High Accuracy Attitude Determination and Control of the DIVA Mission

Small satellite missions for scientific purpose gained more and more importance during the last years in Germany. There are several missions inorbit or scheduled for launch in the near future like GFZ (1995), Equator-S (inorbit, launch Dec 1997), ABRIXAS (launch April 1999) and other missions are on the horizon. One of these highly ambitious projects is the small astrometry satellite DIVA (German Interferometer for Multichannel Photometry and Astrometry). DIVA will be the successor of the HIPPARCOS satellite and a pathfinder to much larger missions like GAIA (Global Astrometric Interferometer for Astrophysics), FAME (Fizeau Astrometric Mapping Explorer) and SIM (Space Interferometry Mission). This work deals with the recent results from spacecraft dynamics simulations performed for the 150 kg DIVA satellite. The attitude determination requirements are quite ambitious for a small satellite mission, while the control requirements of 5 arcmin are moderate. The attitude determination system has to provide attitude information to the scientific payload with an accuracy of 1 arcsec in scan direction and 3 arcsec perpendicular to it using a Kalman filter that relies on attitude sensors and the scientific telescope. The scientific goal is a post mission scientific attitude accuracy of about 0.8 mas. The attitude control system relies on a cold gas system for first acquisition, safe mode and coarse attitude control. For fine attitude control the system is supported by utilizing the solar radiation pressure to minimize the thruster actions. Therefore, the solar panel alignment will be adjusted to support the nominal scanning law.