Which of these attributes best describes the NAVSTAR GPS satellite constellation?

• A. 66 cross-linked satellites in LEO providing global navigation and timing data.
• B. 24 satellites in MEO arrayed in 6 orbital planes, each inclined 55 degrees, providing position, navigation, and timing data.
• C. 3-5 satellites in GEO, sending navigation and timing data to global locations between 60 degrees N and 60 degrees S latitudes.
• D. 24 satellites in sun-synchronous orbits, providing navigation and timing data globally, as the earth rotates beneath them.

Answer: (B)

The GPS constellation is designed for reliable global navigation and precise timing using satellites in medium Earth orbit, arranged to keep the geometry favorable from any point on Earth. The description that fits this is 24 satellites in medium Earth orbit distributed across six orbital planes, each inclined about 55 degrees, providing position, navigation, and timing data. That setup ensures at least four satellites are visible from anywhere on the globe, which is enough to solve for your three spatial coordinates plus the receiver's clock error, delivering accurate positioning and timing. The other configurations—low-Earth-orbit clusters, a small geostationary group, or sun-synchronous orbits—don’t match how GPS is designed to provide global, continuous PNT coverage.