## 3 Approaches for Quantifying the Precision, Stability, Accuracy, and Uncertainty of COSMIC-2 Data
### Stability Estimate  
With a SI-traceable global positioning system, RO is the only remote sensing technology that can provide long term stable measurements no matter how long it stays in orbit [44; 45; 46]. In this section, we compared UCAR consistently processed neutral atmospheric profiles from KOMPSAT-5 (launched in 2013), COSMIC (launched in 2006), TSX (launched in 2010), MetOp-A (launched in 2006), and MetOp-B (launched in 2012) to those collocated COSMIC-2 to quantify the stability of COSMIC-2 data. Figure 6a,b show the fractional refractivity comparisons for COSMIC-2-COSMIC pairs for October 2019 and COSMIC-2-KOMPSAT-5 pairs, respectively. Since only one COSMIC receiver (FM#1) is available in October 2019, the number of collocated COSMIC-2 and COSMIC pairs is limited. The RO-RO pairs are collected within 200 km at around 4 km altitude of the tangent point and 1.5 h apart within \(45^{\circ}\) N to \(45^{\circ}\) S. In total, we have around 500 collocation pairs in this comparison for COSMIC-2 and COSMIC collocated pairs and about the same number for COSMIC-2 and KOMPSAT-5 pairs. The mean difference is in the red line, and the standard deviation of the mean is in the green line. The SEM is in the horizontal black lines superimposed on the mean difference line. Figure 6a shows that although COSMIC has been in orbit since June 2006, where COSMIC-2 was in orbit for just four months, the mean fractional refractivity differences in the height ranging from 5 km to 35 km are all within the SEM. Figure 6: The fractional refractivity comparisons for **(a)** COSMIC-2-COSMIC pairs for October 2019 and **(b)** COSMIC-2-KOMPSAT-5 pairs. The mean differences are in the red line, and the standard deviations of the mean difference are in the green line. The sample numbers are in the blue line. The SEM is in the horizontal black lines superimposed on the mean. The mean fractional refractivity difference between 5 km and 35 km ranges from \(-0.5\%\) (at 5 km) to 0.4% (at 35 km), and their mean difference is about 0.15% from the surface to 45 km altitude. This result demonstrates that COSMIC data quality does not change after almost 14 years since the launch (stability of the data) and the consistent data quality between COSMIC-2 and COSMIC. Because the ray path for COSMIC and COSMIC-2 pairs may be of significantly different azimuth angles, the standard deviation (std) to the mean fractional refractivity differences is relatively large in the troposphere. The relatively large standard deviation above 30 km and the small difference from 35 km to 45 km (\(-0.5\%\)) may be owing to RO retrieval uncertainty caused by residual ionospheric signals. The comparison results between COSMIC-2 and KOMPSAT-5 pairs (Figure 6b) are similar to those from COSMIC-2 and COSMIC collocated pairs. The mean difference is equal to 0.12% from the surface to 40 km, with a std of 1.5%. Note that the small COSMIC-2-KOMPSAT-5 difference between 15 and 20 km altitudes (\(-0.3\%\)) is mainly due to the COSMIC2 L2P tracking errors. CDAAC has fixed the errors in the latest products (personal communication). To further examine the stability of COSMIC-2 data since the launch, we compared COSMIC-2 data to all other available RO missions (i.e., COSMIC, KOMPSAT-5, TSX, MetOp-A, and -B GRAS) from October 2019 to January 2020. All the COSMIC-2-RO pairs are within 2 h and 300 km. Figure 7 shows the monthly fractional refractivity at five height regions, respectively. Note that there is no collocated COSMIC data in January 2020 since the number of COSMIC occultations decreases to about 100 at that month. Figure 7 shows that the largest fractional refractivity standard deviation occurs at the 0-5 km (Figure 7a) and 40-60 km (Figure 7e) regions. The mean fractional refractivity values are comparable at different height regions and between satellites. The largest standard deviation occurs at 0-5 km because of the considerable moisture variation. The uncertainty of ionosphere electron density mainly causes considerable refractivity retrieval uncertainty from 40 km to 60 km.