Abstract: A system and method for correcting pointing errors induced by an inclined orbit (14) for a satellite (20) that is adapted to be in an orbit having zero inclination (12). The present invention generates a yaw correcting cosine rate command (34) that is integrated (36) by a steering law having input parameters dependent upon the angle of inclination, &THgr;, to derive a sinusoidal yaw offset. The present invention also generates a roll correcting cosine rate command (54) that is integrated (56) according to a steering law having input parameters dependent upon the angle of inclination, &THgr;, to derive a sinusoidal roll offset.

Abstract: A method, apparatus, article of manufacture, and a memory structure for controlling an attitude and a nutation of a spacecraft spinning about a first axis. The method comprises the steps of measuring a spacecraft attitude rate &ohgr;1(t) about a second axis substantially orthogonal to the first axis; generating a rate correction value &agr;&ohgr;1(t)/&lgr;0, wherein &lgr;0 is an inertial nutation frequency of the spinning spacecraft, and &agr; is a proportionality factor; determining a spacecraft attitude angle &phgr;2(t) about a third axis substantially orthogonal to the first axis and the second axis; generating an error signal e(t) as &phgr;2(t)+&agr;&ohgr;1(t)/&lgr;0; and applying a torque to rotate the spacecraft along the second axis. The article of manufacture comprises a program storage device storing instructions implementing the instructions described above.

Abstract: A method is provided for controlling the attitude of a momentum biased spacecraft about all three mutually orthogonal axes during a thruster firing maneuver without separately sensing angular displacement, angular rate, or angular acceleration about the spacecraft yaw axis. The method includes a transient phase during which a yaw feedforward torque is computed based on the known pitch/yaw coupling due to thruster geometry, and a steady state phase, during which feedback gains are adjusted to estimate yaw attitude based on coupling of a steady state disturbance torque about the roll axis of the spacecraft and a steady state disturbance torque about the yaw axis of the spacecraft.

Abstract: Spacecraft cost, volume and weight are reduced with gyroscope calibration methods that can be effected with structures (e.g., a single-axis Sun sensor and a wheel system) which are typically carried by spacecraft for other purposes. In a method embodiment, these methods include an initial step 1) of calibrating a selected gyroscope and subsequent steps for each uncalibrated gyroscope of: 2) slewing the spacecraft by a slew angle and controlling attitude with the uncalibrated gyroscope for a selected time period to thereby couple its drift into the inertial axis of a calibrated gyroscope, 3) backslewing the spacecraft to return the calibrated gyroscope to its inertial axis, and 4) measuring an error angle generated by the uncalibrated gyroscope during the selected time period (e.g., with a Sun sensor or with the calibrated gyroscope).

Abstract: A method of simultaneously controlling East/West and North/South positioning and unloading momentum of a spacecraft while orbiting an object. The spacecraft has a thruster array and a momentum accumulator. The method entails moving said spacecraft towards a node of the orbit. At a predetermined position on the orbit, separate from the node, a thruster of the thruster array is fired so as to control the orbital position of the spacecraft. While the thruster is being fired, momentum is dumped from the momentum accumulator at the predetermined position so that any loss in control in the attitude of the spacecraft is reduced.

Abstract: An earth chord sensor on board a satellite produces an electrical pulse as the satellite spins and sweeps the sensor across the earth. If the satellite is spinning at a low spin rate or if the sensor is measuring a long earth chord, the sensor produces an irregular pulse with sharp voltage peaks rather than a near trapezoidal pulse. The present invention digitizes the output of the earth chord sensor and a processor processes the digitized output through a data buffer to detect the leading edge peak and trailing edge peak of the pulse. The processor may detect the peaks through parabolic curve fitting among the digitized data points of the sensor output. The processor calculates the earth chord time as the time width of the pulse between the detected leading edge peak and trailing edge peak of the pulse and transmits, by telemetry, the calculated earth chord time to a ground station for use in satellite positioning.

Abstract: Methods for performing attitude maneuvers of a spinning satellite without the use of thrusters, or with a minimal number of thrusters, are disclosed. The attitude maneuvers are primarily achieved through the use of gimballed momentum wheels and solar wing drives. Various maneuvers can be performed depending on whether the satellite has near-zero net momentum or significant net momentum. The maneuvers include sun acquisition, sun hold, Earth acquisition and inversion.

Abstract: Solar tabs are provided which have thermal emissivity and absorptivity properties designed to increase the attitude-control torques that can be generated with rotatable wings in a spacecraft. These increased torques enhance the ability to offset attitude-disturbance torques which typically tend to alter the attitude of a spacecraft from a desired attitude. The tabs include highly absorptive and nonemissive front faces, highly emissive back faces and a high thermal conductivity between the front faces and the back faces.

Abstract: A method of simultaneously performing a translational maneuver of a spacecraft by a thruster and dumping momentum from the spacecraft during a time period P. The method entails aligning the thruster along a thrust vector which is fixed during the time period P, wherein the thrust vector is aligned with the center of mass of the spacecraft at a time P/2, and firing the thruster throughout the time period P.

Abstract: Methods and systems for stabilizing satellite spin about an intermediate inertia axis (Z) are disclosed. A set of gyros (22) sense the X component and the Y component of the angular velocity of the satellite body. A single degree of freedom momentum wheel (26) has a fixed transverse orientation with respect to the intermediate axis in order to store momentum. In one embodiment, the momentum wheel (26) is oriented to store momentum parallel to the Y axis. A tachometer (30) senses the rotation rate of the momentum wheel (26). A processor (24) forms a control signal representative of a control torque to be applied to the momentum wheel (26). The control torque is based upon the X component and the Y component of angular velocity of the satellite, and the angular velocity of the momentum wheel (26).

Abstract: A method of attitude control for spacecraft with flexible structures utilizes an estimator/state controller pair with on-board time-varying gain scheduling. The control method includes an attitude estimator (100) for each axis, which uses rate input from inertial reference sensors (4, 5, 6) to produce estimates (37, 38, 39) of each of the state variables. The estimator employs a predictor-corrector structure which computes initial rate and position estimates for each sample interval and corrects these values by weighing them with iteratively-calculated time-varying gains according to equations 35 and 36. The state controller (40) for each axis operates on these inputs, combining them with position and rate commands (41, 42) and weighing the results with time-varying gains calculated iteratively for each sample period according to equations 46, 47, and 48. The final result is a commanded control acceleration (50) which is forwarded to a thruster modulation logic.

Abstract: The three axes thruster modulation (8) of the present invention accepts three axes of input torque commands or angular acceleration commands and generates thruster selection and thruster timing (40) information which is used to fire thrusters (48) for the purpose of spacecraft attitude control and velocity change maneuvers. The modulation logic (8) works in all three axes simultaneously and is suitable for use with an arbitrary thruster configuration, including a configuration in which individual thrusters or thruster groups do not produce torques about mutually orthogonal axes. After thruster selection and on-times have been determined, the modulation logic (8) uses this information to compute a best estimate of the actual rate change (42) which is then compared to the commanded rate change (44) to develop a residual unfired rate change.

Abstract: A method of sun search and acquisition is disclosed for a spacecraft (18) which is stabilized in three axes. The method utilizes three axes gyro rate and integrated rate sensing together with three simple slit type sensors (10, 20, 30, 40). The method starts from an arbitrary attitude and from body rates up to the limits of the gyro rate sensors and is comprised of a rate nulling step followed by two consecutive simple search procedures. One search procedure is about the pitch axis until alignment of the sun with the roll/pitch plane and the other search procedure is about the yaw axis until alignment of the sun with the roll axis. The sun acquisition culminates in pointing the roll axis of the spacecraft (18) toward the sun. By using slit type sun sensors (10, 20, 30, 40) versus a wide field of view sensor it is easier to find a mounting location on a spacecraft (18) which is free from spurious reflections off other parts of the spacecraft.

Abstract: A method for acquiring Earth-pointing attitude of a three-axis, body-stabilized spacecraft orbiting the Earth in a prescribed orbit plane, e.g. a geosynchronous communications satellite, including the steps of aligning the roll axis of the spacecraft with the sun line (which is the vector directed from the spacecraft to the Sun); then, orienting the spacecraft such that the angle formed between the yaw axis and the sun line is equal to the Earth-Sun angle (which is the angle formed between the sun line and a vector directed from the origin of the spacecraft internal coordinate system to the Earth); then, orienting the spacecraft such that the yaw axis is aligned with the center of the Earth; and finally, rotating the spacecraft about its yaw axis until its pitch axis is oriented at a desired attitude relative to the orbit plane, e.g., normal to the orbit plane, to thereby complete acquisition of the Earth-pointing attitude.

Abstract: A nutation sensor and nutation control system using a relative spin phase sensor in a dual-spin stabilized satellite is disclosed. The relative spin phase sensor provides index pulses indicative of the relative spin phase and rate between the satellite rotor and platform. Nutation of the satellite perturbs the relative spin position of the rotor and platform and means are provided for analyzing index pulse arrive times to calculate nutation of the satellite. In the nutation control system, means are provided to apply a nutation damping torque to the satellite.

Abstract: 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.