The SMOS payload will be carried on a standard spacecraft bus called PROTEUS, which stands for "Plateforme Reconfigurable pour l'Observation, les Télecommunications et les Usages Scientifiques", which development was initiated in 1996 by the French space agency (CNES) and Thales Alenia Space. It is a generic multi-mission platform utilized by JASON1 (2001), soon by CALIPSO (2006) and COROT (2006). Thanks to with well-defined interfaces so that with limited adaptations, the SMOS scientific payload will be be mounted on the top of the spacecraft. The PROTEUS system architecture consists not only in a generic platform, but also in the different required means to validate and integrate it (SW and Systems validation benches, AIT GSE). The Bus design has been driven by such concerns as simplification of the system architecture by suppressing the subsystem level and replacing it by functional chains directly merged at system level and selection as far as possible of off the shelf equipment units.
Although the reduced spacecraft bus size, measuring just 1 m x 1 m x 1 m, it acts as a service module accommodating all the subsystems that are required for the satellite functions. Main platform capabilities are gathered in following figure.
|Mass||Platform up to 300 kg, Payloads up to 350 kg|
|Orbit||any LEO from 20° to SSO, 700 km to 1500 km|
|Launchers||Any small SL launcher - associated fairing volume|
|Power||Up to 300 W Platform + 350 W Payload|
|Orbital capacity||Up to 120 m/s for a 500 kg spacecraft|
|Autonomy||Ability wrt 1 ground station, operated during working days only|
|Pointing accuracy||0.05° bias + 0.05° 3s per axis + 1 .E-3°/s low frequency stability|
|Payload IF||Dedicated MIL STD 1553 bus 160 kbps + dedicated TM/TC|
|Data storage||500 Mbits Bus + 2 Gbits Payload, End of life|
|TM/TC||S band, 800 kbps TM, 4 kbps TC, CCSDS|
|Orbitography||Use of an on board [no-lexicon]GPS [/no-lexicon]receiver: no ground station angular measurements, nor ranging|
|Lifetime||3 years. All elements subject to aging + radiations sized to 5 years|
The PROTEUS Platform to SMOS mission completed the full mechanical integration in March 2006.
The platform mechanical structure with the differents sub systems are shown on the picture in front of a joint ESA/CNES/AAS team in clean room at Alactel Alenia Space facility in cannes.
For AIT coming operations, the 4 lateral panels are opened. Four small reaction wheels, for generating torque for attitude adjustment, surroundes the hydrazine tank mounted on the center in the base structure. This hydrazine monopropellant system is devoted to the four 1-Newton thrusters that are mounted on the base of the spacecraft (not visible on the photo). The generic harness already manufactured including heaters of the thermal control has been also integrated. The specific harness devoted to the SMOS mission and ensuring interface with the payload will be integrated in the last steps of integration activities late 2006.
On the right panel, 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. On same panel, TMTC equipment using S Band insure through two antennae a spherical coverage with respective TC-TM rates at 4 kbps and 800 kbps.
On the top structure, the electrical on-board command and data handling architecture is centralised on a Data Handling Unit, including a 3 Gbits mass memory storage capacity, plus a MIL 1553 bus dedicated to Payload Remote Terminals. It manages the satellite operational modes and interfaces with the payload central processor unit, forwarding payload commands received from the ground and supplying all auxiliary satellite data that are needed by the payload to fulfil its scientific measurements.
Additional activites will complete the remaining missing subsystems: two solar panels will be driven by two Solar Drive Mechanisms mounted on lateral panels. After launch, when the spacecraft has separated from the launcher, there will be an automatic start-up sequence, which will result in the deployment of 2 symmetrical solar arrays wing with classical silicon cells. The energy is stored in a lithium-ion battery (non mounted on the first front panel) and distributed through a non-regulated electrical bus. Nominal attitude control is based on a autonomous Star Tracker type - lost in space acquisition capacity - which provides accurate attitude information for both the instrument measurements and the satellite attitude control. This start tracker will be accommodated on the payload, when delivered, to minimize thermo-elastics concerns.
The platform thermal-control subsystem relies on passive radiators and active regulation by heaters. Thermal concept is based on 3 platform separated zones: battery, propulsion, platform equipment units and Star Trackers located outside, thermally decoupled from the Payload. Each zone has its radiators, sizable from one mission to the other depending on pointing and dedicated heaters driven by SW.