24 satellites are needed for global performance, yet there are 30. What are the extra six and what is their function?

• A. Extra satellites for redundancy in case of maintenance
• B. Extra satellites to provide more precise measurements
• C. Orbital planes; they ensure there are always 5 satellites in view at all times from all areas on the surface
• D. Ground-based backup satellites

Answer: (C)

The main idea is how a GNSS satellite constellation is arranged to ensure global coverage. Satellites are positioned in multiple orbital planes around the Earth, and the number of planes plus how many satellites sit in each plane determine how many satellites you can see from any location on the surface.

If there are 30 satellites, they’re effectively distributed as six orbital planes with five satellites in each plane. This setup guarantees that, no matter where you are, at least five satellites are above the horizon and available for a position fix. Having five visible satellites improves the geometry and reliability of the solution (better dilution of precision and integrity) compared to fewer satellites. The extra six aren’t ground backups or just for more precise measurements in isolation; their purpose is to ensure a constant minimum number of visible satellites globally.