STEREO observatory trade studies and resulting architecture

The Solar Terrestrial Relations Observatory (STEREO) mission is part of NASA´s Solar Terrestrial Probes (STP) program. The mission calls for two identically instrumented observatories to be placed in heliocentric orbit, one leading the Earth, the other lagging. The APL is responsible for mission design, integration and providing the two spacecraft (without instruments). Due to cost constraints, both spacecraft are single-string with redundancy added where it would have the greatest benefit. Redundancy does not include a duplication of the spacecraft´s avionics. In order to maximize heritage, the STEREO spacecraft avionics are based on the APL-designed Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) spacecraft. TIMED is a fully redundant spacecraft. Due to the symmetry of the avionics architecture, the initial STEREO design was based on dividing the TIMED spacecraft avionics in half. This provided a functional, though not optimal design. As one might suspect, there were numerous potential single point failures and no redundancy. This paper discusses the STEREO architecture as it pertains to avionics and the functional allocation amongst its processors. After a series of trades, functions were reallocated, providing a level of hardware and software robustness that was not present in the initial STEREO design. This paper details these trades, as well as the architecture of the two spacecraft. An emphasis is placed on safing and autonomous control of the spacecraft in the face of contingencies. Spacecraft modes and mode transitions are also described