## 7 Conclusions and Future Work
Although it was not shown in this paper, the receiver quality for different flight modules is very close. Although using slightly different receivers, the precision of Spire STRATOS receivers is of the same quality as those of COSMIC-2 TGRS receivers. 4. The effect of the SNR on the Spire retrieval accuracy. The UCAR Spire retrievals were consistent with those from the STAR-independent derived BA retrievals. The independent statistical analysis and validation from ERA5 and the direct bending angle profile comparison from these SRO cases above 35 km suggested: (i) the RO bending angle profiles retrieved from the GPS satellites were, in general, better than those from the GLONASS satellites; (ii) significant uncertainty existed for the RObending angle profiles from GLONASS, which may indicate a potential RO phase issue related to the clock, residual ionospheric effects, receiver noise, and orbit determination errors for GLONASS. We validated the Spire temperature and water vapor profiles by comparing them with collocated radiosonde in situ data. Generally, over the height region between 8 km and 16.5 km, the RS41 RAOB matched the Spire temperature profiles very well, with temperature biases of \(<\)0.02 K. Over the height range from 17.8 to 26.4 km, the temperature biases were \(\sim\)\(-\)0.034 K, with RS41 RAOB being warmer. 5. Estimates of the error covariance matrix for NWP. Below 9 km, the RO observation errors increased with a decreasing height and peaked at around 2 km. The magnitude of the relative BA errors in the troposphere for Spire was very close to that from COSMIC-2, KOMPSAT-5, and GeoOptics missions (also see [55]). In the lower troposphere, their peak values at a 2 km altitude over oceans (\(\sim\)15%) were more significant than those over land (\(\sim\)12%). These significant errors in the lower troposphere may have resulted from a greater RO retrieval uncertainty relative to the middle and upper troposphere, due to multipath propagation and a smaller SNR over mid-latitudes for COSMIC-2. The COSMIC-2 retrieval uncertainty was slightly more significant over oceans at mid-latitudes (45\({}^{\circ}\)N-30\({}^{\circ}\)N and 30\({}^{\circ}\)S-45\({}^{\circ}\)S), which may also be owed to COSMIC-2's SNR being lower in those latitudinal zones. The GNSS RO landscape has been rapidly evolving. Besides GeoOptics and Spire, more commercial RO data are available on the market (i.e., PlanetQ and others). More than 100 GNSS RO sensors are currently in orbit to track more signals (i.e., GPS, GLONASS, QZSS, IRNSS, GALILEO, Beidou, and GPS III). To use all the above different tracking and receiver systems for climate and NWP applications, we need to carefully examine the quality of each receiver and the inter-consistency of the retrieved data products. NOAA STAR has become a GNSS RO DSC (see [[https://www.star.nesdis.noaa.gov/smcd/GNSSRO/RO/index.php](https://www.star.nesdis.noaa.gov/smcd/GNSSRO/RO/index.php)]([https://www.star.nesdis.noaa.gov/smcd/GNSSRO/RO/index.php](https://www.star.nesdis.noaa.gov/smcd/GNSSRO/RO/index.php)) (access on 1 June 2023) and [31; 32; 33; 44; 45]). We will continue developing RO processing packages for PlanetQ and other RO missions and use our validation system (see [45]) to provide independent validation results. This will be for future work. Conceptualization, S.-p.H. ; methodology, S.-p.H., X.S. and Y.C. ; software, X.Z. and X.J. ; validation, X.Z., X.J. and W.M. ; formal analysis, X.Z. and X.J. ; investigation, X.Z. ; resources, X.Z. ; data curation, X.J. ; writing--original draft preparation, S.-p.H. ; writing--review and editing, S.-p.H., X.S. and Y.C. ; visualization, X.Z. ; supervision, S.-p.H. ; project administration, S.-p.H. ; funding acquisition, S.-p.H. All authors have read and agreed to the published version of the manuscript. This study was supported by NOAA grant NA19 NES4320002 (Cooperative Institute for Satellite Earth System Studies--CISESS) at the University of Maryland/ESSIC. The data presented in this study are available on request from the corresponding author.