Research On High-precision Tropospheric Mapping Function And Horizontal Gradient Modeling Methods For BDS/GNSS

As one major error source in space geodetic techniques,such as BDS/GNSS and VLBI,the tropospheric delay requires high-precision treatments relying on the zenith delay,mapping function and horizontal gradient models established through numerical weather model.The latest generation of mapping function（VMF3）,horizontal gradient（GRAD）and empirical tropospheric model（GPT3）not only uncover the domestic GNSS networks,such as CMONOC and NBASS,but also have problems of low-precision at low elevation angles and bad applicability in extreme weather scenarios as well as regions with complex terrain,limited by their modeling methods and spatiotemporal resolutions（6 h and 1°×1°）.In June 2018,ECMWF released the latest generation of global reanalysis product ERA5 with spatiotemporal resolutions of 1h and 0.25°×0.25°.It has become a key scientific and technological problem that how to using the latest ERA5 reanalysis establish a set of high precision,high spatiotemporal resolution mapping function and horizontal gradient models including the domestic GNSS networks.This dissertation aiming at the key problem carries out research on BDS/GNSS high-precision tropospheric mapping function and horizontal gradient modeling methods from four aspects of tropospheric delay ray-tracing technique,mapping function coefficient determination method,horizontal gradient modeling method and tropospheric model establishment.The main contents and contributions are as follows:1)The fast tropospheric delay ray-tracing method based on the latest ERA5 reanalysis was implemented,and the potential gains of ERA5 instead of ERA-Interim（ERAI）for tropospheric delay calculation were analyzed at 312 IGS stations covering the year of 2018.The results show that:the diurnal variation and accuracy of ERA5 ZTD are significantly better than ERAI with ZTD accuracy improved by 20.3%from 13.8 mm to 11.0 mm.The ERA5 slant wet delay at 5° elevation angle is significantly different from ERAI with RMS of 51.9 mm, corresponding to mapping factor difference and equivalent station height error RMS of 146.1×10^-3 and 10.4 mm.2)A combined mapping function coefficient determination method based on the continued fraction（MFlsmcom）was proposed and compared with the commonly used fast method（MFfast）during 40 days in 2020 at 905 GNSS stations.The results show that:the MFlsmcom method can improve the hydrostatic and wet modeling accuracy at 4°elevation angles from 3.6 mm and 3.0 mm of the MFfast method to 1.6 mm and 1.3 mm by 55.6%and 56.7%,respectively.The MFlsmcom method was also verified in GNSS PPP of 107 MGEX stations during 40 days in 2020.The results show that:the MFlsmcom method has similar coordinate repeatability to the MFfast method,while there are significant PPP height and ZTD biases between two methods at some stations,with maximum biases of-2.2 mm and 1.1 mm.3)A multi-parameterized horizontal gradient modeling method（ETILTING）is proposed with considering high-order variation terms and evaluated through the horizontal gradient modeling of 905 GNSS stations during 40 days in 2020.The results show that:the ETILTING method improves the modeling accuracy of hydrostatic and wet horizontal gradient at 4°elevation angles from 5.3 mm and 10.6 mm of the TILTING method to 1.6 mm and 4.9 mm by 69.8%and 53.8%, respectively.Otherwise,The PPP performance of the ETILTING and TILTING methods was verified at 107 MGEX stations.The results show that:the ETILTING method improves the N,E and U coordinate repeatability from 0.94 mm,0.94 mm and 3.11 mm of the TILTING method to 0.92 mm,0.89 mm and 3.06 mm,and there are significant PPP height and ZTD biases between two methods at some stations,with maximum biases of 8.6 mm and-4.5 mm.4)The hourly site-wise mapping function（WMF）and horizontal gradient（WHG） products from 2016 to 2020 as well as the site-wise tropospheric diurnal variation model（WTM）covering 1,583 stations including IGS,CMONOC and NBASS networks were established by using the ERA5 product and the proposed MFlsmcom and ETILTING methods.The accuracy of WMF,WHG and WTM was compared with the latest VMF3,GRAD and GPT3 at 524 GNSS stations in 2020.The results show that:the accuracy of WMF and WHG at 5°elevation angles is 0.8 mm and 5.6 mm improved by 10.5 mm and 16.6 mm relative to VMF3 and GRAD,while the mapping function and horizontal gradient accuracy of WTM is 20.7 mm and 34.8 mm and has an increase of 0.9 mm and 0.9 mm versus GPT3.5)The PPP performance using GPT3,WTM,VMF3 and WMF mapping functions was verified by GPS data processing of 6 domestic IGS stations from 2016 to 2020 as well as BDS data processing of 31 global MGEX stations in 2020.The results show that:the height coordinate repeatability for WTM and WMF is 5.25 mm and4.97 mm and slightly better than GPT3 and WTM.There are non-negligible biases between PPP height and ZTD of the four mapping functions,especially for GPT3and WTM,with maximum biases of-1.79 mm and 0.94 mm.Furthermore,the PPP performance of the four mapping functions was analyzed by GNSS data processing of 9 CMONOC stations in 12 days before and after the Henan rainstorm.The results show that:the height repeatability using the four mapping functions is6.22 mm,6.22 mm,5.95 mm and 5.88 mm,respectively,and the maximum height and ZTD biases are-2.42 mm and 1.23 mm.