## 2 Data and Methodology
### UCAR RO Data, STAR-Processed Spire Data, Global Radiosonde Data, and Reanalysis Data  
This study examines the Spire data processed by UCAR. The other RO missions used in the Spire-RO comparisons, including COSMIC-1, COSMIC-2, KOMPSAT-5, PAZ, Metop-B, Metop-C, and TerraSAR-X, are also downloaded from UCAR. A general description of the UCAR inversion package and quality control procedures are detailed at [[http://cdaac-www.cosmic.ucar.edu/cdaac/doc/documents/[PERSON]_newroom.pdf](http://cdaac-www.cosmic.ucar.edu/cdaac/doc/documents/[PERSON]_newroom.pdf)]([http://cdaac-www.cosmic.ucar.edu/cdaac/doc/documents/[PERSON]_newroom.pdf](http://cdaac-www.cosmic.ucar.edu/cdaac/doc/documents/[PERSON]_newroom.pdf)) (access on 1 June 2023), and are not further described. Any UCAR data flagged as \"bad\" are not used in this study. To further evaluate the UCAR Spire RO data, we compared the UCAR Spire data with those derived from the STAR independently developed inversion package. The STAR RO package was designed to quantify the observation uncertainty propagation from the clock error and geometry determination to the bending angle/refractivity and temperature/humidity profiles [33]. The package consists of the GNSS RO L1a (phase and pseudo-range) to L1b (excess phase) processing modules (see Appendix), Full-Spectrum Inversion (FSI) based from L1b to L2 (bending angle and refractivity) data inversion module [43], and the neutral atmospheric temperature and humidity profiles derived by using the one-dimension variational (1D-Var) inversion module [45]. We compared the STAR-processed Spire bending angle data with those from UCAR to quantify the retrieval structure uncertainty due to the differences in the algorithm implementations [22; 23; 24; 25]. In this study, we will evaluate the UCAR Spire RO temperature and water vapor accuracy and uncertainty through a comparison with Vaisala RS41 radiosonde observation (RAOB) data, which were downloaded from the National Center of Environmental Prediction (NCEP) Automated Data Processing (ADP) Global Upper Air Observational Weather Data (ds351.0, [[https://rda.ucar.edu/datasets/ds351.0](https://rda.ucar.edu/datasets/ds351.0)]([https://rda.ucar.edu/datasets/ds351.0](https://rda.ucar.edu/datasets/ds351.0)), accessed on 20 July 2023). As mentioned in [26; 31], the RAOB measurement quality may vary with the sensor types. The RAOB temperature measurements are affected by the solar radiation effect on the sensor [26]. Introduced in 2013, the Vaisala RS41 is currently the most accurate radiosonde observation system, which has an improved precision and accuracy compared to RS92 (see [26; 31]). With an advanced manufacturer's correction, the RS41 radiation effect on the sensor is significantly diminished compared to RS92. The NCEP RAOB dataset contains information such as pressure, geopotential height, air temperature, dew point temperature, wind direction, and wind speed. Including a few significant levels, the radiosonde data are available at up to 20 mandatory levels, ranging from 1000 millibars to about 10 millibars. Complete daily data can be downloaded in Binary Universal Form for the Representation  
Figure 5: Latitudinal distribution for COSMIC-2 L1 SNR from 15 February to 15 March 2022, for (**a**) GPS and (**b**) GLONASS. of Meteorological Data (BUFR) format. The software library NCEPLIBS-buff (also often referred to as BUFRLIB) is used in this study to decode the BUFR radiosonde data. This study also uses ERA5 reanalysis model data as the background for the observation-minus-background (O-B) (Spire vs. ERA5) bending angle/refractivity difference and uncertainty analysis. The data used in this paper have been gridded to a regular latitude-longitude grid of 0.25 degrees for the reanalysis. ERA5 is updated daily with a latency of about five days. The background data from ERA5 used for the comparison with Spire are refractivity as a function of geopotential height and bending angle as a function of impact parameter.