# Assessment of ZTD Derived from COSMIC Occultation Data with ECWMF, Radiosondes, and GNSS
## 4 Conclusions  
We obtained ZTDs (occZTD, sonZTD, echZTD) by integrating refractivity from atmPrf, sonPrf, and echPrf from CDAAC/COSMIC, and gnsZTD derived from ground-based GNSS-ZTD. The statistics in 2008 showed 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. The STDs in 2012 are slightly higher than that in 2008. For the temporal and spatial inconsistency between occZTD and sonZTD, the STD of \(\delta\)ZTD\({}_{2}\) is greatly higher than that of \(\delta\)ZTD\({}_{1}\). Since \(\delta\)ZTD\({}_{3}\) includes errors from the correction method and matching constraints, its bias and STD have increased compared to \(\delta\)ZTD\({}_{2}\). The spatial characteristics of the differences between ZTDs derived from COSMIC RO data and ZTDs from ECWMF and GPS-ZTD are discussed:  
1. In the Northern and Southern Hemispheres, \(\delta\)ZTD\({}_{1}\) increases with latitude; the ECMWF model can match the RO observation data better in the Northern Hemisphere than in the Southern Hemisphere; generally, the accuracy on land is higher than that in the marine region. 2. The accuracy of \(\delta\)ZTD\({}_{3}\) increases as the horizontal or vertical matching threshold value increases. 3. The relationship between the accuracy of \(\delta\)ZTD\({}_{3}\) and the altitudes of GPS stations is not obvious. However, similar to \(\delta\)ZTD\({}_{1}\), \(\delta\)ZTD\({}_{3}\) increases from high to low latitudes  
Figure 11: Latitudinal variations in (**a**) occultation event frequency; and (**b**) residual ZTDs of matched atmPrf and gnsZTD in 2008.  
in the Northern and Southern Hemispheres; the gnszTD can match the occultation observation data better in the Northern Hemisphere than in the Southern Hemisphere. This work assesses ZTD derived from COSMIC occultation data with data from ECWMF, radiosondes, and GNSS. The accuracy of \(\delta\)ZTD\({}_{3}\) at different horizontal distances from GPS stations can be used to evaluate the applicable region of occultation data and quantify the attenuation performance of occultation data for tropospheric delay errors in dynamic high-precision positioning, while the richness and uniformity of the occultation events to horizontal and vertical distances are related to the number and orbital design of low-orbit satellites (LEO) equipped with occultation equipment. COSMIC-2's satellite can capture a lower level of atmospheric information with a higher gain of occultation antennas than that of COSMIC-1 and provide more atmospheric occultation observation data for its support to GPS/GLONASS constellations [31]. We look forward to better verification results from the atmosphere RO products from COSMIC-2 and other LEO constellations. Conceptualization, N.F. and P.G. ; methodology, M.J. and F.L. ; software, N.F. ; validation, N.F., F.L. and M.W. ; formal analysis, M.J. and N.F. ; investigation, C.H. ; writing--original draft preparation, N.F. ; writing--review and editing, M.J., F.L., J.W., Z.W. and L.K. ; visualization, N.F. ; supervision, F.L. and P.G. ; funding acquisition, P.G. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Science Foundation of China (grant numbers are U1831116 and 11903064), the National Key R&D Plan (grant numbers are 2018 YFC1406104, 2020 YFA0713501), and the opening project of Shanghai Key Laboratory of Space Navigation and Positioning Techniques (grant number is NO.KFKT_201906). Not applicable. The COSMIC atmosphere profiles used in the present work can be downloaded from the website [[https://cdaac-www.cosmic.ucar.edu/cdaac/products.html](https://cdaac-www.cosmic.ucar.edu/cdaac/products.html)]([https://cdaac-www.cosmic.ucar.edu/cdaac/products.html](https://cdaac-www.cosmic.ucar.edu/cdaac/products.html)) (accessed on 1 January 2021).