New methods of determining spacecraft attitude

Abstract

The IUE spacecraft was launched with prime and redundant mechanical Panoramic Attitude Sensors (PAS) to determine coarse spacecraft pointing. Attitude determination typically took at least 24 hours. After launch both systems failed. A new method was developed which required pointing the spacecraft at the antisolar position. After the failure of the 4th IUE gyro, it was no longer possible to point in the antisolar direction. A second method was developed which utilizes IUE's ability to track the sun with a solid state two-dimensional sun sensor. Attitude determination can now be completed in several hours. An hour is required for coarse position measurement and several more hours are needed, using a small 15 arc minute square finder camera, for final attitude confirmation. These methods should be of use for other spacecraft where weight is critical or there is a desire to avoid mechanical devices.