The Submillimeter Wave Astronomy Satellite Attitude Control Software Design

The Submilimeter Wave Astronomy Satellite (SWAS) was selected for flight by NASA in 1989 as a part of the Small Explorer (SMEX) program. SWAS's primary science objective is to conduct high spectral resolution surveys of galactic molecular clouds. The SWAS Attitude Control System (ACS) is three-axis controlled, zero momentum stabilized, and is capable of performing subarcminute pointing. This paper will discuss the design of the software components which comprise the Attitude Control System Software. The paper first describes the high level requirements which governed the architecture of the Attitude Control System Software. Next, the context in which the software resides is explained. This environment includes: the Spacecraft Computer System (SCS), Attitude Control Electronics (ACE) with sensors and actuators, and a CCD star tracker. The Attitude Control software architecture is described and each of the processes are outlined. These processes include: main, sensor data collection and conversion, star tracker data collection and star matching, attitude determination, attitude control, command input, telemetry output, and failure detection and handling. The final discussion in this paper addresses the design issues allowing it to be easily modified for reuse on subsequent missions.