## 3 Assessment of Spire Data Characteristics
### Precision  
Precision is one of the most crucial qualities for using RO data in climate studies [1; 2; 3; 4; 5; 20; 21; 22; 23; 24; 25]. For climate analyses, the precision of RO data is quantified as the mean differences of the retrieved variables derived for SRO pairs when viewing the same emitter. The uncertainty of the mean (standard deviation, STD) at the vertical level indicates how well one can use RO data to detect climate signals. In [32] and [50], it was demonstrated how SRO pairs are matched. The coplanar (SRO pairs) eliminate the potential comparison uncertainty owing to the viewing geometry. From about 26 to 30 satellites were in the Spire constellation over the DO3 and DO4 periods. Therefore, Spire provided plenty of opportunities for inter-comparisons of the RO observations between two flight modules from the SRO pairs. We collocated these observations from two receivers only 20 min and 200 km apart, but viewed the identical emitters. Figure 9 shows the fractional bending angle, dry temperature, and water vapor mixing ratio comparisons between the collocated Spire S128 and S119 receivers. The mean differences are very close to zero from the surface to a 40 km altitude for all three physical quantities. The standard deviations from the bending angle, dry temperature, and water vapor mixing ratio are similar to those from other RO missions, such as COSMIC-2 and COSMIC-1 [31]. Although it is not shown, the mean bending angle difference and STD of the differences for the SRO pairs between any Spire flight modules are consistent with those between S128 and S119. This indicates that the Spire receiver quality has a high precision. Because the Spire SNR range varies when tracking different GNSS systems (see Figure 4), Figure 10 also examines how the Spire precision may differ with different SNR ranges. To see how the Spire precision may change when tracking other GNSS systems, we compare the fractional mean BA difference for Spire S124 and S120 from the surface to 40 km \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & **10\({}^{\circ}\)N–** & **30\({}^{\circ}\)N–** & **10\({}^{\circ}\)S–** & **45\({}^{\circ}\)N–** & **30\({}^{\circ}\)S–** & **60\({}^{\circ}\)N–** & **45\({}^{\circ}\)S–** & **90\({}^{\circ}\)N–** & **60\({}^{\circ}\)S–** \\  & **10\({}^{\circ}\)S** & **10\({}^{\circ}\)N** & **30\({}^{\circ}\)S** & **30\({}^{\circ}\)N** & **45\({}^{\circ}\)S** & **45\({}^{\circ}\)N** & **60\({}^{\circ}\)S** & **60\({}^{\circ}\)N** & **90\({}^{\circ}\)S** \\ \hline COSMIC- & \multirow{2}{*}{0.85 km} & \multirow{2}{*}{0.90 km} & \multirow{2}{*}{0.75 km} & \multirow{2}{*}{1.35 km} & \multirow{2}{*}{1.10 km} & \multirow{2}{*}{} & \multirow{2}{*}{} & \multirow{2}{*}{} & \multirow{2}{*}{} \\
2 & & & & & & & & & \\ \hline Spire & 0.90 km & 0.90 km & 0.75 km & 0.80 km & 0.55 km & 0.45 km & 0.25 km & 0.45 km & 0.20 km \\ \hline KOMPSAT- & \multirow{2}{*}{1.85 km} & \multirow{2}{*}{1.50 km} & \multirow{2}{*}{1.15 km} & \multirow{2}{*}{0.40 km} & \multirow{2}{*}{0.95 km} & \multirow{2}{*}{0.35 km} & \multirow{2}{*}{0.40 km} & \multirow{2}{*}{0.25 km} & \multirow{2}{*}{0.20 km} \\
5 & & & & & & & & & \\ \hline PAZ & 2.65 km & 1.85 km & 2.05 km & 0.90 km & 1.30 km & 0.45 km & 0.45 km & 0.35 km & 0.35 km \\ \hline \end{tabular}
\end{table} 
Table 1: The lowest penetration heights of 80% of the total data for different RO missions in different latitudinal zones. Figure 9: The mean difference (in red line) and standard deviation (in green line) for (**a**) fractional bending angle, (**b**) dry temperature, and (**c**) water vapor mixing ratio comparisons for the Spire S128 and S119 DO3 SRO pairs. The corresponding numbers of observations are in blue lines. altitudes, but separated with GPS, GLONASS, and GALILEO, respectively (Figure 10a). The corresponding standard deviations and sample numbers at each vertical level are shown in Figure 10b,c, respectively.