## 3 Approaches for Quantifying the Precision, Stability, Accuracy, and Uncertainty of COSMIC-2 Data
### COSMIC-2 Precision  
Precision is a measure of the repeatability of the measurement when both instruments are viewing the same target. For climate study, the precision is defined as the mean difference of the collocated measurements [41; 42]. COSMIC and CHAMP temperature precision is within 0.1 K in the lower stratosphere [16; 42]. Figure 4: The COSMIC-2 penetration depth over mid-latitude in North Hemisphere (45\({}^{\circ}\) N-30\({}^{\circ}\) N and 30\({}^{\circ}\) N-10\({}^{\circ}\) N) and tropical region (10\({}^{\circ}\) N-10\({}^{\circ}\) S), and mid-latitude Southern Hemisphere (10\({}^{\circ}\) S-30\({}^{\circ}\) S and 30\({}^{\circ}\) S-45\({}^{\circ}\) S) over oceans. We also compute the SEM, which is in a vertical line superimposed to the mean. Figure 5a-c shows the mean difference of refractivity, dry temperature, and water vapor mixing ratio collected from DOY 211 through 300 of 2019 from COSMIC-2 Flight Module 4 (FM4) and FM1 receivers where they are within 1200s and a distance separation at the ray tangent point of about 300 km apart. Figure 5a-c show that the mean difference for FM4-FM1 fractional refractivity and dry temperature from surface to 40 km altitude, and for water vapor mixing ratio from the surface to 20 km, is equal to \(-\)0.01%, 0.04K, and 0 g/kg, respectively. \begin{table}
\begin{tabular}{l l l l l l} \hline  & **45\({}^{\circ}\) N-30\({}^{\circ}\) N** & **30\({}^{\circ}\) N-10\({}^{\circ}\) N** & **10\({}^{\circ}\) N-10\({}^{\circ}\) S** & **10\({}^{\circ}\) S-30\({}^{\circ}\) S** & **30\({}^{\circ}\) S-45\({}^{\circ}\) S** \\ \hline COSMIC-2 0-500 \(\eta\)\(\psi\) group & 1.2 & 1.2 & 1.3 & 1.2 & 0.8 \\ \hline COSMIC-2 500-1000 \(\eta\)\(\psi\) group & 1.1 & 0.9 & 1.2 & 0.9 & 0.7 \\ \hline COSMIC-2 1000-1500 \(\eta\)\(\psi\) group & 0.9 & 0.7 & 0.8 & 0.7 & 0.6 \\ \hline COSMIC-2 1500-2000 \(\psi\)\(\psi\) group & 0.8 & 0.5 & 0.5 & 0.5 & 0.5 \\ \hline COSMIC-2 \(>\) 2000 \(\eta\)\(\psi\) group & 0.6 & 0.4 & 0.3 & 0.4 & 0.4 \\ \hline COSMIC-2 & 1.0 & 0.8 & 0.8 & 0.7 & 0.6 \\ \hline COSMIC & 1.0 & 1.1 & 1.2 & 0.9 & 0.7 \\ \hline KOMPSAT-5 & 1.0 & 1.2 & 1.3 & 0.9 & 0.6 \\ \hline MetOp-A & 4.6 & 4.4 & 3.7 & 4.0 & 2.0 \\ \hline MetOp-B & 3.1 & 3.4 & 3.3 & 4.4 & 1.9 \\ \hline MetOp-C & 4.6 & 4.6 & 4.2 & 4.4 & 1.8 \\ \hline \end{tabular}
\end{table} 
Table 1: The corresponding penetration heights for 80% of accumulated samples collected from June to October 2019 at the 45\({}^{\circ}\) N-30\({}^{\circ}\) N and 30\({}^{\circ}\) N-10\({}^{\circ}\) N, 10\({}^{\circ}\) N-10\({}^{\circ}\) S, 10\({}^{\circ}\) S-30\({}^{\circ}\) S, and 30\({}^{\circ}\) S-45\({}^{\circ}\) S latitude bands for COSMIC-2, COSMIC, KOMPSAT-5, and MetOp -A/-B, -C GRAS. COSMIC-2 penetration depths are separated into the 0-500 \(\eta\)\(\psi\) group, 500–1000 \(\eta\)\(\psi\) group, 1000–1500 \(\eta\)\(\psi\) group, 1500–2000 \(\eta\)\(\psi\) group, and \(>\)2000 \(\eta\)\(\psi\) group. Figure 5: The mean difference (in red line) and Median Absolute Deviation (MAD) (in green line) of (**a**) refractivity, (**b**) dry temperature, and (**c**) water vapor mixing ratio collected from DOY 211 through 300 of 2019 from COSMIC-2 FM4 and FM1 receivers. The mean difference (precision) for (**d**) refractivity, (**e**) dry temperature, and (**f**) water vapor mixing ratio for the same data pairs but on a smaller \(x\)-axis scale (the range of \(x\)-axis is from \(-\)1 unit to 1 unit) compared to those for (**a**)–(**c**), respectively. Figure 5d-f are the same as Figure 5a-b but on a smaller scale. The SEM at each vertical level is also computed and superimposed on the mean. COSMIC-2 fractional refractivity precision varies from \(-0.2\%\) to \(0.2\%\) below 8 km and is within \(0.1\%\) between 8 km and 30 km altitude. The mean precision for COSMIC-2 temperature is within 0.2 K, between 8 km and 35 km. The precision for the COSMIC-2 water vapor mixing ratio is within 0.15 g/kg below 5 km.