Galileo will provide horizontal and vertical position measurements with sub-meter accuracy. Basic services with a lower precision will be available for free and open to anyone with a receiver compatible with Galileo. Full accuracy services will be available for government and military users, but also to all paying commercial customers. The Galileo constellation also provides global Search and Rescue Function with feedback to the user.
Serial production of the Galileo satellites suffered a one-year delay due to tooling and production issues, pushing the launch of the first two FOC satellites into 2014. The first 22 FOC satellites are booked for launches on five Soyuz rockets (two satellites per launch vehicle) and three Ariane 5 ECE vehicles (four satellites per launcher). Initial operations of the constellation will begin when the first 18 satellites are operational.
Each Galileo FOC satellite weighs 732.8 Kilograms and measures 2.91 by 1.7 by 1.4 meters in dimensions when its solar arrays are stowed - the core satellite body is 2.5 by 1.2 by 1.1m in size. In orbit, with both arrays extended, the satellite has a span of 14.67 meters from tip to tip.
The satellite consists of seven modules including a plug-and-play propulsion module for a simplified production and integration as part of the serial production of Galileo satellites.
The Galileo satellites are equipped with a Hydrazine monopropellant propulsion system consisting of a central Hydrazine tank and two thruster banks, each containing four 1-Newton thrusters. The propulsion system is used for orbit adjustments and constellation maintenance, attitude control and the maneuver to a disposal orbit at the end of the satellite's mission.
The heart of the Galileo satellites are four clocks - two passive hydrogen maser clocks and two Rubidium clocks. The hydrogen maser clocks are atomic clocks that use the ultra-stable 1.4 GHz transition in hydrogen atoms to achieve a timing accuracy of under 0.45 nanoseconds of drift over a 12-hour period. Rubidium atomic clocks are commonly used in space applications due to their robustness and reliability, but they achieve a lower accuracy of <1.8 nanoseconds over 12 hours.
The satellites are configured to run one hydrogen maser clock in primary mode and a Rubidium clock as hot backup. Should the hydrogen maser encounter any problem, an instantaneous switchover to the Rubidium clock would be performed.
In case of a failure of the primary hydrogen maser, the two spare clocks would automatically start up. On ground command, the secondary hydrogen maser could be activated to take over within a period of days as part of a highly redundant system that ensures that the satellites provide continuous timing signals.
Signals sent by the satellites include pilot signals which are data-free signals that only include a ranging code, not modulated by a navigation data stream. The ranging code is a Pseudo-Random Noise (PRN) sequence of 0s and 1s that allow the receiver to determine the signal's travel time. Data signals include binary-coded messages containing information on the satellite ephemeris (position and velocity), clock bias parameters for error correction, satellite health status and other complementary information.
Up to 150 active beacons can be received by one satellite simultaneously. The distress message is then modulated onto the L6 signal at 1,544 MHz and transmitted to dedicated ground stations with a latency of less than ten minutes. The position of the distress beacon is calculated with an accuracy of at least five Kilometers and can reach an accuracy of a few meters if the terminal is equipped with a Galileo receiver. Unique to Galileo is that the satellites provide feedback to the distress beacons acknowledging the reception of the signal.
The Galileo satellites use S- and C-Band for housekeeping communications. Two S-Band antennas are installed on the satellite for the transmission of telemetry data to ground stations and the uplink of commands for satellite operations. The S-Band terminal also receives and transmits ranging signals that provide precise orbit determination. C-Band is used to uplink mission data from Galileo uplink stations including clock bias data and integrity data about how well each satellite is functioning. These messages are relayed via the data signals in the navigation bands to allow receivers to correct for known errors.