Hughes Aircraft Company

Hughes Electronics Corporation

Loren I Slafer

John F Yocum, Jr

Douglas J Bender

John W Smay

Michael F Barsky

William F Hummel

Thomas M Tanner

Paul D Williams

Sibnath Basuthakur

James H Green

Bernard C Soriano

George B Semeniuk

Joseph M Allard

Carl L Chen

A Stationkeeping Mode for a spacecraft having a gimballed momentum wheel for attitude control in which the stationkeeping is divided into short duration open-loop stationkeeping maneuvers to correct orbital errors, and interlaced with closed-loop momentum management operations to manage momentum stored in the momentum wheel. The sequence of open-loop pulses are spaced to deadbeat a given dominant flexible motion of the spacecraft. Between and during the thruster pulses, the gimbaled momentum wheel is maintained in the closed-loop feedback control using gyro sensor or other inertial reference measurements. Limitations on the range of the momentum storage device and its torque capability are minimized by combining the maneuver thruster pulses with pulses from other thrusters, or by reducing the fire time of one of the maneuver thrusters, to minimize momentum buildup. Thus, thrusters are used for changing orbital velocity and managing momentum through open-loop pulsing with closed-loop momentum refinement, and the momentum storage device controls the attitude with closed-loop attitude feedback.

Stationkeeping method utilizing open-loop thruster pulses and closed-loop authority limited momentum storage devices

A feedforward disturbance compensation system and method for use with a spacecraft that compensates for predictable disturbances exerted on the spacecraft. The invention comprises a model of the disturbance that is to be compensated, means for knowing when the disturbance will occur, and means for applying the feedforward compensation to the spacecraft in accordance with the model (a compensation profile). The present invention reduces pointing transients through the application of open-loop, feedforward compensation of actuators used to point the spacecraft. The shape of the feedforward profile is determined by an in-orbit test or is predicted by analytical means. For the in-orbit test case that compensates for an eclipse, for example, the spacecraft is flown through the eclipse once to determine the size and shape of the thermal shock disturbance and this information is stored in a spacecraft control processor. The application of the feedforward compensation is synchronized with the start and end of each day's eclipse. This may be achieved by sensing the change in battery discharge current that occurs in solar panels on the spacecraft, for example. On subsequent days, the invention anticipates the occurrence of the disturbance using the stored data. When the start or end of eclipse is detected, compensation for the expected disturbance is automatically applied to spacecraft actuators. The invention can also compensate for a large variety of known periodic disturbances other than thermal shock. These disturbances may be external or internal to the spacecraft. The present invention can also provide compensation by fine tuning the compensation magnitude and duration of the compensation, and is also applicable to both linear and nonlinear control systems.

Spacecraft disturbance compensation using feedforward control

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

A system located on board a geosynchronous satellite for detecting spin axis precession and developing thruster control signals for restoring the spin axis to the desired attitude. Fuel economy is enhanced by performing the major attitude correction at a time when the fuel expended contributes to that required for orbit inclination control.

Method for controlling the spin axis attitude of a spinning spacecraft

A pointing apparatus for a dual-spin spacecraft utilizing a first sensor for sensing the time of arrival of an inertial attitude reference as the spinning portion of the spacecraft rotates, and a second sensor for sensing the time of arrival of an index reference which relates the position of the despun portion with the spinning portion. A digital processor estimates the spin rate and phase of the spinning portion from the inertial attitude reference time of arrival, estimates the relative spin rate and phase between the spinning portion and the despun portion from the index reference time of arrival, and estimates the bearing friction bias torque on the motor means which controls the pointing direction of the despun portion of the spacecraft. The spinning portion spin rate and phase estimates are added with the relative spin rate and phase estimates to produce an estimate of the despun portion spin rate and phase, and the despun portion spin rate and phase estimates and the friction bias torque estimates are subtracted from commanded despun portion spin rate, phase and friction bias torque states. A torque command is generated for controlling the motor means from the subtracted estimates.

Precision platform pointing controller for a dual-spin spacecraft

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Los Angeles, CA, United States of America

Loren I Slafer