ESA GNC Conference Papers Repository

FORMOSAT-3 mission, also known as COSMIC (Constellation Observing Systems for Meteorology, Ionosphere, and Climate), is an international collaboration of Taiwan and the United States to deploy a constellation of six micro satellites equipped with GPS occultation receivers (GOX) in low Earth orbits to detect GPS signal variations as they passing through the ionosphere and atmosphere. The six satellites were launched by one Minotaur launch vehicle into space on April 15, 2006. Since then, data associated with occultation occurrences of the constellation had been collected, processed, and assimilated into numerical weather models for real-time weather prediction by various agencies around the world. In addition to GOX, other two scientific payloads onboard each FORMOSAT-3 satellite are a tri-band beacon (TBB) and a tiny ionospheric photometer (TIP). The required constellation is in six orbital planes with 800 km altitude and 30 deg right ascension of ascending node apart. The Minotaur launch vehicle places the six FORMOSAT-3 satellites into a 515 km circular parking orbit. With each satellite raises itself to higher altitude one by one, six orbital planes will form after a period of time with different right ascension of the ascending node (RAAN) utilizing the nodal precession due to the oblateness effect of the Earth. The Attitude Control Subsystem (ACS) of the FORMOSAT-3 was designed, with only coarse sensors and no rate sensors, to maintain satellite in the Local-Vertical-Local-Horizontal (LVLH) attitude for scientific instrument operation. During orbit raising operation, the ACS maneuvers Satellite to align thruster to the anti-velocity direction for firing thruster. However, just after the successful launch, the ACS encountered series of major problems. For example, the bus GPS receiver failed to deliver or to deliver incorrect navigation information, the ACS was unable to maintain the proper attitude, and the ACS was failed to perform orbit raising operation. These problems were in the areas of validation philosophy, software implementation, design issue, and hardware interface, etc. This paper describes how the operation team solved the problems step by step and finally the ACS was able to maintain desired attitudes and to perform orbital raising operation for each of the six satellites.