Galileo Service Interruption Planned for System Upgrade

Operators of Galileo’s Ground Mission Segment at the Galileo Control Centre in Fucino, Italy. These operators oversee the navigation services of the Galileo constellation. Photo: Galileo

Galileo’s navigation messages will shortly stop being updated to enable the migration of a new release for Galileo’s ground mission segment, announced the European Space Agency.

Although the actual navigation signals will continue to be transmitted, the generation and uplink of the navigation message — which renews the contents of the signal — will be interrupted during the last week of January for about five weeks.

The accuracy of the navigation message received by users will slowly degrade, or in case of a reset in the satellite signal generator the message content will be dummy material. The users will be informed accordingly through a warning flag in the disseminated message, or through the online Notice Advisory to Galileo Users.

“The main benefits of this migration from V1.2 to V2.0 of the Galileo Ground Mission Segment are better overall performance and availability, increased robustness and improved operability,” explained ESA’s Martin Hollreiser, who is overseeing the mission segment’s development with Thales Alenia Space France as prime contractor. “The latter is achieved through enhanced operator interfaces, increased access to performance data and the automation of procedures.

“Various ‘non-conformances’ identified by operators over time have been fixed, while overall security has been further strengthened through treatment plans,” Hollreiser said. “This is in particular true for the Public Regulated Service, or PRS, the most secure Galileo class of signal. Finally, the number of Galileo Sensor Stations will increase from 12 to 15 worldwide and the number of Galileo Uplink Stations from four to five.

“The new Sensor Stations will be on Santa Maria in the Azores, Ascension Island in the mid-Atlantic and Kiruna in the Swedish Arctic. The additional Uplink Station will be on Papeete, in French Polynesia. Such system improvements have always been part of our planning since the contract began in 2011.”

Galileo’s worldwide ground mission segment is one of the most complex developments ever undertaken by ESA, with twin European Galileo Control Centres and a network of sensor and uplink stations deployed on remote sites across the world. They are all interconnected via a robust satcoms realtime network.

While satellite control and housekeeping are performed by Galileo’s Ground Control Segment in Oberpfaffenhofen, Germany, the Ground Mission Segment that provides the navigation and timing services and related performances is operated from a separate centre in Fucino, Italy.

Galileo’s ground segment encompasses twin European Galileo Control Centers and uplink stations on remote sites across the world. They are all interconnected via a robust satcom network. It includes Galileo Sensor Stations (GSSs) to provide coverage for clock synchronization and orbit measurements; uplink Stations (ULSs) to uplink navigation and integrity data to the Galileo satellite navigation payloads for rebroadcast to users; Telemetry, Tracking and Command Stations (TT&Cs) to manage the satellite platforms; an In-Orbit Test (IOT) site at Redu for satellite payload testing and a trio of Medium-Earth Orbit Local User Terminals (MEOLUTs) for search and rescue activities.

A user’s satnav receiver needs signals from at least four satellites. It actually measures the time the signal takes to reach its antenna from the satellites, then converts time into distance to compute its spatial position as the intersection of the four resulting spheres. To reach sufficient level of precision, the time synchronization of the overall system clocks must stay accurate to a few billionths of a second, and any drift in satellite orbits must be corrected for.

This is the task of the Ground Mission Segment in the control center, which continuously receives signal measurements from the sensor stations around the world. Based on these measurements, it computes exact satellite orbits and time synchronization for the satellites’ onboard atomic clocks.

These corrections are incorporated into the ephemerides included in an updated navigation message, which is uplinked to the satellites for dissemination to the user. The updated ephemerides information in the navigation signal provides to the user receivers the information required to compute their exact position.

The new Papette Uplink Station in Tahiti, French Polynesia, used for uplinking navigation messages for rebroadcast to users from Galileo satellites. Photo: Galileo