# Assessment of ZTD Derived from COSMIC Occultation Data with ECWMF, Radiosondes, and GNSS
###### Abstract  
Global Navigation Satellite System (GNSS) signals generate slant tropospheric delays when they pass through the atmosphere, which is recognized as the main source of error in many spatial geodetic applications. The zenith tropospheric delay (ZTD) derived from radio occultation data is of great significance to atmospheric research and meteorology and needs to be assessed in the use of precision positioning. Based on the atmPrf, sonPrf, and echPrf data from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) Data Analysis and Archiving Center (CDAAC) from 1 January to 31 December 2008 and 2012, we obtained the ZTDs of the radio occultation data (occZTD) and the corresponding radiosonde (sonZTD) and ECWMF data (ech2 TD). The ZTDs derived from ground-based global positioning system (GPS) observations from the International GNSS Service (IGS) were corrected to the lowest tangent point height of the matched radio occultation profile by the barometric height formula (gnsZTD). The statistical results show that the absolute values of the bias between occZTD and echZTD, sonZTD, or gnsZTD are less than 5 mm, and the standard deviations are approximately 20 mm or less, indicating that occZTD had significant accuracy in the GNSS positioning model even when the local spherical symmetry assumption error was introduced when the Abel inversion algorithm was used to obtain the refractive index profile of atmPrf. The effects of the horizontal/vertical matching resolution and the variation in the station height/latitude on the biases of occZTD and gnsZTD were analyzed. The results can be used to quantify the performance of radio occultation data for tropospheric delay error correction in dynamic high-precision positioning. 2022 9 July 2022 12 July 2022 9 July 20  
delay (ZTD) is recognized as the main source of error in many spatial geodetic applications, and has a great impact on navigation, positioning, and its applications [4; 5; 6; 7; 8; 9]. Many researchers have used the ZTD derived from meteorological data to verify the ground-based ZTD (GNSS-ZTD) measured by GNSS and have concluded that they are basically consistent with each other [4; 10]. From the comparison of the ZTD calculated from the European Center for Medium Range Weather Forecast (ECMWF) by the integration method at GPS stations with that from GPS stations of the China Crustal Movement Observation Network (CMONOC), [PERSON] found that the bias ranged from 11.5 to \(-\)28.6 mm with a corresponding average of \(-\)10.5 mm, while the largest root mean square of difference (RMSD) was 35.4 mm with an average of 24.3 mm [4], which supports the feasibility and reliability of computing the tropospheric delays and establishing the ZTD prediction model over China for navigation and positioning with ECMWF and National Centers for Environmental Prediction (NCEP) data. Other researchers have also used high-precision GNSS-ZTD to verify the ZTD derived from data obtained by many meteorological observation systems [11; 12]. When evaluating the accuracy of the precipitable water (PW) from ground-based GPS, [PERSON] found that GPS-sensed PW agreed with the dual-channel water vapor radiometer (WVR) observations with root mean square error (RMSE) accuracy of 1-2 mm, and the best agreement in PW between Microwave Radiometer and Radiosondes was approximately 2.2 mm for the nine baseline cases [13]. At the same time, GPS-ZTD can be used to construct an empirical model of ZTD related to time and location for precision point positioning (PPP). The Shanghai Astronomical observatory tropospheric delay model--Extended (SHAtropE) was developed based on the ZTD time series of the continuous GNSS sites from CMONOC and GNSS sites of surrounding areas [9]. SHAtropE was applied in the static PPP.