Study On The Tropospheric Error Model For Global Navigation Satellite System And GNSS Meteorology

Space Geodesy is a metrology science,and a data feeder for many other disciplines,from climate,weather forecasting,oceanography,and geophysics.The most employed space geodesy tools are based on the use of electromagnetic high-frequency signals.All these signals have to propagate not only in space but through the Earth atmosphere to reach the ground tracking stations.The delays caused by plasmas are dependent of the radiowave frequency,and can be corrected by using multiple-frequency radio links.The delays caused by neutral matter,essentially lower atmospheres,are independent of the frequency,and cannot be corrected in this way.In space geodesy,neutral atmosphere error is also refered as tropospheric error.The tropospheric error must be corrected by using external information or models.If tropospheric delays are a nuisance for geodesists,they are a treasure for scientists doing atmosphere and climate-related studies(in the form of precipitable water estimates).Water vapor is a main contributor of the greenhouse effects.Besides,water vapor can induce an amplification loop initiated by carbon dioxide as a primary greenhouse gas.Water vapor also plays an important role in energy exchange and weather processes.Mapping functions and gradients in GNSS and VLBI applications were introduced in the sixties and seventies to model the microwave propagation delays in the troposphere,and they were proven to be effective tools for these applications.In this work,we revisited the physical and mathematical basis of these tools in the context of meteorology and climate application and assessed the accuracy of different aspects of the troposphere propagation models in space geodesy.In the end,we proposed a new parameterization method for the tropospheric error,which is suitable for multi-GNSS meteorology applications.The contents of this thesis include:(1)ZTD is the most widely used product from GNSS for meteorology applications.We compared zenithal total delays(ZTD)estimated value from Bei Dou/GPS combined signals,with ZTD estimates from GPS-only signals,and also with ZTD estimates from the IGS analysis center CODE(CODE products),to assess the intrinsic accuracies of these ZTD estimates.We found that the GPS-only ZTD estimates show a very good agreement with the CODE final ZTD products,but that a systematic negative bias of around 3 mm is showing up between the ZTD estimates from combined GPS/Bei Dou data with respect to GPS-only data.This indicates that the accuracy of Beidou satellite orbits and clock errors still need to be improved w.r.t.to IGS standards.(2)We assessed,in the framework of GNSS meteorology,the accuracy of GNSS propagation delays corresponding to the Saastamoinen zenith hydrostatic delay(ZHD)with reference to radiosonde ray-tracing delays over a 3-year period on 28 globally distributed sites.The results show that the Saastamoinen ZHD estimates have a mean RMS error of 1.7 mm with respect to the radiosonde.We also detected some seasonal signatures in these Saastamoinen ZHD estimates.This indicates that the Saastamoinen model,based on the hydrostatic assumption and ground pressure is insufficient to capture the full variability of the ZHD estimates over time with the accuracy needed for GNSS meteorology.(3)We studied,the accuracy of GNSS propagation delays corresponding to the most widely used and accurately VMF1/VMF3(hydrostatic and wet)mapping functions,with reference to radiosonde ray-tracing delays.We found that VMF3 slant hydrostatic delay(SHD)estimates outperform the corresponding VMF1 SHD estimates(equivalent SHD RMS error of 4.8 mm for VMF3 versus 7.1 mm for VMF1 at 5°elevation angle),with respect to the radiosonde SHD estimates.Unexpectedly,the situation is the opposite for the VMF3 slant wet delay(SWD)estimates compared to VMF1 SWD estimates(equivalent SWD RMS error of 11.4 mm for VMF3 versus 7.0mm for VMF1 at 5° elevation angle).Our general conclusion is that the joint approach using ZHD models and mapping functions must be revisited,at least in the framework of GNSS meteorology.(4)We introduced a new method to obtain the line-of-sight wet refractivity from a single GNSS receiver,which takes full use of the tropospheric zenith delay and gradients results.We assume that the wet refractivity is mainly governed by a scale height(exponential law)and that the departures from the decaying exponential can be mapped as a set of low degree 3D Zernike functions and Chebyshev polynomials.This work demonstrates examples of inversion with data acquired at the IGS station in Tahiti,French Polynesia.We also discussed the strengths and limitations of this method,the temporal and spatial resolution of this method.In the end,we also gave examples of meteorology and deep space calibration application of this new approach.