GCMS

Orbital Planes & Walker Pattern

Galileo uses a Walker 24/3/1 constellation: 24 operational satellites distributed across 3 orbital planes (A, B, C), each separated by 120° in Right Ascension of Ascending Node (RAAN). With 8 satellites per plane spaced 45° apart, every point on Earth always has at least 4 satellites above the horizon — the minimum needed for a 3D position fix.

All satellites orbit at 23,222 km altitude (Medium Earth Orbit) with 56° inclination to the equator. This inclination provides optimal coverage over Europe while maintaining global service. Each satellite completes one orbit in about 14 hours 7 minutes.

Additional satellites in auxiliary or extended slots provide redundancy and improve geometry. Doresa and Milena occupy eccentric extended orbits after their 2014 launch anomaly.

Ground Segment Architecture

The Galileo ground segment forms a global network with distinct station roles:

GCC — Galileo Control Centres (Fucino, Oberpfaffenhofen) are the nerve centres: they compute navigation messages, monitor constellation health, and plan all orbital manoeuvres.

ULS — Uplink Stations transmit navigation message corrections to the satellites for rebroadcast to users. Distributed globally to ensure every satellite receives fresh data each orbit.

TTC — Telemetry, Tracking & Command stations handle the housekeeping link: they receive satellite health telemetry, track precise orbits via ranging, and send platform commands.

GSS — Galileo Sensor Stations monitor the actual navigation signals as received on the ground. Their measurements feed back into orbit determination and clock correction at the GCCs.

Eclipse & Sun Geometry

A satellite enters eclipse when Earth blocks the Sun, cutting off solar power. The satellite must run entirely on batteries, and thermal conditions shift as the spacecraft cools without solar heating.

The sun beta angle (β) is the angle between the orbital plane and the Sun direction. When |β| > ~14° at Galileo altitude, the orbit never crosses Earth's shadow and the satellite stays sunlit continuously. When β is near zero, the satellite passes through the longest eclipses — up to ~56 minutes per orbit.

Eclipse seasons occur roughly twice per year when the orbital plane is nearly edge-on to the Sun. Operations teams prepare by pre-charging batteries and reducing non-essential loads.

Galileo Timeline