## 4 Quantify the Spire Retrieval Accuracy and Uncertainty
### Assessment of the Spire Bending Angle Accuracy by Comparing with ERA5  
With a similar lowest penetration height to that from COSMIC-2 (see Section 3.1), Spire data can also explore the atmosphere in the lower troposphere. Figure 15 shows the Spire-ERA5 and COSMIC-2-ERA5 BA fractional differences in different latitudinal zones from 45\({}^{\circ}\)N to 45\({}^{\circ}\)S. Here, the Spire-COSMIC-2 8 RO pairs (compared to the ERA5 profiles interpolated onto the Spire-COSMIC-2 locations and times) are used in the comparisons. Figure 15 shows that, although the Spire data are of a lower SNR, there is no noticeable difference for the Spire-ERA5 and COSMIC-2-ERA5 pairs in different latitudinal zones. The Spire-COSMIC-2 8 RO pairs from 1 to 31 August 2022 are used in the comparisons. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Mean** & **Standard Deviation** \\ \hline GPS & 0.06\% & 0.73\% \\ \hline GLONASS & 0.06\% & 0.78\% \\ \hline GALILEO & 0.06\% & 0.69\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The means and standard deviations for the fractional refractivity difference between STAR and UCAR Spire for GPS, GLONASS, and GALILEO. We also compare the Spire-ERA5 and COSMIC-2-ERA5 BA comparisons for different SNR groups. Again, the mean differences for Spire-ERA5 and COSMIC-2-ERA5 have no significant differences among the five SNR groups. Assessment of Spire Temperature and Water Vapor Accuracy by Comparing to RS41 Radiosonde Observation  
Spire's geographic location is determined by the longitude and latitude of the perigee point at the occultation point. This study collected collocated Spire retrievals and RS41  
Figure 14: Bending angle profile comparison between STAR and UCAR Spire for GPS (in red line), GLONASS (in orange line), and GALILEO (in blue line) for (**a**) the fractional mean difference, (**b**) the standard deviation, and (**c**) the observation numbers from surface to 40 km altitude. Figure 15: Spire–ERA5 mean BA fractional difference and corresponding standard deviations for (**a**) 45\({}^{\circ}\)N to 45\({}^{\circ}\)S, (**b**) 45\({}^{\circ}\)N to 30\({}^{\circ}\)N, (**c**) 30\({}^{\circ}\)N to 30\({}^{\circ}\)S, and (**d**) 30\({}^{\circ}\)S to 45\({}^{\circ}\)S. Similar to (**a–d**), we also compare the COSMIC-2 and ERA5 BA fractional differences and corresponding standard deviations in (**e**) 45\({}^{\circ}\)N to 45\({}^{\circ}\)S, (**f**) 45\({}^{\circ}\)N to 30\({}^{\circ}\)N, (**g**) 30\({}^{\circ}\)N to 30\({}^{\circ}\)S, and (**h**) 30\({}^{\circ}\)S to 45\({}^{\circ}\)S.  
RAOB data within the position and time ranges of 150 km and 2 h, respectively. Any Spire RO profiles with a bad attribute flag (equal to 1) were excluded. We collected 16 months of Spire and RAOB data covering the DO3 and DO4 periods. About 19,000 collocated Spire and RS41 pairs were collected for this study. We use the hydrostatic equation to convert the RAOB pressure level into the geometric height. Then, we interpolate the Spire temperature data to the radiosonde's geometric heights at 8, 9.6, 11, 12.5, 14.5, 16.5, 17.5, 20, 23, 26.5, and 30 km altitudes for the temperature comparison. RO and RAOB humidity data are also interpolated to fixed levels for comparison, and the vertical sampling interval is 0.6 km over the height region of interest below 8.4 km. Using this approach, we maintain a high vertical resolution below 8.4 km for the humidity comparison. Figure 16 shows the height-dependent mean temperature/humidity differences (UCAR Spire-RS41) and associated uncertainties. Table 5 shows the mean temperature differences over three height layers, e.g., 8-11 km, 12.5-16.5 km, and 17.8-26.4 km altitudes, which are derived from the comparison of the retrieval with the RS41 RAOB observations shown in Figure 16a. Figure 16: (**a**) Differences (dash lines) and uncertainties (dot lines) of Spire temperature profiles retrieved by UCAR wetPf2 compared to RS41 RAOB data.