Hughes Aircraft Company

Sibnath Basuthakur

Loren I Slafer

Bernard C Soriano

A method and system (12) for controlling the attitude of a spacecraft during its transfer orbit using an on-board, stand-alone, three-axes attitude determination and control system. The system utilizes a set of on-board sensors to define two independent angular measurements, which will initially identify the z-axis orientation of the spacecraft from an arbitrary attitude after launch vehicle separation. A set of three-axis gyros are then bias calibrated in order to measure the transverse rates of the spacecraft. The three-axis attitude of the spacecraft is continously determined by integrating the gyro outputs even if the Earth or Sun is not visible by an on-board sensor. A state estimator model is provided to determine the three-axis attitude of the spacecraft in the presence of large wobble and nutation. The system also utilizes a linear combination of the estimated attitude, rate and acceleration states to generate commanded rate increments with a pulse-width frequency modulator. The pulse-width frequency modulator seeks to linearize the inherently non-linear thruster duty cycle.

On-board three-axes attitude determination and control system

The estimation and compensation of fixed, but unknown uncertainties of a thruster (22) for improving satellite attitude estimation and control as well as eliminating the need for nutation damping devices is disclosed. A thruster (22) is fired and an attitude state is estimated using an estimator (116). A commanded rate change (122) is calculated, which is modified using a compensating feedforward command (126) as updated by a fixed uncertainty estimator (124) unless an estimated rate is less than an absolute value of a threshold rate. The modified commanded rate change (130) is used to incrementally adjust thruster fire on-time (104) until a predetermined satellite trajectory is reached.

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Rancho Palos Verde, CA, United States of America

Sibnath Basuthakur