## 3 Assessment of Spire Data Characteristics
### Stability  
To quantify the stability of the Spire data, we compare the fractional bending angle profiles for the Spire-PAZ, Spire-KOMPSAT-5, Spire-Metop-B, Spire-Metop-C, and Spire-Terrsar-X SRO pairs during the DO4 period in this section (Figure 12). Although the satellite SNR and equator crossing time differ, we do not see a systematic difference for each Spire-RO comparison. Note that, with a higher SNR, the Spire-Metop-B/-C GRAS STD is smaller than that of Spire-other-RO-missions, especially above a 30 km altitude. Table 2 lists the mean BA fractional differences and corresponding standard deviations for the Spire-PAZ, Spire-KOMPSAT-5, Spire-Metop-B, Spire-Metop-C, and Spire-Terrsar-X SRO pairs from the surface to a 40 km altitude. With a higher SNR, the STD for Spire-Metop-B and Spire-Metop-C is smaller than that of the others above a 30 km altitude. The STDs for the Spire-Metop-B and Spire-Metop-B and Spire-Metop-C SRO pairs are 1.94% and 2.89%, respectively, and those for the Spire-PAZ, Spire-KOMPSAT-5, and Spire-Terrsar-X SRO pairs are 3.25%, 3.20%, and 3.35%, respectively. Figure 10: The DO3 SRO fractional BA comparison for Spire S120 and S124 receivers 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 12: The fractional BA Spire–RO comparison for (**a**) mean differences, (**b**) the standard deviations, and (**c**) observation numbers for the Spire–PAZ, Spire–KOMPSAT-5, Spire–Metop-B, Spire–Metop-C, and Spire–TerraSAR-X SRO pairs during the DO4 period. The bending angle profiles generally show a good agreement between Spire and COSMIC-2 from 8 to 40 km, with an absolute mean of less than 0.2% and a standard deviation close to 4% for all five groups (see below). Above 30 km, the mean differences are minimal (less than 0.5%), but the standard deviations gradually increase with the height and reach 5% at 40 km. Below 10 km, the mean relative bending angle difference increases with a decreasing height and more significant oscillation toward the surface, owing to the sharp decrease in the sample pairs. At the same time, the standard deviations dramatically increase toward the lower-impact height and reach the maximum (\(\sim\)20%) at about 3 km. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Mean Difference** & **Standard Deviation** \\ \hline Spire–PAZ & 0.01\% & 3.25\% \\ \hline Spire–KOMSAT-5 & 0.18\% & 3.20\% \\ \hline Spire–Metop-B GRAS & 0.12\% & 1.94\% \\ \hline Spire–Metop-C GRAS & 0.06\% & 2.89\% \\ \hline Spire–TerraSAR-X & \(-\)0.13\% & 3.35\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The mean BA fractional differences and the corresponding standard deviations for the Spire–PAZ, Spire–KOMSAT-5, Spire–Metop-B, Spire–Metop-C, and Spire–TerraSAR-X SRO pairs from surface to 40 km altitude. Figure 13: Bending angle profile comparison between COSMIC-2 and Spire for (**a**) fractional BA profile differences, (**b**) the standard deviations, and (**c**) vertical observation numbers for five COSMIC-2 SNR groups (i.e., 0–500 \(v/v\), 500–1000 \(v/v\), 1000–1500 \(v/v\), 1500–2000 \(v/v\), and \(>\)2000 \(v/v\)). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Mean** & **Standard Deviation** & **Sample Number at** \\ \hline
0–500 \(v/v\) & 0.12\% & 4.15\% & 364 \\ \hline
500–1000 \(v/v\) & 0.10\% & 4.14\% & 1727 \\ \hline
1000–1500 \(v/v\) & 0.24\% & 3.97\% & 2559 \\ \hline
1500–2000 \(v/v\) & 0.16\% & 3.87\% & 2325 \\ \hline \(>\)2000 \(v/v\) & 0.19\% & 4.07\% & 332 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The mean fractional BA differences, standard deviations of the mean differences, and sample numbers at 10 km altitude for Spire–COSMIC-2 SRO pairs for different COSMIC-2 SNR groups (i.e., 0–500 \(v/v\), 500–1000 \(v/v\), 1000–1500 \(v/v\), 1500–2000 \(v/v\), and \(>\)2000 \(v/v\)).