GALILEO - Understanding GPS: Principles and Applications - page 583

Figure 10.11 GALILEO satellite. (Courtesy of ESA.)

In normal mode, the AOCS sensor/actuator configuration is based on Earth and
Sun sensors for attitude sensing together with reaction wheels for control. The
angular momentum accumulated by the reaction wheels will be unloaded with mag-
neto-torquers. In all other modes, a single-axis gyro is used for additional rate sens-
ing, together with monopropellant thrusters for impulse and attitude control.
Several of the modes can also operate in redundant configurations.
The propulsion subsystem is based on monopropellant thrusters. The baseline
of direct injection means that no boost from a lower transfer orbit is needed. The
propulsion subsystem is equipped with a set of eight thrusters; each thruster pro-
vides, under beginning of life (BOL) conditions, a nominal thrust of 1N. The propel-
lant and pressurant masses are stored in a common tank. The tank is sized for up to
78 kg of usable monopropellant-grade hydrazine.
The power subsystem is responsible for the generation, storage, conditioning,
and distribution of relevant power to the satellite. A power subsystem classical regu-
lated 50-V bus architecture has been selected and it is composed of:

• A power conditioning and distribution unit;
• A single Li-Ion battery;
• Two solar array wings, each one composed of three panels (2,300 mm × 1,000
mm) equipped with silicon high-efficiency cells, providing 1,525W at 53V in
EOL conditions, including one string failure as required by the power budget.
Total solar array area is around 14 m 2 .

The TT&C subsystem will provide redundant command reception, telemetry
transmission, and turnaround ranging at S-band. The TT&C subsystem operates in
two modes:

• ESA standard TT&C mode to be used during LEOP;
• Spread-spectrum mode to be used for nominal operations.