Global Sampling Characteristics and Coverage Complementarity of 12 GNSS Radio Occultation Constellations

Abstract. This study uses observations from COSMIC-2, FY-3, GRACE-C/D, KOMPSAT-5, MetOp-A/C, PAZ, PlanetiQ, Sentinel-6A, Spire, TerraSAR-X/TanDEM-X, TM-1, and YY-1 during 1–7 January 2026 to compare their spatial distributions, grid coverage, local-time sampling, and retrieval accuracy; it also assesses the benefits of multi-mission integration and the potential effects of COSMIC-2 retirement on the integrated observing capability. The results show that orbital configuration determines latitudinal and local-time sampling, whereas event abundance and constellation size primarily control grid coverage. Large constellations such as YY-1 and TM-1 provide the strongest global continuity, while missions with different orbital inclinations offer complementary sampling across latitude bands. Combining all missions substantially reduces spatial gaps and provides near-global coverage at 1.0° and 2.0° resolutions. Within 6 h windows, joint coverage reaches 70.98 % at 2.0° resolution. The combined observations also broaden local-time sampling, whereas COSMIC-2 remains particularly important for equatorial coverage. Therefore, the retirement of COSMIC-2 would reduce equatorial sampling capability and should be considered in the development of future multi-mission GNSS-RO observing systems. Comparisons with ERA5 indicate broadly consistent retrieval accuracy among most missions between 10 and 35 km, with larger differences in the lower troposphere. These findings demonstrate the value of integrating missions with diverse orbital configurations and highlight the need to sustain complementary sampling after the retirement of key missions.