# Assessment of ZTD Derived from COSMIC Occultation Data with ECWMF, Radiosondes, and GNSS
## 3 Results and Discussions
### Residual Variations in Matched ZTDs between occZTD and gnsZTD  
When comparing occZTD and gnsZTD, two kinds of errors need to be considered. One is the error caused by the matching process. Since gnsZTD can provide a continuous time-stable ZTD, the matching error mainly comes from the uncertainty of the spatial position. This paper analyzes the spatial variation characteristics of \(\delta\)ZTD\({}_{3}\) caused by horizontal and vertical matching threshold values. The other is the ZTD errors affected by the locations of GPS stations. This section analyzes the characteristics of \(\delta\)ZTD\({}_{3}\) variations related to the altitude and latitude distribution of GPS stations. To analyze the effect of the error caused by the inconsistency in the vertical height between the lowest point of the occultation profile and the ground station, \(\delta\)ZTD\({}_{3}\) obtained by a filter with a horizontal distance of 300 km was analyzed. Figure 8a shows the number of occultation observations and the height difference (\(\delta\)h, km) between the lowest level of matched RO profiles and GNSS stations. It can be seen from Figure 8a that because COSMIC uses open-loop data tracking, the percent of \(\delta\)ZTD\({}_{3}\) with the \(\delta\)h within 1 km reaches 48.9%, and that with the \(\delta\)h within 3 km reaches more than 90%. Figure 8b shows the statistical precision of different \(\delta\)hs. Because ZTD is sensitive to \(\delta\)h, when \(\delta\)h is in the range of (0.0,3.0) km, its standard deviation increases. We used \(\delta\)ZTD\({}_{3}\) obtained by a filter with a vertical distance of 3 km to analyze the error caused by the horizontal distance (\(\delta\)r, km) between the lowest point of the occultation profile and the GPS station. Figure 9 shows that as \(\delta\)r increases, the bias does not change significantly, but the standard deviation increases from 16.2 mm to 33.8 mm. We used \(\delta\)ZTD\({}_{3}\) obtained by a filter with a horizontal distance of 300 km and a vertical distance of 1 km to compare and analyze the relationship between \(\delta\)ZTD\({}_{3}\) and HSL (km). Figure 10a shows the range of GPS station heights and the amounts of matched occultation data. Figure 10a shows that the HSL data within 1 km reached 88.5% of observations, and the GPS station height within 2 km was close to 97.2%. Figure 10b shows the \(\delta\)ZTD\({}_{3}\) statistics corresponding to different values of HSL. When \(\delta\)h is less than 1 km, the bias and standard deviation fluctuate as the station height increases, although gnsZTD is related to the station height, temperature, and water vapor. The absolute value of the bias is less than 10 mm, and the standard deviation is less than 25 mm. There is no significant trend in HSL. Figure 7: Statistics and distribution of residual ZTDs of atmPrf and ecDrrf in 2008 in (**a**) the south temperate (ST) zone; and (**b**) the north temperate (NT) zone. Blue indicates land and red indicates sea. Figure 8: \(\delta\)h’s variations in (**a**) frequency and (**b**) residual ZTDs of matched atmPrf and gnsZTD in 2008. Red square mark biases of residual ZTDs of matched atmPrf and gnsZTD in \(\delta\)h ranges, and blue error bars in \(y\)-axis indicate STDs. Red circles mark the location of the \(x\)-axis ticks. Figure 9: \(\delta\)r’s variations in residual ZTDs of matched atmPrf and gnsZTD in 2008. Red square mark biases of residual ZTDs of matched atmPrf and gnsZTD in \(\delta\)r ranges, and blue error bars in \(y\)-axis indicate STDs. Red circles marked the location of the \(x\)-axis ticks. Figure 11 shows the total and used amounts, and statistics of \(\delta\)ZTD\({}_{3}\) data in each latitude region for reducing the gross error by the 3-sigma principle to calculate the regional errors at different latitudes. Figure 11a shows that the data are mainly distributed in the mid-latitude region of the Northern Hemisphere (approximately 40%). Figure 11b shows the variation in \(\delta\)ZTD\({}_{3}\) with latitude.