Analytical derivation and verification of zero-gyro control for the IUE satellite

The International Ultraviolet Explorer (IUE) satellite was launched January 26, 1978 into a geosynchronous orbit over South America. From its stationary position, the telescope maintains continuous communication with the control centers at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and at the European Space Agency's (ESA's) Villagranca del Castillo Satellite Tracking Station in Spain. Since its launch in 1978, the satellite has gradually lost four of the original six gyroscopes in the Inertial Reference Assembly (IRA). In August 1985, the fourth of the original six gyros failed and a two-gyro system developed by NASA-GSFC was uplinked to the satellite and is currently in use. A one-gyro system also developed by NASA-GSFC is ready for use in case of another gyro failure. In the event that the sixth gyro should also fail, a zero-gyro system is being developed. The goal of this system is to provide interial target pointing without the use of gyroscopes. The satellite has sun sensors to provide attitude information about two of the three axes. It relies upon the exchange of reaction wheel momenta to determine angular position and rate of the third axis.