Attitude estimation in the presence of atmospheric refraction

This paper discusses the attitude estimation of an optical sensor in the presence of atmospheric refraction. This optical sensor consists of a focal plane array (FPA) mounted within a telescope looking through the atmosphere. A pointing and stabilization system provides a stabilized scan for the telescope to view the volume of interest, and it measures and reports the telescope line-of-sight (LOS). Nominal LOS attitude is determined from gyro measurements with a strap down inertial navigation system. A steller update filter, implemented as a linearized Kalman filter, is also required to bound attitude determination bias errors which would otherwise continue to grow with time. This stellar update filter utilizes star sightings to predict the angular bias errors, the LOS orientation error, and the refraction-induced error by processing star residuals and atmospheric density residuals. Simulation results indicate that initial angular measurement bias errors can be reduced to meet tight performance requirements with a telescope scan pattern that first scans at high elevation to estimate angular bias errors, then scans at low elevation to estimate atmospheric refraction parameters.