SMOS mission satellite
The SMOS instrument MIRAS is carried on a standard 'spacecraft' bus called Proteus, which was developed by the French space agency CNES (Centre National d'Etudes Spatiales) and Alcatel Alenia Space. It is a generic platform with well-defined interfaces so that with a little modification, the SMOS scientific instrument was mounted on the top of the spacecraft through four interface pods.
Although the spacecraft bus is small, measuring just one cubic metre, it acts as a service module accommodating all the subsystems that are required for the satellite to function.
After launch, when the spacecraft separateded from the launcher, there was an automatic start-up sequence, which resulted in the deployment of two symmetrical solar arrays. A Sun-synchronous, dawn-dusk orbit is required to obtain the optimum data on soil moisture and ocean salinity. The solar arrays will always be illuminated, except for short eclipse periods in winter.
Proteus uses a GPS receiver for orbit determination and control, which provides satellite position information, and a hydrazine monopropellant system for four 1-Newton thrusters that are mounted on the base of the spacecraft. Nominal attitude control is based on a gryo-stellar concept. The Star Tracker is accommodated on the payload and provides accurate attitude information for both the instrument measurements and the satellite attitude control. Three 2-axis gyroscopes are used to measure the change in the spacecraft orientation, and thus provide the accurate attitude knowledge needed to fulfil stability and pointing requirements. Four small reaction wheels generate torque for attitude adjustment. In safe mode, a less precise attitude is maintained using magnetic and solar measurements, namely with two 3-axis magnetometers and eight coarse Sun sensors, while magnetotorquers act as the only actuators.
SMOS was launched by one of the modified Russian Intercontinental Ballistic Missile (ICBM) SS-19 launchers, which were decommissioned as a consequence of the Strategic Arms Reduction Treaty (START). Launch was from the Plesetsk Cosmodrome in northern Russia on
Last update: 2 November 2009
SMOS: ESA's water mission 
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