# Global GNSS-RO Electron Density in the Lower Ionosphere
## Appendix B GNSS-RO Data Availability and Statistics
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**LEO** & **Mission** & **Init, Final** & **Sun-Syn** & **Lat** & **Top RO** & **Tracked** & **Max No.** \\
**Satellites** & **Lifetime** & **Alt (km)** & **(Asc ECT (1))** & **Coverage** & **Ht (km)** & **GNSS** & **ROs/day** \\ \hline CHAMP & 2001–2008 & 450,330 & varying & 90\({}^{\circ}\)S/N & 140 & G & \(\sim\)250 \\ COSMIC-1 (2) constellation & 2006–2020 & 525,810 & varying & 90\({}^{\circ}\)S/N & 130 & G & \(\sim\)4000 \\ SAC-C & 2000–2013 & 705 & 10:30 & 90\({}^{\circ}\)S/N & 90 & G & \(\sim\)300 \\ MetOp-A & 2006–2021 & 820 & 19:00 (3) & 90\({}^{\circ}\)S/N & 90 & G & \(\sim\)720 \\ MetOp-B & 2012– & 820 & 19:00 & 90\({}^{\circ}\)S/N & 90 & G & \(\sim\)700 \\ MetOp-C & 2018– & 820 & 19:00 & 90\({}^{\circ}\)S/N & 90 & G & \(\sim\)650 \\ C/NOPS & 2008–2015 & 850,350 & varying & 37\({}^{\circ}\)S/N & 170 & G & \(\sim\)320 \\ KOMPSAT-5 & 2015– & 560 & 06:00 & 90\({}^{\circ}\)S/N & 135 & G & \(\sim\)600 \\ TSX & 2009– & 520 & 18:00 & 90\({}^{\circ}\)S/N & 135 & G & \(\sim\)320 \\ TDX & 2016– & 520 & 18:00 & 90\({}^{\circ}\)S/N & 135 & G & \(\sim\)320 \\ GRACE & 2007–2017 & 475,300 & varying & 90\({}^{\circ}\)S/N & 140 & G & \(\sim\)230 \\ FY-3C (4) & 2013– & 838,850 & 22:00 & 90\({}^{\circ}\)S/N & 135 & G & \(\sim\)530 \\ FY-3D & 2017– & 838 & 13:30 & 90\({}^{\circ}\)S/N & 135 & G & \(\sim\)600 \\ FY-3E & 2021– & 830 & 05:30 & 90\({}^{\circ}\)S/N & - & G & - \\ COSMIC-2 (5) constellation & 2019– & 715,545 & varying & 44\({}^{\circ}\)S/N & 90–130 & G,R & \(\sim\)6500 \\ PAZ & 2018– & 520 & 18:00 & 90\({}^{\circ}\)S/N & 135 & G & \(\sim\)320 \\ Sentinel-6A & 2020– & 1336 & varying & 90\({}^{\circ}\)S/N & 80 & G,R & \(\sim\)830 \\ Spire (6) constellation & 2018– & varying & varying & 90\({}^{\circ}\)S/N & 170–400 & G,R,EJ & \(\sim\)4000 (7) \\ Other (9) Constellations & 2020– & varying & varying & varying & varying & varying \\ \hline \end{tabular} \({}^{()}\) Ascending-orbit equator crossing time (Asc ECT), \({}^{(2)}\) The COSMIC-3 spacecraft never reached the intended orbital altitude and was operated at 725 km for the rest of its mission. \({}^{(3)}\) The spacecraft started to drift away from the Sun-sync orbit since \(\sim\)2021. A scan experiment with the high-rate data up to \(h_{t}=300\) km was conducted  
\end{table} 
Table 1: Summary of GNSS-RO data in this study. between 2020.161 and 2020.254. [4] The FY-3C spacecraft started to drift away from the Sun-sync orbit (SSO) from 2016 (see Appendix B). [5] The COMSIC2 NRT data contain GNSS-RO profiles from GPS and GLONASS. [6] The Spire constellation acquires RO profiles from GPS, GLONASS, Galileo, and QZSS before 2021, and BDS since 2022. [6] NOAA's Commercial Weather Data Pilot (CWDP) NRT data published by CDAAC from September 2021 to the present. The daily RO number is from November 2021 statistics. [7] NASA's Commercial SmallSat Data Acquisition (CSDA) data from November 2019 to the present. The daily RO number is from November 2021 statistics, of which approximately 880 are overlapped with the NOAA CWDP profile. [5] Other commercial GNSS-RO data include the constellations such as GeoOptics and PlanetIQ. A great boost in diurnal coverages comes from the recent COSMIC-2 and Spire constellation. Both constellations are capable of sampling a full LST range within a month, but COSMIC-2 covers only low latitudes (<44 degS/N) with a good longitudinal sampling. On the other hand, the Spire constellation has a good pole-to-pole coverage with a smaller number of longitudinal samples at low latitudes, which was a similar situation to COSMIC-1. The combined COSMIC-2/Spire constellation, therefore, will provide an unprecedented spatiotemporal coverage for studying the ionosphere in the years to come. In ionospheric limb sounding, the orbit altitude of RO receivers can have a significant effect on the retrieval, because of the extensive vertical range of ionosphere. Satellites are often orbiting inside the ionosphere.