Altitude Reconstitution of a Small Scientific Satellite using a Limited Set of On-Board Sensors

The ALEXIS spacecraft carries a limited set of attitude instruments on board. The scientific mission of ALEXIS requires "after the fact" knowledge of the spacecraft attitude at all times. Reconstitution of the attitude is performed as one of the permanent tasks of the microcomputer based ground station. ALEXIS carries a fine sun-sensor, an Infrared horizon crossing indicator and a magnetometer. Because of the orbital and attitude configurations horizon and sun will not be observed during sizable fractions of some orbits. The attitude will be estimated at all times by an extended fitter making use of whatever data is available. The state vector includes disturbance and misalignment terms. Fiber initialization uses the first available set of valid data from sun sensor and magnetometer, for a deterministic fix insuring rapid convergence. Magnetic field measurements provide a ceiling to the possible errors when the optical sensors are not operating, and can be used as back up. Detailed simulations of the filter were run to assess its performance. The simulations include major disturbance torques, misalignments, and earth oblateness effects. They show that the filter maintains knowledge of the attitude well within the mission data requirements. As a result, with a minimal amount of low cost instrumentation, without a star tracker on board, ALEXIS attitude and rates can be reconstituted for all times from a simple microcomputer based ground station. This illustrates a practical trade-off between hardware and software.