of the grid file,compared to GPT2w.The Saastamoinen model is found to be better than the Hopfield model,however,with a global systematic error of about-3 mm.Therefore a modified version of Saastamoinen model is fitted using the least square method,with an accuracy at sub-millimeter level on the global,comparing to the result of WHURT.3)We have assessed the applicability of three classical mapping functions over China,including NMF,VMF1 and GMF,by using WHURT,based on meteorological data from 8 radiosonde stations carefully selected in China.The errors of NMF show seasonal biases and is found to be greatly latitude-dependent.Errors of hNMF and wNMF increase with latitude positively and negatively,respectively.The VMF1 performs the best in most stations,howerver,there is also some seasonal biases in some stations,such as ZHHH and ZGCS.The accuracy of the GMF is between that of the NMF and VMF1,which is more close to the VMF1.4)We have discussed the basic theory of the fitting of the mapping function,and have studied the significant digit of the coefficients of the three-order continued fraction.Based on the Era-Interiam NWM product with high spatial and temporal resolution,which is provided by the ECMWF,slant delays on various elevations and azimuths are calculated by the WHURT,and results show that the non-negligible anisotropy commonly exists among the hydrostatic and non-hydrostaic components of the tropospheric delays,which is even more serious during the lower elevation section and under the sever weather conditions.Three new mapping functions named WHMF1,WHMF2 and WHMF3 respectively,are fitted using the least square method.The WHMF2 gets the highest precision in the lower elevation section,and is proven to be superior than VMF1 on many aspects,including the satellite residuals,inner precision of baseline component,and the accuracy of the estimated ZTD.5)Several GNSS baseline experiments are implemented.The result of the zero-baseline experiment shows that,the influence of zenith delay estimation to the N and E component is in a sub-millimeters level,and influence to the U component is less than 2mm,which can be reduced significantly by averaging results between several epochs.The result of a 1200m long baseline with a height difference of 156m shows that,the estimation of the zenith delay is necessary,without which will result to a bias of 8 mm for the height difference.