Global sampling characteristics and coverage complementarity of 12 GNSS radio occultation constellations

This study analyzes observations from COSMIC-2, FY-3, GRACE-C/D, KOMPSAT-5, MetOp-B/C, PAZ, PlanetiQ, Sentinel-6A, Spire, TerraSAR-X/TanDEM-X, TM-1, and YY-1 collected during 1–7 January 2026 to compare their spatial distributions, grid-cell occupancy, area-weighted surface coverage, and local-time sampling; quantify the sampling gains achieved through multi-mission integration; and assess changes in sampling when COSMIC-2 is excluded. 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 area-weighted global surface coverage of 99.71 % and 99.99 % at 1.0 and 2.0° resolutions,respectively. Within 6 h windows, the mean area-weighted joint coverage reaches 76.79 % at 2.0° resolution. Multi-mission integration also broadens local-time cycle. Excluding COSMIC-2 reduces the number of RO events within 5° S–5° N by 21.75 %, although the remaining 11 missions retain most of the mid- and high-latitude sampling. These results characterize the sampling complementarity and resilience of the multi-mission observing system.