Performance Evaluation of the Fiber Optic Gyroscopes integrated into the SOFIA Telescope Assembly

SOFIA, the Stratospheric Observatory for Infrared Astronomy, is an airborne observatory that will study the universe in the infrared spectrum: A Boeing 747-SP aircraft carries a 2.5 m telescope designed to make sensitive infrared measurements of a wide range of astronomical objects. During science observations, three low-noise fiber optic gyroscopes are used as fast feedback sensors to stabilize the telescope inertially. They are a vital component of the telescope pointing control system in order to fulfill the stringent pointing stability requirements. The paper introduces SOFIA and presents the results of recent gyroscope performance testing. It describes firstly long-term performance measurement after integration of the gyroscopes into the telescope assembly of the aircraft. The noise analysis of the gyroscopes is performed via Power Spectral Density and Allan Variance analysis. For the gyroscopes' application within the SOFIA telescope system, the two quantified noise parameters of interest are the angular random walk and the bias stability. In addition, various system effects caused by other telescope subsystems, aircraft systems or ground support equipment are identified in the gyroscope data along with the noise originated within the gyroscopes. These effects are characterized depending on their source and the specific telescope operation mode. Furthermore, the aircraft based data are compared to calibration measurements from the manufacturer, which were generated on an isolated rate table. The calibration data were obtained over a period of 9 years and include results after a recent replacement of the power units. They show a very constant sensor performance. In preparation for the first science observations in flight, several observation testing campaigns of the fully integrated telescope were completed on the ground in 2008. These included the testing of the three gyroscopes. The three telescope guiding cameras and one science instrument mounted on the focal plane of the telescope could be used to assess the gyroscopes' performance. The centroid position information from the imaged stars served as an inertial reference while the telescope was stabilized inertially using the gyroscope measurements as internal feedback signal. Although such centroid position information is affected by atmospheric disturbances, the centroid positions provided a high-resolution inertial attitude for absolute bias measurements of the gyroscopes. The absolute bias variations obtained are presented in regard to short period of times, as well as to day-to-day variations.