Theory And Methodology Of Real-time And Post Of GNSS Estimated Zenith Tropospheric Delay

As a type of meteorological data and one of the main error resources for GNSS precise positioning,the tropospheric delay plays an important role in the predictions for severe weather events such as typhoons and heavy precipitation events.In recent years,the rapid development of the Global Navigation Satellite Systems(GNSS)has made it become one of the most reliable technique to remotely sense the zenith tropospheric delay(ZTD),due to its high spatio-temporal resolution,all-weather operability,global coverage and low cost.Therefore,this work focuses on retrieving real-time and post-processed ZTD estimates using high-accuracy multi-GNSS systems.The main works of this paper are as follows.(1)In this study,GNSS data from January 1 2019 to January 31 2019 are processed using Precise Point Positioning(PPP)at globally distributed stations.The performances of seven widely used global ocean tide loading(OTL)models are assessed and their influences on the GNSS-ZTD are evaluated by comparing against the ZTDs obtained from their respective co-located radiosonde profiles(RS).The results indicate that the inclusion or exclusion of the OTL effect will lead to a difference in the ZTD estimates of up to 3-15 millimeters for island stations,and up to 1-2 millimeters for inland stations.The differences of the ZTD determined using different OTL models is quite small,with a root-mean-square(RMS)value below 1.5mm at most stations.The comparison between the GNSS-ZTD and RS-ZTD indicates that the use of OTL models can improve the accuracy of GNSS-ZTD.(2)The results also indicate that the adoption of a smaller cutoff elevation,e.g.3° and 7°,can significantly reduce the differences between the ZTDs determined using GNSS and radiosonde techniques,in comparison to using a cutoff elevation of 15°.(3)To analyze the accuracy of ZTD estimates from GNSS post-processed observations,we selected ten GNSS tracking stations with uniform distribution worldwide,which can receive data from multi-GNSS systems provided by IGS.Different strategies including using single-GNSS,dual-GNSS and multi-GNSS observations to obtain ZTD were all performed to make an effective comparison.Compared with the ZTD estimates derived from using single-GNSS observations,the use of dual-GNSS observations can considerably improve the accuracy of GNSS-ZTD.Moreover,the performance of retrieving ZTD using multi-GNSS observations outperformed the others,with the RMS error of only 1 cm.(4)In this study,four GNSS tracking stations were selected,which can receive real-time data from multi-GNSS systems provided by IGS.By comparing the real-time ZTD estimates from single-GNSS and multi-GNSS observations with the post-processed ZTD estimates from IGS,it can be discovered that the ZTD estimates from single-GNSS observations only have a RMS error of around 1 cm.It is noted that the GPS outperformed other GNSS systems when using single-GNSS observations to retrieve ZTD.However,in comparison to single-GNSS,the adoption of multi-GNSS observations to obtain ZTD estimates clearly has a higher accuracy with a smaller RMS error.(5)This work also analyzed the accuracy of real-time and post-processed ZTD estimates using BDS-2 and BDS-3 by taking advantages of the completion of the cutting-edge Beidou system.It is shown that with the use of BDS-3,the accuracy of real-time and post-processed ZTD estimates retrieved from BDS-2 can both be significantly improved.