Real-Time Optimal Attitude Estimation Using Horizon Sensor and Magnetometer Data

摘要

Full three-axis knowledge was desired for a momentum bias spacecraft employing only a horizon sensor for attitude determination. This sensor provided adequate knowledge for rotations about two axes but no information for rotations about the nadir axis. Though satisfactory for many applications, proximity and rendezvous operations of a thrusting vehicle near the Shuttle are best supported with three-axis knowledge. The solution devised takes three-axis magnetometer outputs, used on the spacecraft to scale magnetic torquer commands, and horizon sensor outputs to establish an overdetermined attitude matrix. The attitude matrix is solved by minimizing a chi-square loss function relating the measured vectors to internally modeled reference vectors. The reference vectors are provided by a Keplarian propagator modified for oblateness effects and the 1995 World Magnetic Model 12th-order geomagnetic field coefficients. The approach is implemented on a laptop computer for real-time flight operations. Ground simulations indicate three-sigma errors of about 1.3 deg in pitch and roll and 2.6 deg in yaw. Tests conducted on the Shuttle's robot arm prior to release of the spacecraft on STS-69 and STS-80 confirmed these expectations. The system performed well during free flight and provided valuable attitude knowledge during perturbations due to tip-off and thruster alignment uncertainties.